At 10.4 mM bentazon treatment, the tolerance index of susceptible (S) inbred corn, line TN89, at one-leaf stage decreased to 27% of that of the control, whereas > 90% of tolerance index of control was maintained in tolerant (T) line LU21. More than fourfold of malondialdehyde (MDA) accumulated in S line within 7 d after treatment (DAT), but only a slight accumulation of MDA was found in T line. 14C-Bentazon application experiment indicated that there was no difference in bentazon absorption between T and S lines. However, bentazon metabolism in T line was more active than that in susceptible TN89. The metabolite, 6-glucose-bentazon in T line rapidly accumulated to the maximum 3 DAT, whereas this conjugate actually decreased in S line. Assay of in vitro activity of bentazon-6-hydroxylase showed that it was decreased in both lines with development and that this activity in T line at two-leaf stage was ca. 50% higher than that in S line. It is suggested that the higher bentazon tolerance in LU21 is primarily associated with an active bentazon metabolism, partially due to a higher bentazon-6-hydroxylase activity coupled with glucosylation.

Response of glyphosate-resistant cotton to various rates of topically applied glyphosate was investigated in growth chamber experiments to determine the relationship between glyphosate rate and boll abscission. Glyphosate at 0, 0.56, 1.12, or 2.24 kg ai/ha was applied to all exposed foliage at the 12-leaf growth stage. Immediately after this treatment, 14C-glyphosate was applied to the three uppermost fully expanded leaves at 0, 37, 74, or 148 kBq per leaf for the 0, 0.56, 1.12, or 2.24 kg/ha treatment, respectively. After glyphosate application, glyphosate accumulated in reproductive tissue, and bolls were abscised. Abscission increased as the amount of glyphosate translocated to fruiting sites increased.

Glyphosate and glufosinate are now options for postemergence weed control in herbicide-resistant corn and soybean. Velvetleaf is one of the more difficult to control annual weeds with these herbicides at commonly used rates. Ammonium sulfate (AMS) is generally used with these herbicides to overcome hard water antagonism and to increase herbicide activity. Greenhouse and field trials were conducted with commercial adjuvants that might substitute for AMS. The adjuvants were evaluated in deionized water, tap water, and deionized water containing 500 mg/L CaCO3. In the absence of AMS, hard water reduced velvetleaf control with both herbicides. Regardless of water source, AMS increased velvetleaf control with both glyphosate and glufosinate. Several adjuvants increased velvetleaf control with either herbicide; however, none were superior to 2% w/v AMS. Other adjuvants decreased velvetleaf control with either herbicide.

Laboratory and greenhouse experiments were conducted to determine quinclorac efficacy as influenced by surfactants, methylated seed oil (MSO), basic pH compounds, and salts in the spray carrier water. Quinclorac efficacy for green foxtail control generally increased with an increase in linear alcohol ethoxylate (LAE) surfactant carbon-chain length and percentage of ethoxylation. With LAE surfactants, quinclorac phytotoxicity to green foxtail was nearly doubled (average from 44 to 81%) when triethanolamine (TEA) was included in the spray mixture. Combination of LAE surfactants with TEA also enhanced quinclorac absorption. Enhancement of quinclorac absorption and phytotoxicity by LAE surfactants and TEA was related to spray deposits that had close contact with the cuticle and without apparent quinclorac crystals. Sodium and calcium ions strongly antagonized quinclorac efficacy when applied with a block copolymer surfactant or MSO. Ammonium sulfate or ammonium nitrate adjuvants were more effective than urea–ammonium nitrate liquid fertilizer in overcoming antagonism from salts in spray carrier waters. These results demonstrate the potential for maximizing quinclorac efficacy by careful selection of surfactants, nitrogen fertilizer, and basic pH additives.

Field experiments were conducted to examine the influence of spray volume on glyphosate efficacy in relation to glyphosate rate, formulation, ammonium sulfate addition, and type of sprayer nozzle. Using several grass species it was shown that glyphosate efficacy increased as spray volume decreased from 190 to 23 L/ha. To obtain equal efficacy, glyphosate rates can be reduced by at least one-third when glyphosate is applied in 23 or 47 L/ha spray volume compared with 94 or 190 L/ha. The amount of surfactant in formulated glyphosate at 35 to 140 g ae/ha rates was insufficient when glyphosate was applied in 94 or 190 L/ha spray volumes. Additional surfactant enhanced glyphosate efficacy at these rates when applied in 94 or 190 L/ha spray volume, but efficacy was still less than when applied in 23 or 47 L/ha without additional surfactant. Thus, low spray volumes maximized glyphosate efficacy primarily through high herbicide concentration in the spray deposit and reduced salts from the carrier to antagonize efficacy. Glyphosate applied in 23 L/ha spray volume with drift-reducing nozzles provided control equal to that provided by glyphosate applied with standard flat-fan nozzles. Grass control also was equal from several glyphosate formulations that contained surfactants, regardless of spray volume.

Field experiments were conducted at three locations in 2000 and 2001 to evaluate what effect soil characteristics have on sensitivity of two corn hybrids to soil-applied isoxaflutole plus flufenacet. Soil types were a Flanagan silt loam with 3.9% organic matter (OM), a Drummer silty clay loam with 5.0% OM, and a Cisne silt loam with 2.1% OM. Soil pH was adjusted to four target levels, < 6.0, 6.0 to 6.5, 6.6 to 7.0, and > 7.0, at each location. Isoxaflutole plus flufenacet treatments consisted of the recommended rate (1 ×) and two times (2 ×), and four times (4 ×) the recommended rates based on soil type. Additional treatments included 1 × and 2 × isoxaflutole plus flufenacet with 1.1 kg/ha atrazine and S-metolachor plus atrazine as a control treatment. Corn hybrid ‘Garst 8366’ (G8366) was more sensitive to soil-applied isoxaflutole plus flufenacet compared with ‘Garst 8600’ (G8600). Corn injury occurred only at two of three locations. Addition of atrazine to the 1 × and 2 × isoxaflutole plus flufenacet treatments showed a trend (P = 0.15) toward increased injury and reduced yield only for the G8366 on the Cisne soil. The 4 × rate of the premixture reduced yield 9 to 49% at two locations. Yields were reduced at the low-OM (2.1%) location, regardless of hybrid or application rate. At two locations, there was a negative linear relationship between soil pH and corn grain yield, particularly at the 4 × rate. Corn visual injury and yield reductions were greatest from herbicide applications to low-OM soils with high pH levels.

The classification of herbicides by site of action, published in 1997, has been revised. The classification system uses a numbering system for a herbicide's site of action, chemical family, and common name. Regulatory agencies in the United States and Canada have published labeling guidelines based on the classification to aid in herbicide resistance management.

Herbicide-resistant cultivars may improve some aspects of weed management in wheat. However, negative consequences such as gene transfer among species, increased development of resistant weeds, or less effective volunteer wheat control may result from their use. Therefore, we reviewed literature on volunteer (self-sown) wheat seedling emergence and seed longevity in soil for insight in managing herbicide-resistant wheat. Data from classical burial studies suggested that wheat seeds were short-lived in soil, persisting less than 1 yr. Yet, in field studies, volunteer wheat seedlings were still emerging 16 mo after harvest; occasionally, seedlings have been observed 2 yr after harvest. Volunteer wheat emergence was extremely variable; causes of the variability are numerous and include genotypic, environmental, and production factors. This variability makes it difficult to predict volunteer wheat infestations in future years. Diverse cropping systems will enable producers to accrue the benefits of herbicide-resistant cultivars and yet still manage wheat volunteers, minimize gene flow by pollen, and avoid transfer of herbicide resistance. In regions where alternative crops are not viable, a key concern will be controlling volunteers and gene transfer in the next wheat crop.

Red rice has long been a troublesome, conspecific weed of cultivated rice. Rice varieties carrying certain herbicide-resistant traits acquired through genetic modification (herbicide-resistant varieties) now offer new options for red rice control. In concert with this innovation is the risk of gene flow, which can result in the transfer of that specific herbicide resistance to red rice and thus render this weed control measure ineffective. Gene flow in concept is simple, however, the parameters that determine the establishment of a new trait in a weed population are complex. Cross-pollination to make hybrid seed and the subsequent fate of those hybrid families in the general weed population are some of the biological factors that influence gene flow between red rice and cultivated rice. Natural outcrossing among rice plants is generally low. Most of the pollen dispersal studies published to date indicated that rice × rice outcrossing rates were less than 1.0%. Numerous reports summarized in this study suggest that outcrossing rates between rice and red rice can be highly variable but usually are similar to or lower than this level. However, once hybrids form, they may introgress into a red rice population within only a few generations. If hybrid seed families are to persist and establish herbicide-resistant red rice populations, they must successfully compete in the crop–weed complex. The ability to survive a herbicide applied to a herbicide-resistant rice variety would be a strong selective advantage for these hybrid families. Thus, the well-established principles of weed resistance management appear to be relevant for herbicide-resistant crop systems and should be used in combination with practices to minimize coincident flowering to mitigate the potential impact of gene flow from herbicide-resistant rice into red rice. For the rice–red rice crop–weed complex, there are both biological factors and agricultural practices that can work together to preserve these new weed control options.

“One Queen Artemisia, as old stories tell, When deprived of her husband she loved so well, In respect for the love and affection he show'd her, She reduc'd him to dust and she drank up the powder.”
Another on the said Occasion (lines 1 to 4), Robert Burns (1759 to 1796).

Biennial wormwood (Artemisia biennis Willd.) is an aggressive and prolific seed-producing plant of the Asteraceae. The genus Artemisia belongs to the Anthemidaeae, or mayweed, tribe and includes various annual, biennial, and perennial herbs and shrubs that are often aromatic and bitter (Hall and Clements 1923; McArthur and Welch 1986). There is little consensus about the exact number of species within the genus Artemisia (Stebbins 1974); estimates include “over 100” (Munz and Keck 1968), “nearly 300” (Kelsey and Shafizadeh 1979), “346” sensu stricto (Ling 1994), “about 400” (Greger 1982), and “more than 400” (Polyakov 1995) species.