Late-season weed emergence in California rice fields complicates decisions concerning the timing of control measures. The objective of this study was to predict the emergence of three problematic weed species in rice using thermal time models. Smallflower umbrella sedge, barnyardgrass, and bearded sprangletop seedlings were counted and removed daily at three locations across the Sacramento Valley rice-growing region in 2018. The accumulation of thermal time (growing degree days; GDD) commenced with the initial flooding of the fields at each location, utilizing the specific base temperatures corresponding to each species. The pattern of emergence as a function of GDD was modeled with a Weibull function. Root-mean-square values for comparing actual and model-predicted cumulative emergence values were 6% to 23%. Percent cumulative emergence initially increased rapidly for smallflower umbrella sedge and reached 90% emergence with accumulation of 13 GDD. Barnyardgrass emerged after smallflower umbrella sedge and reached 90% emergence with an accumulation of 124 GDD. Bearded sprangletop had a delay of 64 GDD compared to barnyardgrass to reach first emergence and reached 90% emergence at 215 GDD. The period of weed emergence at all field sites differed across the three species and led to a continuous spectrum of weed emergence over time. This study characterizes the emergence of three economically important rice weeds and provides useful information for the timing of weed management. Typical herbicide applications on the day of seeding may have less efficacy on the late-emerging weeds, causing reduced weed control. Delayed herbicide application, overlay of residual herbicides, or use of herbicides with longer residual activity are suggested to control late-emerging weeds.

Understanding the effect of phosphorus (P) fertilization on weed interference with sweet corn is important for deciding appropriate fertilization levels and weed control programs. Field experiments were conducted in 2020 and 2021 in Belle Glade, FL, to determine the influence of P fertilization levels (0 or residual P, 62.5, and 120 kg P2O5 ha−1) on the critical period of weed control (CPWC) in sweet corn on organic soils. Experimental plots were subjected to increased duration of weed interference and weed-free period treatments for each P fertilization level. The beginning and end of the CPWC based on 5% and 10% acceptable yield loss (AYL) levels were determined by fitting log-logistic and Gompertz models to represent the increasing duration of weed interference and duration of the weed-free period, respectively. The log-logistic curves did not estimate the beginning of the CPWC at 5% AYL for 0 and 125 kg P2O5 ha−1 because the estimated upper limits of the curves were lower than the 95% relative yield used for estimation of 5% AYL. Based on a 10% AYL level, the length of the CPWC in sweet corn under optimum P fertilization levels was estimated to be 27 d, from the 6- to 7-leaf stage until the silking stage of growth. Reducing P fertilization by 50% increased the CPWC to 36 d, from the 5-leaf stage until the silking to blister stage of growth. Lack of P fertilization increased the CPWC to 64 d, from sweet corn emergence until the blister to milk stage of growth. These results show that the beginning of the CPWC in sweet corn is delayed and the end is shortened as P fertilization level increases. Therefore reduction in P fertilization will require a more intensive weed management program for sweet corn because of the prolonged duration of the CPWC.

Alfalfa (Medicago sativa L.) hectares in Michigan are declining partly due to reliance on corn (Zea mays L.) silage as a continuous feed source. By interseeding corn and alfalfa, farmers can replace the low alfalfa yield in the establishment year with corn silage while simultaneously establishing alfalfa. A randomized split-block field study was conducted in East Lansing, MI, over 3 yr (2019 to 2021) to determine the critical period of weed control (CPWC) in the interseeded corn and alfalfa system using two corn hybrids with differing leaf architecture (pendulum vs. upright). Whole plots were assigned to corn hybrids interseeded with alfalfa, and subplots were assigned to a surrogate weed, Japanese millet [Echinochloa esculenta (A. Braun) H. Scholz], for the duration of competition treatments. Weed-free and weedy plots were included as controls. At the end of the interseeding year, corn was harvested, while alfalfa was harvested the following year. The CPWC is made up of two components: the critical timing of weed removal (CTWR) and the critical weed-free period (CWFP). Corn hybrid had no impact on the CTWR or CWFP for interseeded corn or alfalfa. Averaged across hybrids, the CTWR was 303 growing degree days (GDD), and CWFP was estimated to be greater than the study duration. The CTWR in the first cutting of alfalfa was estimated to be 369 GDD. The CWFP was estimated to be 394 GDD for a 5% acceptable yield loss for the first alfalfa cutting. Identification of the CPWC in the interseeded system will increase adoption and interest in other interseeded systems that can mitigate potential negative environmental and economic impacts of monoculture agriculture.

Transgenic crops are being developed with herbicide resistance traits to expand innovative weed management solutions for crop producers. Soybean with traits that confer resistance to the hydroxyphenylpyruvate dioxygenase herbicide isoxaflutole is under development and will provide a novel herbicide mode of action for weed management in soybean. Ten field experiments were conducted over 2 years (2017 and 2018) on five soil textures with isoxaflutole-resistant soybean to evaluate annual weed control using one- and two-pass herbicide programs. The one-pass weed control programs included isoxaflutole plus metribuzin, applied PRE, at a low rate (52.5 + 210 g ai ha−1), medium rate (79 + 316 g ai ha−1), and high rate (105 + 420 g ai ha−1); and glyphosate applied early postemergence (EPOST) or late postemergence (LPOST). The two-pass weed control programs included isoxaflutole plus metribuzin, applied PRE, followed by glyphosate applied LPOST, and glyphosate applied EPOST followed by LPOST. At 4 weeks after the LPOST application, control of common lambsquarters, pigweed species, common ragweed, and velvetleaf was variable at 25% to 69%, 49% to 86%, and 71% to 95% at the low, medium, and high rates of isoxaflutole plus metribuzin, respectively. Isoxaflutole plus metribuzin at the low, medium, and high rates controlled grass species evaluated (i.e., barnyardgrass, foxtail, crabgrass, and witchgrass) 85% to 97%, 75% to 99%, and 86% to 100%, respectively. All two-pass weed management programs provided 98% to 100% control of all species. Weed control improved as the rate of isoxaflutole plus metribuzin increased. Two-pass programs provided excellent, full-season annual grass and broadleaf weed control in isoxaflutole-resistant soybean.

The development of a linuron-free weed management strategy for carrot production is essential as a result of the herbicide reevaluation programs launched by the Pest Management Regulatory Agency in Canada for herbicides registered before 1995 and the discovery of linuron-resistant pigweed species in Ontario. Field trials were conducted in one of Ontario’s main carrot-growing regions on high organic soils in 2016 and 2017. Pigweed species seedlings were effectively controlled with PRE treatments of prometryn, pendimethalin, S-metolachlor, or glufosinate. POST treatments of pyroxasulfone and metribuzin followed by predetermined biologically effective dose (≥90% control of pigweed seedlings) of acifluorfen, oxyfluorfen, fluthiacet-methyl, and fomesafen achieved excellent crop selectivity and commercially acceptable pigweed species seedling control under field conditions. Carfentrazone-ethyl or fomesafen applied PRE severely reduced seedling emergence and yield in the wet growing season of 2017. This study demonstrated clearly that an alternative linuron-free strategy can be developed for carrots. The strategy of exploring the potential to use the biologically effective dose of selected herbicides to achieve crop selectivity and control of pigweed species seedlings was verified.

The critical period for weed control (CPWC) identifies the phase of the crop growth cycle when weed interference results in unacceptable yield losses; however, the effect of planting date on CPWC is not well understood. Field studies were conducted in 2004 and 2005 at Urbana, IL, to determine CPWC in sweet corn for early May (EARLY) and late-June (LATE) planting dates. A quantitative series of treatments of both increasing duration of interference and length of weed-free period were imposed within each planting-date main plot. The beginning and end of the CPWC, based on 5% loss of marketable ear mass, was determined by fitting logistic and Gompertz equations to the relative yield data representing increasing duration of weed interference and weed-free periods, respectively. Weed interference stressed the crop more quickly and to a greater extent in EARLY, relative to LATE. At a 5% yield-loss level, duration of weed interference for 160 and 662 growing-degree days (GDD) from crop emergence marked the beginning of the CPWC for EARLY and LATE, respectively. When maintained weed-free for 320 and 134 GDD, weeds emerging later caused yield losses of less than 5% for EARLY and LATE, respectively. Weed densities exceeded 85 plants m−2 for the duration of the experiments and predominant species included barnyardgrass, common lambsquarters, common purslane, redroot pigweed, and velvetleaf. Weed canopy height and total aboveground weed biomass were 300% and 500% higher, respectively, for EARLY compared with LATE. Interactions between planting date and CPWC indicate the need to consider planting date in the optimization of integrated weed management systems for sweet corn. In this study, weed management in mid-June–planted sweet corn could have been less intensive than early May–planted corn, reducing herbicide use and risk of herbicide carryover to sensitive rotation crops.

Weed control in lentil is difficult because lentil is a poor competitor with weeds and few POST broadleaf herbicides are available. Imadazolinone-tolerant lentils have more herbicide options, but the optimum timing for herbicide application is not known. The critical period of weed control (CPWC) is the period in a crop's life cycle when weeds must be controlled in order to prevent yield loss. The objective of this research was to determine the CPWC for lentil. We made lentil remain weedy or weed-free from 0 to 11 aboveground nodes to investigate the durations of weed interference and weed-free period, respectively. It was found that lentil has a CPWC beginning at the five-node stage and continuing to the 10-node stage. There was an inverse relationship between weed biomass and lentil yield; that is, lentil yield was highest when weed biomass is minimal. We propose that the CPWC begins when weeds start to accumulate significant biomass and ends with crop canopy closure. Therefore, to maximize lentil yields, growers should consider using a POST residual herbicide that can control weeds during the CPWC.

There is an ever-larger need for designing an integrated weed management (IWM) program largely because of the increase in glyphosate-resistant weeds, not only in the United States but also worldwide. An IWM program involves a combination of various methods (cultural, mechanical, biological, genetic, and chemical) for effective and economical weed control (Swanton and Weise 1991). One of the first steps in designing an IWM program is to identify the critical period for weed control (CPWC), defined as a period in the crop growth cycle during which weeds must be controlled to prevent crop yield losses (Zimdahl 1988).

Field studies were conducted in 2010 and 2011 at Belle Glade, FL, to evaluate the influence of phosphorus (P) applications (98, 196, and 293 kg P ha−1) on the critical period of weed control (CPWC) in lettuce. Natural populations of mixed weed species were allowed to interfere with lettuce in a series of treatments of both increasing duration of weed interference and the duration of weed-free period imposed within 98, 196, and 293 kg P ha−1 levels added to the soil. The beginning and end of the CPWC for each P fertilization level based on a 5% acceptable marketable fresh lettuce yield loss level was determined by fitting log-logistic and Gompertz models to represent the increasing duration of weed interference and the duration of weed-free period, respectively. The CPWC in lettuce was estimated to be 4.6, 3.4, and 2.3 wk at 98, 196, and 293 kg P ha−1, respectively. The beginning of the CPWC was delayed at the highest P fertilization level (293 kg P ha−1), whereas the end of the CPWC was hastened at the same P fertilization level. Our study shows that inadequate levels of P fertilization in lettuce result in the need for more-intensive weed management practices to attain acceptable yields.

Previous research with annual weed species indicates that critical timing of weed removal begins primarily after the two-leaf stage of onion, a time when postemergence (POST) herbicides can first be applied. Volunteer potato is difficult to manage and persists in onion fields of western United States. The purpose of this research was to quantify the duration of volunteer potato interference on yield and market grade of onion as well as potato tuber production. Volunteer potato interference caused a 5% yield loss before onions reached the two-leaf stage, at two of three locations. Relative to weed-free plots, onion bulb diameter was reduced as duration of interference increased, resulting in smaller proportions of marketable bulbs. Volunteer potato produced daughter tubers shortly after emergence, which explains, in part, weed persistence despite removal of shoots with contact herbicides, cultivation, and hand-weeding in onion. Significant losses in onion yield and bulb diameter are likely given current volunteer potato management systems.

The critical period for weed control (CPWC) is the period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses. Field studies were conducted in 1999 and 2000 in eastern Nebraska to evaluate the influence of nitrogen application on the CPWC in dryland corn in competition with a naturally occurring weed population. Nitrogen fertilizer was applied at rates equivalent to 0, 60, and 120 kg N ha−1. A quantitative series of treatments of both increasing duration of weed interference and length of weed-free period were imposed within each nitrogen main plot. The beginning and end of the CPWC based on an arbitrarily 5% acceptable yield loss level were determined by fitting the logistic and Gompertz equations to relative yield data representing increasing duration of weed interference and weed-free period, respectively. Despite an inconsistent response of corn grain yield to applied nitrogen, there was a noticeable influence on the CPWC. The addition of 120 kg N ha−1 delayed the beginning of the CPWC for all site–years when compared with the 0-kg N ha−1 rate and for three of the four site–years when compared with the 60-kg N ha−1 rate. The addition of 120 kg N ha−1 also hastened the end of the CPWC at three of the four site–years when compared with both reduced rates. The yield component most sensitive to both nitrogen and interference from weeds was seed number per ear. Practical implications of this study are that reductions in nitrogen use may create the need for more intensive weed management.

Field studies were conducted to determine the effect of early-season and early- plus late-season acetolactate synthase–resistant Helianthus annuus interference on Glycine max and H. annuus growth and yield at two sites in Missouri. Helianthus annuus densities of 3 plants m−2 were established shortly after G. max emergence in all plots except the weed-free check. To study early-season interference, H. annuus were removed with postemergence glyphosate (0.84 kg ae ha−1) 2, 4, 6, and 8 wk after planting (WAP) and kept weed-free for the rest of the growing season. Glycine max yields were not different with 2, 4, 6, or 8 wk of early-season interference at either location. To study early- plus late-season interference, H. annuus densities were established at 3 plants m−2. They were then removed 2, 4, 6, or 8 WAP with glyphosate and subsequently reestablished at the same density within 2 wk after removal by newly emerging and transplanted H. annuus. These H. annuus were allowed to remain in the field for the remainder of the growing season. This provided a weed-free period of approximately 2 wk during the growing season beginning 2, 4, 6, or 8 WAP. Season-long interference and no-interference treatments were also included. Glycine max yields were reduced 47 to 72% with season-long interference. Helianthus annuus vegetative dry matter was approximately 56% lower at Columbia than at Miami. Glycine max yields tended to increase as the weed-free period was delayed into the growing season. Early-season weed-free periods (2 to 4 and 4 to 6 WAP) allowed H. annuus to become re-established before G. max formed a canopy and resulted in larger amounts of H. annuus biomass and seed production as well as G. max yield losses of 15 to 80%. Re-establishment of H. annuus in 6 to 8 WAP and 8 to 10 WAP weed-free treatments generally resulted in the plants surviving for only a few weeks after establishment and not producing seed or reducing G. max yield.

The critical period for weed control (CPWC) is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses. Knowing the CPWC is useful in making decisions on the need for and timing of weed control and in achieving efficient herbicide use from both biological and economic perspectives. An increase in the use of herbicide-tolerant crops, especially soybean resistant to glyphosate, has stimulated interest in the concept of CPWC. Recently, several studies examined this concept in glyphosate-resistant corn and soybean across the midwestern United States. However, these studies presented various methods for data analysis and reported CPWC on the basis of a variety of crop- or weed-related parameters. The objectives of this study are (1) to provide a review of the concept and studies of the CPWC, (2) to suggest a common method to standardize the process of data analysis, and (3) to invite additional discussions for further debate on the subject. Wide adoption of the suggested method of data analysis will allow easier comparison of the results among sites and between researchers.

Weed management systems in carrot are limited in part by a lack of fundamental understanding of crop–weed interactions. Irrigated field studies were conducted to quantify the effect of volunteer potato density and duration of interference on carrot yield and to determine relationships among weed density, duration of weed growth, and volunteer potato tuber production. A season-long volunteer potato density of 0.06 plants m−2 produced from 150 to 230 g tubers m−2 and resulted in an estimated 5% crop yield loss. At two volunteer potato plants m−2, the same level of crop loss was estimated with a duration of interference of 430 growing degree days (GDD), a time at which the weed had already produced 130 g tubers m−2. Volunteer potato height at the time of weed removal predicted carrot yield loss (R 2 = 0.77) and may be useful for timing of management strategies such as hand weeding. Functional relationships describing carrot–volunteer potato interactions provide simple information that is useful for developing weed management recommendations for carrot, a crop that relies on multiple tactics for managing weeds, and rotational crops that are negatively affected by persistence of volunteer potato.

Field experiments were conducted in 1996, 1997, and 1998 at Lacombe and Lethbridge, AB, to determine the influence of early weed competition on field pea yields. Wild oat and Tartary buckwheat were removed from plots at weekly intervals after pea emergence by hand-weeding and maintained weed-free for the remainder of the growing season. Tartary buckwheat produced slightly more biomass than wild oat at Lacombe, whereas wild oat produced much more biomass than Tartary buckwheat at Lethbridge. Weed-free pea yields at Lacombe were always two- to threefold higher than at Lethbridge. At Lethbridge, early competition with Tartary buckwheat in all years, and with wild oat in 1998, did not reduce pea yields. In 1996 and 1997 at Lethbridge, pea yield reductions due to wild oat began at 2 wk after pea emergence; for the next 2 wk yield loss was linear, decreasing at an average rate of 97 kg/ha per day. Early weed competition led to pea yield losses at Lacombe in all 3 yr. The onset of yield loss at Lacombe over the 3-yr period ranged from 1 to 2 wk after pea emergence; for the next 2 to 3 wk yield loss was linear, decreasing at an average rate of 45 kg/ha per day. Yield losses after full-season weed competition ranged from 40 to 70% at both sites. Usually, the beginning of the critical weed-free period was at 1 or 2 wk after pea emergence. Optimum pea yields usually required weed removal very early in the pea life cycle; weed removal beyond 2 wk after pea emergence often protected only suboptimal yields.

Studies were conducted to evaluate the effects of grass and broadleaf weeds on peanut growth and peanut yield. In separate studies, grass or broadleaf weeds were allowed to compete with peanut for various intervals to determine both the critical timing of weed removal and the critical weed-free period. Hand-weeding and selective herbicides were used at appropriate times to remove and terminate weed growth. These periods were then used to determine the critical period of weed control. The effects of various weedy intervals on peanut yield were also investigated. The critical period of grass weed control was found to be from 4.3 to 9 wk after planting (WAP), whereas the critical period of broadleaf weed control was from 2.6 to 8 WAP. Peanut yields decreased as weed interference intervals for both grass and broadleaf weeds increased, demonstrating the need for control of both grass and broadleaf weeds throughout much of the growing season.

Nitrogen (N) management markedly affects weed competition with crops. The effect of N availability on plant competition varies with a plant's abilities to capture and use N. Accordingly, we expected the N effect on plant competition to change with the relative emergence time of competing individuals. This hypothesis was tested by growing corn and velvetleaf plants in target and neighbor roles and comparing their N responses. Sowing times were varied, so that target plant emergent dates were constant, whereas neighbor plants were sown to emerge 7 d before, with, or 7 d after targets. Seedlings were fertilized daily with 2.5 mmol (“low N”) or 10.0 mmol (“high N”) ammonium nitrate (NH4NO3). Corn had greater total weight, leaf area, and root-to-shoot ratio (RSR) than velvetleaf. Different dry weights may have reflected seed weights; corn seed weight was greater than velvetleaf. Regardless of role, corn and velvetleaf dry weight and leaf area were greater with high N than low N; in contrast, RSR was lower with high N than low N. The RSR response to N availability suggested plant resources were shifted from N foraging, toward competition for photosynthetically active radiation (PAR). In target plants of each species, dry weight and leaf area increased linearly with time between target and neighbor emergence. Conversely, dry weight and leaf area of neighbor plants decreased with time between target and neighbor emergence. The N response of neighbor plants increased with time of emergence between target and neighbor emergence. Larger plants likely required more N to sustain growth than smaller plants; plants that emerged earlier likely had greater daily N requirements than those that emerged later. These results support factoring in emergence when predicting N effects on plant competition. Optimal N provisioning for integrated weed management may differ with emergence times of competing plants.

Knowing the interference potential of common waterhemp in corn could be beneficial in planning waterhemp management strategies. In 2000, 2001, and 2002, field studies were conducted to examine both early- and late-season common waterhemp interference in corn. Early-season interference was determined by removing common waterhemp at the VE (vegetative emergence), V4 (four visible leaf collars), V6, V8, V10, V12, and V14 growth stages of corn for the entire season, and late-season interference was determined by allowing common waterhemp to emerge and compete from the VE, V4, V6, V8, V10, V12, and V14 corn growth stages. The interference potential of common waterhemp varied between the year 2000 and the combined years of 2001–2002. This is probably due to differences in precipitation in May and June in these two environments (297 mm in 2000 compared with 198 mm in 2001–2002). An excess of 590 g m−2 of dry matter and 13,000 and 1,200 seeds per female plant were produced when common waterhemp emerged at V4 and V6 corn, respectively, the 2 yr that corn was drought stressed. When corn was not moisture stressed, common waterhemp that emerged at V4 and V6 corn produced less than 220 g m−2 and less than 500 seeds per female plant. Season-long common waterhemp interference reduced corn yield 74% in 2 yr of the study and 11% in the third. Early-season common waterhemp interference began at V6 corn, with a 4 and 23% yield loss in 2000 and 2001–2002, respectively. Common waterhemp interference from late-season emergence reduced corn yield when emergence occurred before the V8 corn growth stage. Taking into account early- and late-season common waterhemp interference. the critical common waterhemp–free period was around the V6 corn stage to optimize corn yield.

An experiment was conducted at one location in 1999 and two locations in 2000 to determine the critical weed-free period for peach in North Carolina. The cultivars for the three locations were ‘Contender’, ‘Norman’, and ‘Summerprince’. Weed-free intervals of 0, 3, 6, 9, 12, and 15 wk after peach tree bloom were established. Paraquat at 1.1 kg ai/ha plus nonionic surfactant at 0.25% v/v was applied every 10 d, after treatments were initiated at peach bloom, to maintain weed-free plots. Large crabgrass, hairy vetch, and smooth crabgrass were the primary weeds in Contender. Horseweed, smooth crabgrass, and large crabgrass were the primary weeds in Norman. Bermudagrass, smooth pigweed, and common lambsquarters were the primary weeds in Summerprince. No differences in trunk cross-sectional area were observed between the weed-free periods. Maintaining the orchard floor weed-free for 12 wk after peach tree bloom resulted in the greatest fruit size (individual fruit weight and diameter), total yield, and fruit number.

Field experiments were conducted to determine the effect of large crabgrass densities of 0.5 to 8 plants m−1 of row and emergence time on snap bean yield. Large crabgrass was planted either along with snap beans (early) or when the first trifoliate leaf of snap beans was opening (late). Observed yield loss ranged from 46 to 50%, and predicted yield loss ranged from 53 ± 29.3% to 63 ± 18.3%. Relative leaf area was correlated to snap bean yield (r 2 = 0.88 to 0.92). The relative damage coefficient (q), an indication of the competitiveness of large crabgrass with snap bean, was 1.65 ± 1.03 and 1.26 ± 0.72 for early- and late-emerging large crabgrass, respectively. Early-emerging large crabgrass reduced snap bean biomass 10 to 28% and snap bean pod numbers 44 to 60%, depending on the density. Because of intraspecies competition, leaf area index and number of seed for large crabgrass were reduced with increasing density. Emergence of > 2 plants m−1 of large crabgrass with snap beans should be controlled to avoid significant yield loss.