Step 1

The databases of Scopus, Web of Science, and Peanut Science (journal of the American Peanut Research and Education Society) covering 53 yr (from 1970 to July 2022; accessed July 12, 2022) were searched using predefined search terms (Table 1). Peanut Science was included because it is currently not indexed in Scopus or Web of Science but publishes peer-reviewed results of peanut research in the United States.

Table 1. Search terms and exclusion criteria used to identify relevant articles in the databases of Scopus, Web of Science, and Peanut Science (accessed: July 12, 2022).

Step 2

The total record (2,178 peer-reviewed articles) from the three databases was screened to identify the articles’ relevance for the review by refining the search terms based on exclusion criteria (Table 1). This resulted in a refined cohort of 555 peer-reviewed publications.

Step 3

The refined cohort of 555 peer-reviewed publications from the three databases was exported and combined in Excel, with the year of publication as rows and contents (journal, research focus, weeds studied, study type [field, greenhouse, or laboratory], study location, number of site-years, research methods, and abstract) as columns.

Step 4

Duplicates (78 peer-reviewed publications) were removed, and the remaining publications (477) were further screened by two independent researchers for their relevance by reviewing the titles and abstracts. This resulted in 273 unique and relevant publications that were subsequently reviewed. Of the 273 publications reviewed, 81 (30%) were relevant to nonchemical weed management, while 192 (72%) were focused on chemical weed management. Because a review of chemical and nonchemical weed management research in peanut for the last five decades (53 yr: 1970 to 2022) in the United States is too broad to be covered extensively in one paper, only the 81 publications that focused on nonchemical weed management are discussed in the “Research Priority Areas” section in the current paper. The remaining 192 publications focused on chemical weed management are covered in the second part of this publication series.

Geography and Peanut-Growing Regions

More than half of the weed research in peanut (56%) is conducted in the U.S. Southeast (Alabama: 14%; Florida: 13%; and Georgia: 29%) while 23.7% and 19% is conducted in the Virginia–Carolinas (North Carolina, South Carolina, and Virginia) and Southwest (Oklahoma: 3.3%, Texas: 16%) regions, respectively (Figure 1). This level of research corresponds with the importance of peanut in these regions, as about 65%, 17%, and 13% of the total peanut production in the United States is from the Southeast (Alabama, Florida, Georgia, and Mississippi), Southwest, and Virginia–Carolinas regions, respectively (USDA-NASS 2021). Around 85% (231 of 273) of the studies were conducted as individual state trials, with 41% in single sites and 59% in multiple sites. Only 14% of the studies were conducted at multistate levels or in a regionally coordinated manner.

Figure 1. The number of weed studies (1971–2022) from the major peanut-producing states in the United States.

Study Focus and Methods

Weed research in peanut in the United States focused primarily on weed management (88%). Among the remaining 12% (32 of 273) not focused on weed management, the majority (11%) assessed the effect of weed interference and the CPWC in peanut, while others (1%) presented insights on weed distribution. Seventy-two percent of the studies focused on curative weed management approaches based on chemical weed control with emphasis on optimized herbicide programs. Curative weed management with mechanical options (5%) and preventive management approaches (13%) that influence crop competition, including planting timing, planting pattern, row spacing, and the use of competitive cultivars, have received less attention. Despite the increased advocacy for integrated approaches that combine preventive and curative measures as the best way of keeping weed pressure below thresholds that reduce yields and profits (Swanton et al. Reference Swanton, Mahoney, Chandler and Gulden2008), studies on integrated weed management in peanut (8%), although increasing since 2010, remain relatively low (Figure 2).

Figure 2. The number of weed studies (1971–2022) focusing on different weed control methods in peanut in the United States.

Most of the weed research in peanut in the United States (83% of the studies) was conducted as on-station field experiments. Other methodologies, such as on-farm researcher-led experiments (3%), combinations of on-station and on-farm studies (6%), field and greenhouse studies (4%), field and laboratory studies (0.4%), field, greenhouse, and laboratory studies (2%), and observational studies such as weed surveys (0.7%), have been used less frequently.

Weed Types and Species

Most of the weed research studies (60%) in peanut-cropping systems in the United States are focused on broadleaf weeds, while only 23% and 19% are relevant to grasses and Cyperus spp., respectively (Figure 3A). Broadleaf weeds such as morningglory species (Ipomoea spp.) (25%), sicklepod [Senna obtusifolia (L.) Irwin & Barneby] (23%), Florida beggarweed [Desmodium tortuosum (Sw.) DC.] (21%), pigweed species (Amaranthus spp.) (12%), and common lambsquarters (Chenopodium album L.) (9%) are the most prominent in the literature (Figure 3B). The greater research attention on these weed species could be explained by their prevalence in peanut fields, as they are ranked among the most common and problematic weeds in peanut in the United States (Cardina and Brecke Reference Cardina and Brecke1991; Webster and MacDonald Reference Webster and MacDonald2001; Wilcut et al. Reference Wilcut, York, Grichar, Wehtje, Pattee and Stalker1995).

Figure 3. The number of weed studies (1971–2022) focusing on a particular weed type or weed species.

Amaranthus spp. did not receive much attention before the year 2000, unlike other broadleaf weeds such as D. tortuosum and S. obtusifolia, which have been more frequently studied for many years in peanut. However, the number of studies focused on Amaranthus spp. species, particularly A. palmeri, increased steeply thereafter (Figure 3B). This can be attributed to the increased awareness of the need for diversified management options for A. palmeri, which was driven by the evolution of resistance to herbicides commonly used in peanut and rotating crops (Sperry et al. Reference Sperry, Ferrell, Smith, Fernandez, Leon and Smith2017). As discussed earlier, A. palmeri biotypes are resistant to ALS-inhibiting herbicides and also dinitroanilines, glyphosate, triazine, and photosystem II- and hydroxyphenylpyruvate dioxygenase-inhibiting herbicides in some parts of the United States (Bond et al. Reference Bond, Oliver and Stephenson2006; Culpepper et al. Reference Culpepper, Grey, Vencill, Kichler, Webster, Brown, York, Davis and Hanna2006; Heap Reference Heap2023; Norsworthy et al. Reference Norsworthy, Griffith, Scott, Smith and Oliver2008; Ward et al. Reference Ward, Webster and Steckel2013; Wise et al. Reference Wise, Grey, Prostko, Vencill and Webster2009).

It is noteworthy that other broadleaf weed species such as A. artemisiifolia, bristly starbur (Acanthospermum hispidum DC.), coffee senna [Senna occidentalis (L.) Link], common cocklebur (Xanthium strumarium L.), eclipta [Eclipta prostrata (L.) L.], horse purslane (Trianthema portulacastrum L.), prickly sida (Sida spinosa L.), spurred anoda [Anoda cristata (L.) Schltdl.], tropic croton (Croton glandulosus L.), and wild poinsettia (Euphorbia heterophylla L.) were reported in <10% of the weed studies (data not shown). These weed species can be problematic in peanut under certain conditions (Clewis et al. Reference Clewis, Askew and Wilcut2001; Grichar Reference Grichar2007; Place et al. Reference Place, Reberg-Horton, Jordan, Isleib and Wilkerson2012; Royal et al. Reference Royal, Brecke and Colvin1997; Walker et al. Reference Walker, Wells and McGuire1989; Webster and MacDonald Reference Webster and MacDonald2001), but are not a widespread problem (Webster Reference Webster2013), which may justify the low research attention they have received.

Only 1.8% (5 out of 273) of the weed studies were focused on tropical spiderwort (Commelina benghalensis L.) (data not shown). This level of research is not proportionate to the level of importance of C. benghalensis in peanut, as it is identified as one of the most troublesome and difficult weeds to control in peanut in the U.S. Southeast, the major peanut-producing region (Morichetti et al. Reference Morichetti, Ferrell, MacDonald, Sellers and Rowland2012; Webster et al. Reference Webster, Burton, Culpepper, York and Prostko2005). Commelina benghalensis is tolerant to many commonly used herbicides, especially glyphosate (Culpepper et al. Reference Culpepper, Flanders, York and Webster2004; Spader and Vidal Reference Spader and Vidal2000), which suggests the need for more research studies on diversified options for C. benghalensis management in peanut.

The most-studied grass weed species in peanut in the United States are Texas panicum [Urochloa texana (Buckley) R. Webster] (10%), large crabgrass [Digitaria sanguinalis (L.) Scop.] (6%), goosegrass [Eleusine indica (L.) Gaertn.] (3%), broadleaf signalgrass [Urochloa platyphylla (Munro ex C. Wright) R.D. Webster] (3%), bermudagrass [Cynodon dactylon (L.) Pers.] (2%), and fall panicum [Dicanthelium dichotomum (L.) Gould var. dichotomum] (1%), while weed research on sedges has mainly focused on yellow nutsedge (Cyperus esculentus L.) (19%). Although annual grasses are very competitive, they are not considered a major problem in peanut high-input systems because of the availability of residual herbicides, such as flumioxazin, pendimethalin, and S-metolachlor, and postemergence herbicides, such as clethodim, fluazifop-P-butyl, and sethoxydim, that can provide effective control of these weed species (Burke et al. Reference Burke, Price, Wilcut, Jordan, Culpepper and Tredaway-Ducar2004; Johnson and Mullinix Reference Johnson and Mullinix2005). This may justify the lower research attention for grasses compared with broadleaf weeds in peanut. However, these weeds are a major problem in organic peanut production (Johnson and Mullinix Reference Johnson and Mullinix2008) and should be considered research priority species. On the other hand, the dominance of weed studies focused on C. esculentus compared with individual grass weed species is justified by its allelopathic effect and perennial growth habit, which make it difficult to control (Johnson and Mullinix Reference Johnson and Mullinix2003). In addition, the tubers of C. esculentus can be a contamination in the harvested crop. Cyperus esculentus can exert great competition and yield reduction through allelopathy, and there are limited herbicide options for its management (Webster and MacDonald Reference Webster and MacDonald2001; Webster et al. Reference Webster, Burton, Culpepper, York and Prostko2005).

Weed Ecology and Distribution

Only 12% of the published weed research studies on peanut in the United States have focused on weed ecology. The weed ecological research in peanut was conducted mainly on weed distribution, weed interference and competition, and the CPWC.

Weed Distribution

Despite the acknowledged importance of weed distribution research as a valuable tool to identify the problem weeds of an area, understand weed community diversity, and provide direction for future research efforts (Webster and Coble Reference Webster and Coble1997; Webster Reference Webster2001), only 1% (3 out of 273) of the weed research studies in peanut in the United States have focused on weed distribution. However, from 1971 to 2013, the Southern Weed Science Society (SWSS), USA, presented an annual weed survey report of the 10 most common and troublesome weeds in major agronomic crops, including peanut. These reports, compiled annually in the proceedings of the SWSS, provide insights into weed distribution and the relative importance of various weed species associated with peanut for each of the participating southern states (Alabama, Arkansas, Florida, Georgia, Mississippi, North Carolina, Oklahoma, South Carolina, and Virginia). Summaries of these regionally coordinated surveys published by Elmore (Reference Elmore1984) and Webster and Coble (Reference Webster and Coble1997) and a state-specific weed survey from Georgia by Webster and Macdonald (Reference Webster and MacDonald2001) indicated important changes in weed species composition over time in response to production practices. For example, weed surveys conducted in the early 1970s indicated that X. strumarium was the most troublesome weed in peanut; however, by the early 1980s and 1990s, it was ranked the seventh most important species (Elmore Reference Elmore1984; Webster and Coble Reference Webster and Coble1997). Similarly, Cyperus species were previously identified as the most troublesome weed species in peanut in the southern United States in the early 1980s and 1990s, but their relative importance in peanut decreased thereafter (Elmore Reference Elmore1984; Webster and Coble Reference Webster and Coble1997; Webster and Macdonald Reference Webster and MacDonald2001). These shifts in weed species composition and the relative importance of weeds associated with peanut were attributed to the introduction of new herbicide chemistries that provided selective control of some troublesome weed species. For instance, Cyperus species are shade intolerant and become more established after most grasses and small-seeded broadleaf weeds have been controlled with dinitroaniline herbicides, which do not have activity on Cyperus species (Webster and Nichols Reference Webster and Nichols2012). The reduction in the relative importance of Cyperus species over time may be due to the introduction of newer chemistries like imazethapyr, imazapic, and diclosulam, which have good activity on both C. esculentus and purple nutsedge (Cyperus rotundus L.) or the increased application of herbicides such as bentazon and metolachlor that can suppress C. esculentus growth (Grichar Reference Grichar2002; Webster and Coble Reference Webster and Coble1997).

Some differences were reported in the common and troublesome weed species between the peanut-producing regions in the United States (Elmore Reference Elmore1984; Webster and Coble Reference Webster and Coble1997), even within the same state (Webster and Macdonald Reference Webster and MacDonald2001), with some species prevalent in one climatological district but absent in another. For example, C. benghalensis was ranked as the eighth most troublesome weed in peanut in Georgia but was found only in the southwestern and south-central districts and was listed among the top five species in only eight counties (Webster and Macdonald Reference Webster and MacDonald2001). This suggests that weed community composition can vary between farms, states, and regions, thus requiring more tailored weed management strategies. However, several weed species such as D. tortuosum, E. indica, Ipomoea spp., A. palmeri, S. obtusifolia, and U. texana are consistently associated with peanut across different environments and have frequently been identified as being among the most troublesome weeds in peanut for many years (Elmore Reference Elmore1984; Webster and Coble Reference Webster and Coble1997; Webster and Macdonald Reference Webster and MacDonald2001). Their prevalence in peanut-cropping systems is attributed to traits such as hard seed coats, which ensure a persistent seedbank and limits the effectiveness of residual herbicides (e.g., S. obtusifolia); large seed size, which enhances germination from deeper soil depth (e.g., D. tortuosum, Ipomoea spp., and S. obtusifolia); prolific seed production, which increases weed population; and rapid growth, which enhances competition (e.g., A. palmeri and S. obtusifolia) (Lancaster et al. Reference Lancaster, Jordan, Spears, York, Wilcut, Monks, Batts and Brandenburg2005; Webster Reference Webster2001; Webster and MacDonald Reference Webster and MacDonald2001; Webster et al. Reference Webster, Burton, Culpepper, York and Prostko2005; Wilcut et al. Reference Wilcut, York, Grichar, Wehtje, Pattee and Stalker1995). Also, the evolution of resistance to herbicides commonly used in peanut and in rotating crops has favored the widespread distribution of A. palmeri (Poirier et al. Reference Poirier, York, Jordan, Chandi, Everman and Whitaker2014). For instance, herbicide-resistant A. palmeri was reported as the most troublesome weed in peanut in Georgia and Florida (Berger et al. Reference Berger, Ferrell, Dittmar and Leon2015; Webster Reference Webster2013) and was found to occur throughout the peanut-growing regions in the U.S. Southeast (Wise et al. Reference Wise, Grey, Prostko, Vencill and Webster2009).

Weed Interference and Competitive Mechanisms

Peanut–weed interference and weed competitive mechanism studies are important to understand weed dynamics and make appropriate weed management decisions (Jordan et al. Reference Jordan, Wilkerson and Krueger2003; Robinson et al. Reference Robinson, Moffitt, Wilkerson and Jordan2007). Competitive index parameters generated from such studies, along with other factors such as soil moisture status and cost of weed control, have been integrated into computer models and decision aids such as the Herbicide Application Decision Support System (HADSS™) (a trade name registered by North Carolina State University, USA) and computerized economic threshold decision (HERB™) (a trade name registered by North Carolina State University, USA) to accurately predict the level of yield loss at a given weed density and size in order to estimate economic thresholds and devise appropriate weed management strategies (Bennett et al. Reference Bennett, Price, Sturgill, Buol and Wilkerson2003; Scott et al. Reference Scott, Askew, Wilcut and Bennett2002; White and Coble Reference White and Coble1997). These computer decision models have been used to determine the appropriate herbicide and application rate recommendations, thereby improving the profitability of peanut production while minimizing herbicide inputs and reducing environmental impact (Bennett et al. Reference Bennett, Price, Sturgill, Buol and Wilkerson2003; Jordan et al. Reference Jordan, Wilkerson and Krueger2003; Scott et al. Reference Scott, Askew, Wilcut and Bennett2002).

Our systematic review of the literature indicates that 11% of the weed research studies on peanut in the United States focused on assessing the effect of weed interference and the CPWC. These studies showed that weed competition can reduce peanut yield by up to 80% depending on weed species, weed population densities, and the duration of weed interference (Burke et al. Reference Burke, Price, Wilcut, Jordan, Culpepper and Tredaway-Ducar2004; Chamblee et al. Reference Chamblee, Thompson and Coble1982; Everman et al. Reference Everman, Clewis, Thomas, Burke and Wilcut2008b; York and Coble Reference York and Coble1977). Peanut yield decreased with increasing weed density and periods of weed interference, which indicates that weed control is essential throughout much of the growing season (Everman et al. Reference Everman, Clewis, Thomas, Burke and Wilcut2008b). Predicted peanut yield loss from season-long weed interference and density-dependent competitive indices (i value) that indicate potential weed competitiveness using hyperbolic yield loss model (Cousins et al. Reference Cousins, Brain, O’Donnovan and O’Sullivan1987) showed that grasses have greater competitiveness and are more detrimental to peanut yield compared with broadleaf weeds and Cyperus spp. (Table 2). Season-long interference of grass weed species was reported to reduce peanut yield by 7% to greater than 60% (Everman et al. Reference Everman, Clewis, Thomas, Burke and Wilcut2008b; York and Coble Reference York and Coble1977). As few as 1.4 U. platyphylla plants m⁻² (Chamblee et al. Reference Chamblee, Thompson and Coble1982), 0.1 D. dichotomum plants m⁻² (York and Coble Reference York and Coble1977), and 2.2 U. texana plants m⁻² (Johnson and Mullinix Reference Johnson and Mullinix2005) reduced peanut yield by 25%. In contrast, greater densities of C. esculentus (68 plants m⁻²) and broadleaf weeds (D. tortuosum: 6.2 plants m⁻²; horsenettle [Solanum carolinense L.]: 4.2 plants m⁻²; and S. obtusifolia: 7.2 plants m⁻²) were required to cause similar yield reduction in peanut (Hackett et al. Reference Hackett, Murray and Weeks1987; Johnson and Mullinix Reference Johnson and Mullinix2003). However, X. strumarium (Royal et al. Reference Royal, Brecke and Colvin1997), A. artemisiifolia (Clewis et al. Reference Clewis, Askew and Wilcut2001), jimsonweed (Datura stramonium L.) (Price et al. Reference Price, Burke, Askew, Schroeder, Everman and Wilcut2006), C. glandulosus (Thomas et al. Reference Thomas, Askew and Wilcut2004), E. heterophylla (Bridges et al. Reference Bridges, Brecke and Barbour1992), A. palmeri (Burke et al. Reference Burke, Schroeder, Thomas and Wilcut2007), and A. hispidum (Walker et al. Reference Walker, Wells and McGuire1989) are more competitive broadleaf weeds, reducing peanut yield by 25% at much lower densities. Commelina benghalensis was also found to be a highly competitive broadleaf weed in peanut, with season-long interference reducing yield by 51% to 100% (Webster et al. Reference Webster, Faircloth, Flanders, Prostko and Grey2007).

Table 2. Competitiveness of weeds found in peanut in the United States based on Cousin et al.’s (1987) hyperbolic yield loss model [Y = iD/(1 + iD/100)], where D is the weed density per meter of peanut row, and i is the % yield loss as weed density approaches zero. ^a

^a NA, not available in the literature.

Although these weed interference studies were conducted using different peanut cultivars under different growth conditions and cultural practices, the values for yield reduction are similar among grass weed species, which is not the case for broadleaf weeds (Table 2). Apart from competition for growth resources, grass weed species generally reduce peanut yield through the production of a fibrous root system that entangles peanut pods during digging, resulting in excessive harvest losses (Johnson and Mullinix Reference Johnson and Mullinix2005). For example, Johnson and Mullinix (Reference Johnson and Mullinix2005) observed 836 kg ha⁻¹ harvest losses at 2 U. texana plants m⁻¹ of peanut row. Greater competition of the grass weed species may also be attributed to their C₄ metabolism, which confers a higher efficiency in water use, nutrient uptake, and net photosynthesis compared with peanut, which has a C₃ pathway of photosynthetic CO₂ fixation (Procopio et al. Reference Procopio, Santos, Pires, Silva and Mendonça2004; York and Coble Reference York and Coble1977). However, greater variability in peanut yield reduction due to interference among broadleaf weeds (Table 2) may be due to differences in their growth rate, canopy architecture, and shading effects on peanut as influenced by the prevailing growing conditions and cultural practices. Broadleaf weeds that grow very tall and above peanut canopies are generally more competitive and detrimental to peanut yield, because they intercept sunlight at the expense of the crop, leading to reduced phtosynthesis, consequently reducing yield (Barbour et al. Reference Barbour, Bridges and NeSmith1994; Walker et al. Reference Walker, Wells and McGuire1989; Webster et al. Reference Webster, Faircloth, Flanders, Prostko and Grey2007). For example, A. hispidum, with its very dense foliar canopy and greater shading effect on peanut, is at least three times more competitive than D. tortuosum and five times more competitive than S. obtusifolia (Walker et al. Reference Walker, Wells and McGuire1989). Desmodium tortuosum and S. obstutifolia at 1 plant 10 m⁻² reduced peanut yield by 16 to 30 kg ha⁻¹ and 6 to 22 kg ha⁻¹, respectively (Hauser et al. Reference Hauser, Buchanan, Nichols and Patterson1982), whereas 1 A. hispidum plant 7.5 m⁻¹ of crop row reduced peanut yield by 75 kg ha⁻¹ (Walker et al. Reference Walker, Wells and McGuire1989). Broadleaf weeds that form a canopy over peanut can also interfere with fungicide deposition and increase canopy humidity, thereby increasing the activity of plant pathogens and the incidence of foliar and soil-borne diseases, causing greater yield reduction (Royal et al. Reference Royal, Brecke and Colvin1997; Webster et al. Reference Webster, Faircloth, Flanders, Prostko and Grey2007). In 1 of 2 yr of studies, peanut yield was eliminated (100% yield loss) due to interference by C. benghalensis (Webster et al. Reference Webster, Faircloth, Flanders, Prostko and Grey2007). The total yield loss was attributed to the inability of applied maintenance fungicide to contact peanut foliage due to interception by C. benghalensis, which formed a complete canopy above the peanut. Webster et al. (Reference Webster, Faircloth, Flanders, Prostko and Grey2007) concluded that the competitive effect of C. benghalensis is likely complicated by the activities of plant pathogens. Previous studies have also shown that C. benghalensis is an alternate host for several soil-borne pathogens and insects, including aphids (Aphis spp.) that transmit peanut rosette virus disease, and nematodes, such as Meloidogyne, Pratylenchus, and Paratrichodorus species that infect peanut (Agostinho et al. Reference Agostinho, Gravena, Alves, Salgado and Mattos2006; Davis et al. Reference Davis, Webster and Brenneman2006; Desaeger and Rao Reference Desaeger and Rao2000).

CPWC in Peanut

The CPWC is the time interval in the crop growth cycle during which the crop must be kept weed-free to prevent unacceptable yield losses (usually losses greater than 2% to 5%, depending on the expected financial gain and cost of weed control) (Knezevic and Datta Reference Knezevic and Datta2015). Knowledge of the CPWC is essential for identifying the growth stage at which the crop is most vulnerable to weed competition, making appropriate decisions on the timing of weed control, and achieving the efficient use of management practices (Knezevic and Datta Reference Knezevic and Datta2015). Most of the CPWC studies on peanut in the United States have not focused on a mixed population of weed species, but rather on individual weed species. Although peanut exhibits a clear period of vulnerability to weed competition due to its unique growth habit and canopy architecture, studies indicate that the duration of the CPWC in peanut varies by weed species (Table 2).

Depending on the weed community, peanut requires a weed-free period beginning from 2 to 3 wk until 6 to 12 wk after crop emergence to avoid unacceptable yield loss (Everman et al. Reference Everman, Clewis, Thomas, Burke and Wilcut2008b; Wilcut et al. Reference Wilcut, York and Wehtje1994). When the weed community was composed of a mixed population of annual grasses and broadleaf weeds, the CPWC lasted approximately 5 wk, from 3.1 to 7.5 wk after crop emergence (Everman et al. Reference Everman, Clewis, Thomas, Burke and Wilcut2008b). Similarly, when the weed community was predominantly a mixed population of broadleaf weeds, the CPWC lasted approximately 5 wk (from 2.6 to 8 wk after crop emergence) but began earlier and ended later than the CPWC in peanut infested with a mixed population of annual grasses and broadleaf weeds (Everman et al. Reference Everman, Burke, Clewis, Thomas and Wilcut2008a). The earlier start of the CPWC for broadleaf weeds illustrates the need for timely broadleaf weed control early in the crop life cycle to avoid yield loss. If not controlled, broadleaf weed species such as A. artemisiifolia, A. hispidum, X. strumarium (Royal et al. Reference Royal, Brecke and Colvin1997), A. palmeri, D. tortusum, and S. obtusifolia that emerge at or before peanut emergence can outgrow the crop and effectively compete for nutrients, water, and light, thus causing yield losses (Burke et al. Reference Burke, Schroeder, Thomas and Wilcut2007; Walker et al. Reference Walker, Wells and McGuire1989). For example, broadleaf weeds such as A. hispidum and X. strumarium reduced yield by 4% and 8%, respectively, when allowed to compete with peanut for 2 wk after planting (Royal et al. Reference Royal, Brecke and Colvin1997; Walker et al. Reference Walker, Wells and McGuire1989). Amaranthus palmeri that emerged with peanut produced an approximately 10-fold greater number of seeds compared with A. palmeri that emerged 3 wk later, with heavier seed production leading to greater weed problems and yield reduction in subsequent cropping seasons (Mahoney et al. Reference Mahoney, Jordan, Hare, Leon, Roma-Burgos, Vann, Jennings, Everman and Cahoon2021).

In a study investigating the CPWC for individual broadleaf weeds including D. tortuosum and S. obtusifolia, Hauser et al. (Reference Hauser, Buchanan, Nichols and Patterson1982) observed the greatest yield when peanut was kept weed-free for 4 wk after crop emergence. For A. hispidum (Walker et al. Reference Walker, Wells and McGuire1989) and S. carolinense (Hackett et al. Reference Hackett, Murray and Weeks1987), the CPWC was between 2 and 6 wk after peanut emergence. However, when peanut was grown in competition with X. strumarium, a highly competitive broadleaf weed, Royal et al. (Reference Royal, Brecke and Colvin1997) found that the CPWC extended from 2 to 12 wk after crop emergence, which was 4 wk longer than the CPWC observed for a mixed population of less competitive broadleaf weeds reported in a later study by Everman et al. (Reference Everman, Burke, Clewis, Thomas and Wilcut2008a). This indicates that broadleaf weeds can affect peanut yields for an extended period or throughout much of the growing season, depending on the competitiveness of the dominant species.

In peanut infested with a mixed population of annual grasses including B. platyphyla, D. sanguinalis, E. indica, and U. texana, the CPWC occurred between 4.3 and 9 wk after crop emergence, beginning and ending later than the CPWC for peanut infested with a mixed population of broadleaf weeds or grasses and broadleaf weeds combined (Everman et al. Reference Everman, Burke, Clewis, Thomas and Wilcut2008a). However, when peanut was grown in competition with U. platyphyla or D. dichotomum in single species-specific studies, the end of the CPWC was at least 2 wk earlier (Chamblee et al. Reference Chamblee, Thompson and Coble1982; York and Coble Reference York and Coble1977). The variability in the CPWC among individual weed species reflects the differences in the competitive abilities of the weed species. This variability may also be due to the differences in the methodology used to achieve weed-free periods in different studies. CPWC studies in peanut in the United States have utilized hand weeding or hoeing (Bridges et al. Reference Bridges, Brecke and Barbour1992; Hauser et al. Reference Hauser, Buchanan and Ethredge1975; Webster et al. Reference Webster, Faircloth, Flanders, Prostko and Grey2007; York and Coble Reference York and Coble1977), herbicides (Everman et al. Reference Everman, Burke, Clewis, Thomas and Wilcut2008a, Reference Everman, Clewis, Thomas, Burke and Wilcut2008b), and the combination of hand weeding and herbicides (Farris et al. Reference Farris, Gray, Murray and Verhalen2005; Price et al. Reference Price, Burke, Askew, Schroeder, Everman and Wilcut2006) to maintain weed-free periods. Hand hoeing or herbicide application at different intervals or peanut growth stages may impact the CPWC intervals. While hand weeding will immediately terminate weed competition, weeds treated with herbicides, on the other hand, may continue to interfere with peanut for several days after treatment (Ferrell et al. Reference Ferrell, Earl and Vencill2003; Webster et al. Reference Webster, Faircloth, Flanders, Prostko and Grey2007). Therefore, the herbicide mode of action is an important consideration in applying the CPWC intervals for weed management in peanut. Although weeds that emerge before or after the CPWC would not directly impact crop yield, if not controlled, they can reduce harvest efficiency by interfering with peanut digging and also increase the weed seedbank, which can make weed management more problematic in subsequent growing seasons (Burke et al. Reference Burke, Price, Wilcut, Jordan, Culpepper and Tredaway-Ducar2004; Johnson and Mullinix Reference Johnson and Mullinix2003; Mahoney et al. Reference Mahoney, Jordan, Hare, Leon, Roma-Burgos, Vann, Jennings, Everman and Cahoon2021). Hence season-long weed control is often required to maximize peanut yield.

Preventive Weed Management

One of the first steps in achieving effective weed management is to prevent weed establishment, because it is difficult to control weeds once they are established (Chauhan et al. Reference Chauhan, Singh and Mahajan2012). Preventive weed management involves the use of different measures that reduce the buildup of the weed seedbank, weed seedling recruitment, weed interference, and weed seed production (Chauhan et al. Reference Chauhan, Singh and Mahajan2012). It is often considered an easier, less costly, and environmentally friendly weed management option compared with curative options, especially for problematic weeds under the circumstances of limited herbicide options and herbicide resistance (Bajwa et al. Reference Bajwa, Walsh and Chauhan2017; Chauhan et al. Reference Chauhan, Singh and Mahajan2012). However, the literature on weed management in peanut in the United States includes only a few examples of weed management programs centered on preventive measures. Apart from preventive weed management practices used in combination with curative measures, only about 3% of the published weed research studies in peanut in the United States have focused mainly on the impact of preventive measures on weed management: crop rotation (Johnson et al. Reference Johnson, Brenneman, Baker, Johnson, Sumner and Mullinix2001; Leon et al. Reference Leon, Wright and Marois2015; Tiwari et al. Reference Tiwari, Reinhardt Piskáčková, Devkota, Mulvaney, Ferrell and Leon2021), stale seedbed (Johnson and Mullinix Reference Johnson and Mullinix1995, Reference Johnson and Mullinix2000), cover crops (Aulakh et al. Reference Aulakh, Saini, Price, Faircloth, van Santen, Wehtje and Kelton2015; Dobrow et al. Reference Dobrow, Ferrell, Faircloth, MacDonald, Brecke and Erickson2011; Johnson et al. Reference Johnson, Prostko and Mullinix2010; Lassiter et al. Reference Lassiter, Jordan, Wilkerson, Shew and Brandenburg2011; Price et al. Reference Price, Reeves, Patterson, Gamble, Balkcom, Arriaga and Monks2007), row spacing (Johnson et al. Reference Johnson, Prostko and Mullinix2005; Stephenson and Brecke Reference Stephenson and Brecke2011), planting pattern (Besler et al. Reference Besler, Grichar, Senseman, Lemon and Baughman2008; Brecke and Stephenson Reference Brecke and Stephenson2006; Colvin et al. Reference Colvin, Wehtje, Patterson and Walker1985; Grichar et al. Reference Grichar, Colburn and Kearney1994; Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022), planting date (Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022; Linker and Coble Reference Linker and Coble1990), and the use of competitive cultivars (Fiebig et al. Reference Fiebig, Shilling and Knauft1991; Leon et al. Reference Leon, Mulvaney and Tillman2016; Place et al. Reference Place, Reberg-Horton and Jordan2010, Reference Place, Reberg-Horton, Jordan, Isleib and Wilkerson2012). These are examples of weed-preventive measures that have been studied in peanut in the United States, and these measures were mainly tested in combination with curative measures, particularly chemical weed control.

Crop Rotation

Crop rotation is considered an essential component of an effective weed management program. In the United States, peanut is commonly rotated with crops such as corn, cotton, grain sorghum [Sorghum bicolor (L.) Moench. ssp. bicolor], wheat (Triticum aestivum L.), and sometimes soybean (Johnson and Mullinix Reference Johnson and Mullinix1997; Jordan et al. Reference Jordan, Shew, Barnes, Corbett, Alston, Johnson, Ye and Brandenburg2008; Leon et al. Reference Leon, Wright and Marois2015). Nematodes and soil-borne diseases can quickly become an economic problem when peanut is grown in consecutive years, whereas rotating peanut with these crops can improve weed control and reduce the buildup of pests and pathogens that can negatively impact peanut yield (Warren and Coble Reference Warren and Coble1999). For instance, C. esculentus population densities and tubers were reduced with peanut–corn and peanut–cotton rotations compared with fallow (Johnson and Mullinix Reference Johnson and Mullinix1997). Similarly, shoots and tubers of C. rotundus were effectively managed with imazapic with 20% and 7% yield increase in a 3-yr corn and peanut rotation sequence (corn–peanut–corn) compared with peanut grown in consecutive (peanut–peanut–peanut) or alternate years (peanut–corn–peanut), respectively (Warren and Coble Reference Warren and Coble1999). Additionally, rotating peanut with other crops allows effective rotation of herbicides or herbicide modes of action, which can improve weed management (Jordan et al. Reference Jordan, Shew, Barnes, Corbett, Alston, Johnson, Ye and Brandenburg2008). For example, A. artemisiifolia, E. prostrata, and S. obtusifolia can be difficult to control in peanut but can be relatively well controlled in glyphosate-resistant crops, such as corn, cotton, and soybean. Furthermore, diversification of crop rotations can be an essential tool for improving weed management (Owen Reference Owen2008). Evidence from peanut in the United States for this idea is limited. Diversified crop rotations as a weed-preventive measure have only been tested by Leon et al. (Reference Leon, Wright and Marois2015) in north Florida and more recently by Tiwari et al. (Reference Tiwari, Reinhardt Piskáčková, Devkota, Mulvaney, Ferrell and Leon2021) in west Florida. Leon et al. (Reference Leon, Wright and Marois2015) reported that adding bahiagrass (Paspalum notatum Flueggé), a perennial, to the predominant peanut–cotton rotation system of Florida growers modified the structure of the weed community and increased weed species evenness and richness, thereby favoring weed species diversity. Results from their 13-yr rotation study showed that the bahiagrass–bahiagrass–peanut–cotton rotation system had more diverse and dense weed seedbanks and a higher weed frequency than the conventional peanut–cotton–cotton rotation system (Leon et al. Reference Leon, Wright and Marois2015). However, they concluded that adding bahiagrass to the rotation system did not affect weed management in peanut, because the increased weed community with this rotation system was transient and only limited to the first phase of bahiagrass. Thereafter, the weed seedbank structure and density decreased and were similar to the peanut phase, which was suggested to be due to increased seed-predatory activity that possibly resulted from the greater ground cover provided by bahiagrass (Leon et al. Reference Leon, Wright and Marois2015).

Tiwari et al. (Reference Tiwari, Reinhardt Piskáčková, Devkota, Mulvaney, Ferrell and Leon2021) evaluated the effect of winter carinata (Brassica carinata A. Braun)—a recently introduced nonedible winter biofuel crop in the southeastern United States—on summer weed population dynamics in peanut. In that study, winter carinata grown in winter reduced the emergence of smooth pigweed (Amaranthus hybridus L.) and S. obtusifolia by greater than 27% and 25%, respectively, without preemergence herbicide application. With or without preemergence application of S-metolachlor, greater than 40% reduction in A. hybridus emergence was observed after winter carinata harvest compared with winter fallow (Tiwari et al. Reference Tiwari, Reinhardt Piskáčková, Devkota, Mulvaney, Ferrell and Leon2021). This indicates that winter carinata has the potential to enhance integrated weed management strategies in peanut at the rotational level by reducing summer weed seedbanks.

Stale Seedbed and Tillage

The use of stale seedbed tillage is a valuable way to reduce weed pressure, improve weed management, and possibly reduce herbicide inputs (Chauhan et al. Reference Chauhan, Singh and Mahajan2012). In this practice, soil disturbance through tillage is used to stimulate weed seed germination several days, weeks, or months before planting a crop, and emerged weed seedlings are killed using shallow tillage or a nonselective herbicide such as paraquat or glyphosate (Johnson and Mullinix Reference Johnson and Mullinix2000). This practice can provide a weed-free environment for crop emergence and growth early in the growing season, thereby enhancing crop competition with late-emerging weeds (Chauhan et al. Reference Chauhan, Singh and Mahajan2012). In peanut, stale seedbed with shallow tillage of 7.6-cm depth using a power tiller three times at 2-wk intervals was found very effective, resulting in lower densities of weed species such as D. tortuosum, U. texana, and C. esculentus compared with conventional tillage (23-cm deep) or glyphosate (Johnson and Mullinix Reference Johnson and Mullinix1995). In another study, however, Johnson and Mullinix (Reference Johnson and Mullinix2000) demonstrated that stale seedbeds with shallow tillage did not improve weed control compared with a non-tilled control.

Cover Crops

Although peanut production in the United States is mostly in conventional tillage, interest in the conservation-tillage system has increased dramatically in recent years due to its economic and environmental benefits (Price et al. Reference Price, Reeves, Patterson, Gamble, Balkcom, Arriaga and Monks2007). The conservation-tillage system leaves at least 30% of residue cover on the soil surface after planting (SSSA 2020). In conservation-tillage systems, cover crop residues or mulch present on the soil surface protects soil resources before planting peanut and can serve as a preventive weed management measure to provide weed suppression through reduced light transmittance to the soil surface, allelopathy, or direct physical suppression (Lassiter et al. Reference Lassiter, Jordan, Wilkerson, Shew and Brandenburg2011; Price et al. Reference Price, Reeves, Patterson, Gamble, Balkcom, Arriaga and Monks2007). Several cover crops, including black oat (Avena strigosa Schreb.), cereal rye (Secale cereale L.), Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot], oats (Avena sativa L.), triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum]), and winter wheat are easy to establish and can provide high amounts of biomass for weed suppression in peanut (Lassiter et al. Reference Lassiter, Jordan, Wilkerson, Shew and Brandenburg2011; Price et al. Reference Price, Reeves, Patterson, Gamble, Balkcom, Arriaga and Monks2007), but challenges with pegging, digging, and inverting peanut vines have limited the use of this approach for weed management in peanut (Leon et al. Reference Leon, Jordan, Bolfrey-Arku, Dzomeku, Korres, Burgos and Duke2019). Furthermore, even when the cover crop provides high residues, weed suppression is inadequate without other supplementary control measures such as herbicide input. A 4-yr study conducted by Price et al. (Reference Price, Reeves, Patterson, Gamble, Balkcom, Arriaga and Monks2007) in Alabama demonstrated that winter cover crops such as black oat, cereal rye, and wheat were not effective in controlling weeds without a herbicide program in a high-residue conservation-tillage peanut production system. Similarly, studies conducted in Georgia showed that strip tillage with the use of cereal rye provided only moderate control of annual grasses, including southern crabgrass [Digitaria ciliaris (Retz.) Koeler] and U. texana in the absence of herbicide input (Johnson et al. Reference Johnson, Prostko and Mullinix2010). However, high-residue cover crops, including cereal rye, Italian ryegrass, oats, triticale, and wheat, tested in combination with herbicide programs provided greater weed control and yield advantage relative to no cover (Aulakh et al. Reference Aulakh, Saini, Price, Faircloth, van Santen, Wehtje and Kelton2015; Dobrow et al. Reference Dobrow, Ferrell, Faircloth, MacDonald, Brecke and Erickson2011; Lassiter et al. Reference Lassiter, Jordan, Wilkerson, Shew and Brandenburg2011). When the cover crop does not produce adequate biomass to provide a dense layer of residue on the ground for weed suppression, the benefits of this weed-preventive measure are limited, and a comprehensive herbicide program will be required for effective weed management (Dobrow et al. Reference Dobrow, Ferrell, Faircloth, MacDonald, Brecke and Erickson2011; Johnson et al. Reference Johnson, Prostko and Mullinix2010).

Row Spacing, Seeding Rate, and Planting Pattern

One of the most effective approaches to preventive weed management is the use of agronomic practices such as row spacing, seeding rate, and planting pattern to minimize weed interference and enhance crop competitiveness with weeds (Bajwa et al. Reference Bajwa, Walsh and Chauhan2017). Because of the prostrate and initial slow growth habit of peanut, most weeds that establish before peanut plants will overtop and outcompete the crop, reducing harvest efficiency and yield (Burke et al. Reference Burke, Price, Wilcut, Jordan, Culpepper and Tredaway-Ducar2004). Therefore, cultural practices that enhance uniform stand establishment, rapid growth, increased nutrient uptake, elevated plant height, greater dry matter production, rapid canopy closure, greater light interception, and shading of weeds in the understory (below the crop canopy) are important to increase peanut competitiveness against weeds (Burke et al. Reference Burke, Price, Wilcut, Jordan, Culpepper and Tredaway-Ducar2004, Reference Burke, Schroeder, Thomas and Wilcut2007; Johnson et al. Reference Johnson, Prostko and Mullinix2005). The use of narrow row spacing to increase the rate of canopy closure and the competitive ability of peanut has proved highly beneficial in terms of weed suppression and yield improvement in many studies (Brecke and Stephenson Reference Brecke and Stephenson2006; Buchanan and Hauser Reference Buchanan and Hauser1980; Hauser and Buchanan Reference Hauser and Buchanan1981; Johnson et al. Reference Johnson, Prostko and Mullinix2005; Stephenson and Brecke Reference Stephenson and Brecke2011). For instance, due to rapid canopy closure, late-season D. tortuosum and S. obtusifolia biomass was reduced by 28% and 18%, respectively, when peanut was planted at 41-cm row spacing compared with 81-cm row spacing (Buchanan and Hauser Reference Buchanan and Hauser1980). Additionally, the yield benefit for the 41-cm row spacing was about 50% due to better weed suppression and favorable conditions for crop growth that increased yield (Buchanan and Hauser Reference Buchanan and Hauser1980). Similarly, a reduction in row spacing from 81.2 to 20.3 cm decreased S. obtusifolia density and increased peanut yield by up to 15% in studies conducted on two soil types in Alabama and Georgia (Hauser and Buchanan Reference Hauser and Buchanan1981). Also, a 4-yr study conducted in Florida showed that narrow (38-cm) row spacing provided greater browntop millet [Brachiaria ramosa (L.) Stapf] and C. benghalensis control than wide (76-cm) row spacing but did not influence control of pitted morningglory (Ipomoea lacunosa L.) or S. obtusifolia (Stephenson and Brecke Reference Stephenson and Brecke2011). However, Johnson et al. (Reference Johnson, Prostko and Mullinix2005) found that peanut planted in 30-cm rows had greater midseason control of S. obtusifolia and a 25% decrease in total weed density compared with peanut planted in 91-cm rows. Johnson et al. (Reference Johnson, Prostko and Mullinix2005) also reported a 12% increase in peanut yield under the narrow- versus the wide-row peanut system (Stephenson and Brecke Reference Stephenson and Brecke2011). Despite the improved weed control and proven yield increase, peanut seeded in narrow-row patterns is not common, because increased crop canopy commonly associated with narrow row spacing and increased plant population can serve to create and maintain a humid subcanopy environment that can serve to enhance occurrence and severity of diseases such as stem rot (Sclerotium rolfsii Sacc.) (Wehtje et al. Reference Wehtje, Weeks, West, Wells and Pace1994).

Numerous studies have reported the superior weed suppressive ability of peanut using twin rows as a weed-preventive measure compared with single rows (Brecke and Stephenson Reference Brecke and Stephenson2006; Colvin et al. Reference Colvin, Wehtje, Patterson and Walker1985; Grichar et al. Reference Grichar, Colburn and Kearney1994; Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022; Wehtje et al. Reference Wehtje, Walker, Patterson and McGuire1984). The benefits of twin-row spacing are attributed mainly to rapid canopy cover and more efficient use of light and water that give peanut a competitive advantage against weeds (Brecke and Stephenson Reference Brecke and Stephenson2006; Johnson et al. Reference Johnson, Prostko and Mullinix2005; Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022; Place et al. Reference Place, Reberg-Horton and Jordan2010). In a recent study, Kharel et al. (Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022) reported that twin rows spaced 18 or 23 cm apart on 91-cm centers achieved canopy closure 2 wk earlier, resulting in greater S. obtusifolia suppression and an 18% increase in yield compared with single rows spaced 76 cm apart. Similarly, S. obtusifolia control was 9% higher and yield 10% to 43% higher when peanut was seeded in the twin-row planting pattern (rows spaced 18 cm apart on 91-cm centers) compared with peanut planted in the single-row planting pattern (single rows on 91-cm centers) under different herbicide treatments (Lanier et al. Reference Lanier, Lancaster, Jordan, Johnson, Spears and Wells2004). In another study, twin rows spaced 18 cm apart on 91-cm centers reduced competition from D. sanguinalis and U. texana, resulting in greater peanut yield (Wehtje et al. Reference Wehtje, Walker, Patterson and McGuire1984). The twin-row planting pattern has also been reported to provide greater late-season control of C. esculentus (Grichar et al. Reference Grichar, Colburn and Kearney1994), D. tortuosum (Brecke and Stephenson Reference Brecke and Stephenson2006; Colvin et al. Reference Colvin, Wehtje, Patterson and Walker1985), E. prostrata (Place et al. Reference Place, Reberg-Horton and Jordan2010), Ipomoea spp. (Place et al. Reference Place, Reberg-Horton and Jordan2010), S. obtusifolia (Brecke and Stephenson Reference Brecke and Stephenson2006; Colvin et al. Reference Colvin, Wehtje, Patterson and Walker1985; Lanier et al. Reference Lanier, Lancaster, Jordan, Johnson, Spears and Wells2004), U. texana (Colvin et al. Reference Colvin, Wehtje, Patterson and Walker1985), tumbleweed (Salsola tragus L.), and X. strumarium (Brecke and Stephenson Reference Brecke and Stephenson2006) compared with the single-row planting pattern in peanut. However, not all cultivars benefit equally from the twin-row planting pattern. Jordan et al. (Reference Jordan, Place, Brandenburg, Lanier and Carley2010) showed that a bunch-type growing habit cultivar responded more positively to twin-row planting than a cultivar with a prostrate growth habit. Also, apart from weed suppression, yield benefits from twin-row planting from different studies have been inconsistent. Colvin et al. (Reference Colvin, Wehtje, Patterson and Walker1985) and Kharel et al. (Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022) reported greater peanut yield in twin rows compared with single rows. In a similar study, Grichar et al. (Reference Grichar, Colburn and Kearney1994) found no yield benefit with twin-row spacing compared with single-row spacing. Although S. obtusifolia control was greater when peanut was planted in twin rows in both conventional- and conservation-tillage systems, consistent yield increase with twin rows was only observed in conservation tillage (Brecke and Stephenson Reference Brecke and Stephenson2006).

The use of twin-row planting patterns can improve weed control and reduce the incidence of tomato spotted wilt of peanut (Johnson et al. Reference Johnson, Prostko and Mullinix2005), but most of the studies reported that the increase in weed control from a twin-row planting pattern was not sufficient to reduce or eliminate the need for herbicides to protect yield (Brecke and Stephenson Reference Brecke and Stephenson2006; Colvin et al. Reference Colvin, Wehtje, Patterson and Walker1985; Lanier et al. Reference Lanier, Lancaster, Jordan, Johnson, Spears and Wells2004; Place et al. Reference Place, Reberg-Horton and Jordan2010). This suggests that twin-row spacing can only be used as a supplement to a comprehensive weed control program and should not be considered a stand-alone weed management option. Additionally, peanut seed is one of the most costly inputs in peanut production. Increasing the seeding rate in twin rows to enhance weed suppression will further increase the cost of production, as the seeding rate is 10% to 20% greater in narrow rows compared with wide rows (Smith and Smith Reference Smith and Smith2011). Furthermore, increasing plant population with the use of a twin-row pattern can increase the incidence of stem rot disease (Wehtje et al. Reference Wehtje, Weeks, West, Wells and Pace1994). Hence, reaching a balance between disease prevention, improved weed control, and increased yield must be a central goal when choosing peanut planting patterns.

Planting Date

Planting date can have a huge impact on weed management in peanut by affecting weed seed germination, weed growth rate, and crop vegetative growth (Linker and Coble Reference Linker and Coble1990). Peanut planting dates in the United States range between mid-April and late June (Linker and Coble Reference Linker and Coble1990; Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022). During this period, weeds have a well-defined period of emergence determined by soil moisture content and soil temperature (Egley and Paul Reference Egley and Paul1986; Stoller and Wax Reference Stoller and Wax1973). Therefore, peanut planting date can be manipulated as a weed-preventive measure in such a way that the ecological conditions for weed seed germination are not met during the planting timing. The peak of weed emergence and prolonged competition can also be avoided through a well-planned planting date (Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022). Furthermore, the planting date can be adjusted to facilitate faster growth, which can result in rapid canopy closure, increased crop competitiveness, and greater weed suppression (Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022). Gardner and Auma (Reference Gardner and Auma1989) showed that peanut planted earlier in May had greater leaf area index, canopy light interception, and total dry matter than peanut planted late in June, suggesting that planting date can be optimized to enhance the competitive ability of peanut. However, the effect of planting date on weed suppression in peanut can vary depending on environmental conditions and prevailing weed species (Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022; Linker and Coble Reference Linker and Coble1990). For example, in the first year of a 2-yr study, Kharel et al. (Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022) reported that S. obtusifolia biomass was reduced in peanut planted in early May compared with mid-May and early-June planting dates. In the second year of the same study, however, delayed planting resulted in reduced S. obtusifolia density compared with mid- or early-May planting dates (Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022). This inconsistency was attributed to the environmental conditions, which were favorable for longer periods of weed infestation and enhanced S. obtusifolia growth with early planting in one year and late planting in the other (Kharel et al. Reference Kharel, Devkota, Macdonald, Tillman and Mulvaney2022). In addition to the direct effect on weed growth, planting date can affect the efficacy and intensity of herbicide use in peanut. Wehtje et al. (Reference Wehtje, McGuire, Walker and Patterson1986) reported greater herbicide efficacy for the control of U. texana in early-planted peanut compared with late-planted peanut. Hauser et al. (Reference Hauser, Buchanan, Currey, Ethredge, Gorbet, Slaughter and Swann1977) also showed that early-planted peanut required reduced herbicide input for weed control compared with late-planted peanut.

Competitive Cultivars

Competitive crop cultivars play an important role in effective weed management, because they offer some level of weed suppression and are better able to acquire growth resources such as light, nutrients, water, and space (Leon et al. Reference Leon, Mulvaney and Tillman2016). Despite its short stature compared with other row crops, when grown at the right planting density and arrangement, peanut can form a dense canopy that limits light transmittance to the soil surface, thereby reducing weed seed germination and suppressing weed growth. Peanut cultivars can differ in their weed suppressive ability due to differences in growth habits, plant morphology, canopy architecture, and efficiency of light interception (Fiebig et al. Reference Fiebig, Shilling and Knauft1991; Leon et al. Reference Leon, Mulvaney and Tillman2016; Place et al. Reference Place, Reberg-Horton, Jordan, Isleib and Wilkerson2012). However, only a few studies have been conducted in the United States to test the weed competitive ability of peanut cultivars. Fiebig et al. (Reference Fiebig, Shilling and Knauft1991) observed differences in the response of four peanut genotypes to X. strumarium competition that were mostly associated with differences in growth habits and canopy architecture. Xanthium strumarium at distances of 0 to 25 cm from peanut reduced yields 50% for the cultivars ‘NC7’ and 30, 26, and 13% for the Florida breeding lines ‘BL-10’, ‘BL-8’, and ‘F8143B’, respectively (Fiebig et al. Reference Fiebig, Shilling and Knauft1991). Similarly, Hackett et al. (Reference Hackett, Murray and Weeks1987) reported that the tall Spanish peanut cultivar ‘Pronto’ was more competitive and exhibited greater tolerance for S. carolinense interference compared with the runner-type cultivar ‘Florunner.’ Although these studies were conducted using what are now obsolete cultivars, the results indicate that there is potential for developing peanut cultivars with improved competitive ability against weeds. Subsequent studies conducted with other cultivars, however, showed that the morphological response to weed interference was similar among the cultivars despite the variation in growth habit and canopy architecture (Leon et al. Reference Leon, Mulvaney and Tillman2016; Place et al. Reference Place, Reberg-Horton and Jordan2010, Reference Place, Reberg-Horton, Jordan, Isleib and Wilkerson2012). Place et al. (Reference Place, Reberg-Horton, Jordan, Isleib and Wilkerson2012) compared the response to weed interference among eight Virginia market-type genotypes, including ‘NC 10C’, ‘NC-V 11’, ‘NC 12C’, ‘Phillips’, ‘VA 98R’, and breeding lines ‘N99027L’, ‘N01013T’, and ‘N02020J’ and found no clear differences despite variations in canopy coverage among the genotypes. In another study, Place et al. (Reference Place, Reberg-Horton and Jordan2010) observed that the difference in peanut cultivar VA 98R, with runner growth habit, and NC 12C, with excessive vine growth, and an intermediate (between a runner and bunch type growth) had only minor effects on weed control. Leon et al. (Reference Leon, Mulvaney and Tillman2016) also showed that the differences in growth habits among peanut cultivars ‘Bailey’, with an erect and tall canopy height, ‘Georgia-06G’, with a semi-bunch and intermediate height, and advanced breeding line ‘UFT312’, with very prostrate growth and short canopy height, had no effect on weed suppression and weed tolerance. Furthermore, peanut cultivars with early maturity that may allow earlier harvest have not been effective. Increased weed tolerance in peanut in previous studies was attributed to the allocation of more photosynthate resources to vegetative growth than to reproductive growth, which may result in delayed maturity and potentially lower yield (Fiebig et al. Reference Fiebig, Shilling and Knauft1991). Therefore, breeding efforts to increase weed suppression and competitiveness in peanut must be pursued with the goal of identifying lines that better balance the translocation of photoassimilates over vegetative growth and developing cultivars that allow earlier harvest with increased weed tolerance and high yield potential. Such cultivars could help reduce reliance on chemical weed control and serve as a viable component of integrated weed management.