Greenhouse and field studies were conducted to examine the interaction of sethoxydim or fluazifop-P tank mixed with chlorsulfuron or thiameturon for selective weed control in safflower. Under greenhouse conditions, the addition of chlorsulfuron to sethoxydim or fluazifop-P slightly improved the control of green foxtail above that achieved with either herbicide alone. Control of wild oat and wild mustard was similar with the tank mixes and each herbicide alone. In the field, control of wild oat with sethoxydim or fluazifop-P was not altered by adding chlorsulfuron or thiameturon in tank mixes. Similarly, control of common lambsquarters and wild mustard with chlorsulfuron or thiameturon was not affected by adding either sethoxydim or fluazifop-P in tank mixes. Plant height, date of flowering, seed yield, and quality data indicated that safflower tolerated these herbicides applied alone or in combination. Sethoxydim or fluazifop-P tank mixed with chlorsulfuron or thiameturon provide a postemergent alternative for selective control of grass and broadleaf weeds in safflower.

Postemergence control of broadleaf weeds in canola with DPX A7881 was studied under controlled-environment and field conditions. DPX A7881 controlled weeds of the Cruciferae family such as wild mustard, field pennycress, and flixweed, which are not controlled with existing herbicides used for weed control in canola. Weed control was better when DPX A7881 was applied at the two-leaf than at the six-leaf stage of the weeds. At the two-leaf stage, control of flixweed was achieved with 15 g ai/ha of DPX A7881; 20 to 30 g ai/ha was required for control of wild mustard and field pennycress. Control of redroot pigweed required 30 g ai/ha. Common lambsquarters was not controlled in this study with DPX A7881 up to 60 g ai/ha. Control of weeds with DPX A7881 increased canola yields in this study without altering the important quality components: oil content, 1000-kernel weight, and green seed content. DPX A7881 provides a postemergence alternative to existing herbicides for broadleaf weed control in canola.

Field studies were conducted in 1984, 1985, and 1986 to compare standard flat fan nozzles, low-volume flat fan nozzles, rotary controlled droplet applicators, and air-assisted nozzles for postemergence weed control in onions with oxyfluorfen and fluazifop-P. Herbicides applied with flat fan nozzles at 47 or 190 L/ha controlled broadleaf and grass weeds as good or better than with the other nozzles. Oxyfluorfen applied with flat fan nozzles controlled common lambsquarters and ladysthumb best. None of the nozzles improved control of the difficult marsh yellowcress. Carrier volume had minimal effect on weed control. Nozzle and carrier volume did not affect onion yield.

Weed management systems using a chloroacetamide herbicide applied preplant incorporated (PPI) and at cracking (AC) plus lactofen applied AC and early postemergence (POST) controlled at least 98% of common lambsquarters. Similar systems using a chloroacetamide herbicide applied PPI and early POST plus lactofen applied early POST and postemergence (POST) controlled less than 70% of common lambsquarters. All sequential lactofen systems controlled common ragweed completely and at least 86% of morningglory species. Highest peanut yields and net returns were obtained with weed management systems consisting of sequential applications of lactofen applied AC and early POST, or systems that used acifluorfen plus bentazon POST, or lactofen plus bentazon applied early POST or POST.

The first step in appearance of herbicide-resistant weed populations is the appearance of resistant plants or mutants. While efforts are underway to study and predict the spread of resistant plants within weed populations, knowledge of the conditions prevailing at the time of appearance of the first resistant plants is misunderstood. We try to shed some light on this phenomenon using the example of atrazine-resistant common lambsquarters. The population structure and variability, the presence of unusual genotypes that have a high mutation rate, the occurrence of a low-dose resistant phenotype that is the precursor of a high-dose resistant phenotype, and the potential for multiplication and spread are shown to be of major importance in the behavior of herbicide resistance.

Research in Kansas from 1983 to 1986 evaluated early preplant (30 to 45 days) and late preplant (10 to 14 days) herbicide treatments for weed control before ridge-till planting in a soybean and sorghum rotation. Control of fall panicum and common lambsquarters at planting time averaged at least 95% for all early preplant and 92% for late preplant treatments. Where no preplant treatment was used, heavy weed growth in spring delayed soil dry-down, which resulted in poor ridge-till planting conditions and reduced plant stands, and ultimately reduced sorghum grain yields by 24% and soybean yields by 12%. Horsenettle population declined significantly, and honeyvine milkweed population increased. Smooth groundcherry populations fluctuated from year to year with no overall change.

Field studies were conducted in 1987 and 1988 to investigate the control of common lambsquarters in pea and snap bean following preplant incorporated, preemergence, and postemergence applications of imazethapyr at 36 to 69 g ai/ha. The residual activity of imazethapyr on rotational crops of sweet corn, grain sorghum, and cucumber was evaluated. All rates of imazethapyr provided good (>80% initially) control of common lambsquarters. Although imazethapyr caused some injury to both pea and snap bean, yields were not reduced except for snap bean following postemergence applications. Imazethapyr residues resulting from the applications of 36 to 69 g/ha caused visible injury and fresh weight reductions to all three rotational crops.

Field studies were conducted over a four-year period to determine the weed control efficacy of ICIA-0051 and SC-0774 in no-tillage and conventional corn. Treatments consisted of both pre- and postemergence applications of ICIA-0051, alone and in combination with atrazine, and preemergence applications in combination with metolachlor. SC-0774 was applied preemergence, alone and in combination with atrazine. Results indicated good control of triazine-resistant (TR) smooth pigweed and other annual broadleaf weeds present with no adverse effects on crop vigor and yield with ICIA-0051. Generally, the addition of atrazine to ICIA-0051 improved weed control. SC-0774 provided good control of fall panicum, but generally poorer and less consistent control of giant ragweed, common ragweed, common lambsquarters, and ivyleaf and tall morningglory species.

The interaction of DPX-A7881 and clopyralid applied as tank mixtures was studied under controlled environment and field conditions on several commonly occurring weeds in canola. Control of wild mustard and wild buckwheat was not affected by mixing these herbicides. DPX-A7881 controlled wild mustard, and clopyralid controlled wild buckwheat independently of the other herbicide. However, redroot pigweed and common lambsquarters were controlled better with DPX-A7881 plus clopyralid than with either herbicide alone. The herbicides interacted in an additive to synergistic manner on redroot pigweed depending on the rate of each herbicide. DPX-A7881 and clopyralid interacted synergistically at all rates on common lambsquarters. Plant height, date of flowering, seed yield, and quality data all indicated that canola completely tolerated DPX-A7881 and clopyralid applied alone or in combination.

The objectives of this research were to compare the weed control efficacy of liquid, granular, and microencapsulated formulations of preemergence herbicides in moldboard plow, chisel plow, ridge tillage, and no-tillage corn and soybean production systems, and to determine whether herbicide formulation can influence herbicide interception and retention on surface corn residue. Common lambsquarters populations were threefold higher in corn than in soybeans. A mixed population of giant foxtail and green foxtail was highest in the chisel plow and lowest in the ridge tillage system as were total weed numbers. Percent weed control was not influenced by tillage when considered across all herbicide treatments. Weed control was not influenced by herbicide formulation in the moldboard plow, chisel plow, or ridge tillage systems, but granular herbicide applications provided better weed control than liquid applications in the no-tillage system and across various rates of corn residue in an experiment with no tillage variables. Two- to threefold less granular-applied herbicide was intercepted by surface corn residue at the time of application compared to liquid-applied herbicide. Increasing amounts of postapplication rainfall decreased the difference among formulations with regard to both total soil reception of the herbicide and resultant weed control. There was no consistent advantage for the microencapsulated formulation over the other herbicide formulations. Surface corn residue controlled many weeds without the aid of a herbicide and actually contributed to overall weed control even where herbicides were applied. This suggests that the binding of preemergence herbicides on surface crop residue may not be the cause of weed control failures in reduced-tillage systems as is often assumed to be the case.