Field experiments were conducted in 1982 through 1986 to evaluate the effect of six postemergence grass herbicides, clethodim, the methyl ester of diclofop, the butyl ester of fluazifop-P, the methyl ester of haloxyfop, the methyl ester of quizalofop, and sethoxydim, on 31 seedling grass species. All herbicides except diclofop controlled most of the species tested. None of the herbicides controlled rattail fescue or red fescue. Only clethodim and haloxyfop controlled annual bluegrass.

Field experiments were conducted to compare performance of glyphosate with three different boom arrangements in a winter wheat-fallow rotation near North Platte, NE, in 1994 and 1995. One boom was optically controlled, and the other boom was for broadcast herbicide applications. Spraying with both booms at the same time was called “dual boom.” The sprayers were tested during May, June, and July on two weed density levels established by applying glyphosate at 0.42 kg ae/ha with and without atrazine at 0.84 kg ai/ha in October following wheat harvest. The dual-boom and the broadcast herbicide applications were more efficient in controlling weeds than the optically controlled system. The dual boom reduced weed density 4.5-fold compared with the optically controlled sprayer used alone. Horseweed < 8 cm tall was more difficult to control with the optically controlled sprayer than redroot pigweed and kochia because of its cylindrical-shaped growth patterns. Barnyardgrass and green foxtail seedlings with an erect growth pattern were also difficult for the sensors to detect. Poorer control with the optically controlled sprayer was associated with failure to identify small weeds, chlorotic plants, inconsistency among sensors, and too wide a field of view (FOV), as sensors were spaced farther apart than presently recommended. The number of sensors on a boom needs to be increased to improve the performance of the optically controlled sprayer.

DPX-79406 was evaluated for POST annual grass weed control in both controlled environment and field experiments. In controlled environment experiments, green foxtail was most susceptible to DPX-79406; whereas yellow foxtail was least susceptible of the species evaluated. DPX-79406 at 12 g/ha completely controlled six leaf black-seeded proso millet, yellow foxtail, green foxtail, and barnyardgrass. In the field, DPX-79406 at 3.0 to 25.0 g/ha effectively controlled annual grass weeds without injury to three- to six-leaf corn. There was more variation in the effectiveness of DPX-79406 applied in the field. Early POST applications provided less weed control than the late application, especially for barnyardgrass, because of weeds emerging after application. As a result, higher doses were sometimes needed for effective control. In weed-free field trials at two sites in 1990 and 1991, corn tolerated doses up to 75 g/ha of DPX-79406 applied at the three- to six-leaf growth stage. However, doses as low as 18.8 g/ha applied at the six- to nine-leaf growth stage reduced grain yield. In 1991, corn tillering increases and height and yield reductions were related linearly to the dose of DPX-79406 applied during later growth stages. DPX-79406 should be applied early POST in order to avoid crop injury while providing effective weed control.

Triazine-resistant (TR) biotypes of weeds normally susceptible (S) to triazine herbicides first were documented in Ontario in 1974. By 1988, at least nine weed species had TR biotypes, and at least 50% of the corn-producing areas were infested. Corn has been grown the longest in the southwestern Ontario, but TR weeds are only a minor problem there. Predominant agronomic practices in the southwest include crop rotation, atrazine use on 60% of corn land, use of other postemergence herbicides, interrow cultivation, little silage corn, and little manure returned to the land. Corn, particularly grain corn, is new as a large hectareage crop in eastern Ontario. However, more than 75% of corn land in that area is infested with two or more TR weed species. Predominant agronomic practices in the east include continuous corn, nearly all fields are atrazine treated, use of postemergence herbicides is not common, cultivation is rare, 25% of the corn is for silage, and manure from corn silage is returned to all cultivated land. Adequate herbicide programs exist to control TR weeds in corn and other crops. However, the spread from field to field and from farm to farm is often rapid, particularly in eastern Ontario.

Experiments were conducted at two sites for 2 yr in the Pacific Northwest dryland cropping region to determine if seeding rate of small-red lentil could enhance weed control with herbicides and increase lentil seed yield. At Pendleton, OR, and LaCrosse, WA, lentil was planted at 22 or 44 kg ha−1 in one direction in all plots. In one-half of the plots, lentil was cross-seeded at right angles with an additional 22 kg ha−1 to provide seeding rates of 22, 44, 22 + 22, and 44 + 22 kg ha−1. Seeding rate main plots were split into three herbicide treatments and an untreated control. Total weed density was reduced by increasing seeding rate at Pendleton both years when averaged over all herbicide treatments. Seeding rate reduced total weed density to a greater extent when herbicides did not adequately control weeds or when herbicides were not applied at Pendleton in 1992. Increased seeding rate also reduced total weed dry weight at Pendleton in 1992 and 1993 and at LaCrosse in 1993. The suppressive effect of increased seeding rate on weed dry weight was more evident when herbicides were not used or when herbicides gave only partial control. Herbicides generally reduced weed density, but the effectiveness of individual treatments was related to the weed species present and environmental conditions present in each experiment. Lentil aboveground dry weight production increased with seeding rate at both locations; however, only in 1 yr did lentil seed yield increase with seeding rate. The primary benefit from increased seeding rate in this study was to reduce weed density and dry weight.

Buried seeds of witchgrass (Panicum capillare L., # PANCA) exposed to natural seasonal temperature changes in Lexington, KY, for 0 to 35 months exhibited annual dormancy/nondormancy cycles. Seeds were dormant at maturity in early October. During burial in late autumn and winter, fresh seeds and those that had been buried for 1 and 2 years became nondormant. Nondormant seeds germinated from 76 to 100% in light at daily thermoperiods of 15/6, 20/10, 25/15, 30/15, and 35/20 C, while in darkness they germinated from 1 to 24%. In late spring, seeds lost the ability to germinate in darkness, and by late summer 63 to 100% of them had lost the ability to germinate in light. As seeds became nondormant, they germinated (in light) at high (35/20, 30/15 C) and then at lower (25/15, 20/10, and 15/6 C) temperatures. As seeds reentered dormancy, they lost the ability to germinate (in light) at 15/6 C and at higher thermoperiods 2 to 3 months later.

Spring small grains were not as competitive with barnyardgrass and witchgrass as winter wheat. Winter wheat grain yields were greater than spring barley or spring wheat in 1986, 1987, and 1988 and oat in 1986 and 1988. Barnyardgrass, stinkgrass, and witchgrass control with glyphosate plus 2,4-D plus atrazine at 0.6 plus 0.8 plus 1.7 kg ha−1 was usually less when the herbicides were applied to stubble of spring small grain versus winter wheat due to the advanced weed growth at treatment Barnyardgrass and witchgrass were more difficult to control than stinkgrass, redroot pigweed, tumble pigweed, kochia, and tumble thistle. No-till corn planted into winter wheat stubble had fewer barnyardgrass and witchgrass than corn planted into spring wheat stubble. The addition of metolachlor plus atrazine at 1.7 plus 0.6 kg ha−1 eliminated differences among small grain cultivars in weed control in corn. Corn grain yields from winter wheat plots were greater than other small grains in 1989 because of better weed control and more crop residue.

Chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide} was nontoxic to safflower (Carthamus tinctorius L.) when applied postemergence in 1983 and 1984 at 0.018 and 0.035 kg ai/ha. Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] at 1.1 and 1.7 kg ai/ha and pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynyl)benzamide] at 0.8 and 1.1 kg ai/ha were applied previously as preplant soil-incorporated treatments. In both years, safflower was relatively free of weeds where trifluralin was applied alone or in sequential treatments with chlorsulfuron. Pronamide, with or without chlorsulfuron, failed to completely control witchgrass (Panicum capillare L. ♯ PANCA) in 1 yr; thus safflower grain yields were reduced 21 to 35% when compared to weed-free safflower. Chlorsulfuron controlled redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), puncturevine (Tribulus terrestris L. ♯ TRBTE), and common sunflower (Helianthus annuus L. ♯ HELAN).

A survey of 174 fields was conducted to investigate performance of herbicides applied after winter wheat harvest on weeds across western and southern Nebraska during August and September 1998. Glyphosate plus 2,4-D plus atrazine was applied on 32%, glyphosate plus 2,4-D or dicamba on 24%, paraquat plus atrazine on 23%, glyphosate on 8%, ICIA0224 plus 2,4-D or atrazine on 10%, and atrazine plus 2,4-D on 3% of the fields. These treatments controlled 85 to 100% of the weeds except atrazine plus 2,4-D, which controlled 30%. The frequency of occurrence of the most prevalent summer annual grasses was as follows: green foxtail, 65%; barnyardgrass, 46%; stinkgrass, 41%; witchgrass, 39%; and longspine sandbur, 36%. The most common broadleaf weeds and their frequency were redroot pigweed, 32%; tumble pigweed, 30%; tall waterhemp, 28%; and kochia, 25%. Virginia groundcherry, 22%; common milkweed, 11%; yellow woodsorrel, 9%; and field bindweed, 6% were the most common perennial weeds. The five most difficult weeds to control were yellow nutsedge, spotted spurge, Virginia groundcherry, common milkweed, and toothed spurge, with control ratings of 0, 3, 17, 26, and 33%, respectively. These weeds were not controlled with glyphosate or mixtures containing glyphosate. Only 35% of the fields were treated before summer annual grasses had headed. Late applications required higher herbicides rates for effective control.