A naturally occurring prickly lettuce biotype resistant to a 5:1 formulated mixture of chlorsulfuron:metsulfuron (DPX-G8311) was identified in a no-till winter wheat field near Lewiston, ID, in April, 1987. Field and greenhouse studies were established to evaluate its resistance to other sulfonylureas, imidazolinones, and herbicides with alternate sites of action. The resistant biotype resisted eight sulfonylurea herbicides; resisted the imidazolinone herbicides, imazapyr and imazethypyr, but not imazaquin; and resisted no other herbicides included in the studies. The resistant biotype was identified in seven of nine fields on the farm where it was discovered.

Dinitroaniline herbicides are major herbicides used to control annual grass weeds in cool-season turfgrasses. At least three herbicides, benefin, pendimethalin, and trifluralin are labeled preemergence to control weeds in established turfgrass. Prodiamine could be a competitive product. Benefin at 2.2 to 3.4 kg ai/ha, pendimethalin at 1.7 to 3.4 kg ai/ha, prodiamine at 0.6 to 1.7 kg ai/ ha, and benefin plus trifluralin at 1.1 plus 0.6 to 2.3 plus 1.1 kg ai/ha effectively control many annual grass weeds, including large and smooth crabgrass, yellow and green foxtail, barnyardgrass, goosegrass, fall panicum, and annual bluegrass. Most dinitroaniline herbicides provide residual preemergence weed control for the entire growing season. In general, Kentucky bluegrass, perennial ryegrass, tall fescue, red fescue, and other cool-season turfgrasses tolerate most of these herbicides, although higher-than-normal rates of any dinitroaniline herbicide could restrict root growth and cause stand thinning in turfgrass species.

The effects of 14 herbicide treatments, tested in the establishment phase, were investigated on timothy productivity in two field experiments conducted at Melfort, Saskatchewan. Satisfactory weed control achieved with chlorsulfuron, metsulfuron, thifensulfuron, bentazon, diclofop plus bromoxynil, and MCPA/mecoprop/dicamba during the establishment year ensured good forage establishment which in turn caused a ‘residual effect’ for 1 to 3 yr increasing individual cut as well as cumulative forage dry matter yields significantly. In the stand established in 1985, when field pennycress (97 m-2) was the dominant weed in the seedling stage, these treatments increased yields 29 to 52% over a 3-yr period. In the stand established in 1986, when common lambsquarters (27 m-2) was the dominant weed, these treatments increased cumulative yields 24 to 44%. Sethoxydim and fenoxaprop, tested alone, caused significant crop injury in both experiments.

Research was conducted on a sandy clay loam in Georgia during 1987–88 and 1988–89 to determine winter wheat, oats, barley, and Italian ryegrass tolerance to flurtamone. Flurtamone was applied PPI, PRE, and early POST at 0.28, 0.56, and 1.1 kg ai ha-1. Order of tolerance of the four species was barley > wheat > oat > Italian ryegrass. Flurtamone potentially can be used to control Italian ryegrass in winter wheat and barley, but not in oats. In general, wheat and oat injury with flurtamone was PPI > PRE ≥ early POST, particularly at 1.1 kg ha-1.

Newly seeded alfalfa tolerated glyphosate applied from the 8-trifoliolate leaf stage (17 cm tall) to the pre-bud stage (34 cm tall) at 75 or 150 g ae ha-1. Injury symptoms were moderate and temporary. Glyphosate at 300 g ha-1 suppressed alfalfa severely and killed 30% of the plants when applied at the smallest (3-trifoliolate) stage of alfalfa growth. Alfalfa responded similarly to glyphosate and SC-0224.

The behavior and dissipation of 26 dinitroaniline herbicides in soils are reviewed. Some of the compounds also are referred to as dinitrotoluidines, -benzen(e)amines, -benzenediamines, -cumidines, -benzenesulfonamides, -acetophenones, and sulfonylsulfilimines. All dinitroanilines are sorbed to soil particles, particularly to organic or humic substances, and are nearly immobile in soils. Soil K values range from 7 to 117 and KOC values range from 80 to 471 000. Compounds with vapor pressures greater than 50 × 10-6 mm of Hg at ambient temperatures were reported to volatilize and diffuse through, and out of, the soil in significant amounts, depending on temperature and moisture conditions. Greater losses occurred from warm, moist soils than from cool, dry soils. Photodecomposition of dinitroanilines on soil surfaces was low but occurred when the chemicals were present in the vapor state or in aqueous solutions. Bioavailability of the chemicals decreased as organic matter contents of the soils increased. Dinitroaniline herbicides were degraded primarily by soil microorganisms, and fungi were the major organisms involved. Degradation proceeded faster under anaerobic conditions than under aerobic conditions and faster under warm, moist conditions than under cool, dry conditions. Half-life values for dinitroanilines ranged from 7 to 27 days under anaerobic conditions and from 19 to 132 days under aerobic conditions. The chemicals did not affect most soil microflora; and any significant effects reported were of short duration.

Clomazone is used in the Central Great Plains for weed control during fallow in a winter wheat-fallow rotation. Improved precipitation storage during non-crop periods has stimulated new crop rotations such as winter wheat-corn or proso millet-fallow. The objective of this study was to determine if clomazone applied in the fall after winter wheat harvest would injure succeeding spring-planted crops. Greenhouse studies indicated crop tolerance to clomazone was in the order of safflower > corn > proso millet > barley > winter wheat. Clomazone did not affect grain yields of safflower, corn, or proso millet grown at two field sites with different soil textures (silt loam and sandy loam), nor germination of seed from treated plants of these crops. A no-till production system with clomazone increased grain yields for all crops compared to the conventional system where tillage replaced clomazone for fallow weed control.

A biotype of kochia resistant to chlorsulfuron has been isolated from a Kansas wheat field that had been treated with chlorsulfuron for five consecutive years. In greenhouse tests, the biotype also was resistant to both preemergence and postemergence herbicides with the same mode of action including imazapyr, CGA-131036, the methyl ester of metsulfuron, DPX-M6316, DPX-L5300, and the methyl ester of sulfometuron. The degree of cross resistance varied among the herbicides tested. Herbicides with different modes of action, including atrazine, bromoxynil, MCPA, and diuron, effectively controlled the resistant biotype in the greenhouse.

Field experiments were conducted in 1983, 1984, and 1985 at Stoneville, MS, to evaluate the differential growth of 17 biotypes of bermudagrass in two cotton planting patterns. Growth comparisons (parallel = along the row, perpendicular = across the row, and height) were made among biotypes and between solid and skip-row cotton planting patterns, 1- and 2-m row widths, respectively. Biotype growth parameters differed significantly regardless of cotton planting patterns. Bermudagrass growth, perpendicular and parallel to rows, was greater in skip-row than in solid planted cotton by ≥5 and ≥7 wk after planting, respectively. Bermudagrass was taller in skip-row vs. solid planted cotton at ≥18 wk after planting. When averaged over biotypes, growth of bermudagrass was 3 to 4 times greater in skip-row than in solid cotton planting patterns by 18 wk after planting.

A heavy, uncontrolled infestation of field dodder and largeseed dodder suppressed newly seeded alfalfa severely and killed many of the plants. Glyphosate at 75 or 150 g ae ha-1, sprayed over the top of the dodder-infested alfalfa, stopped the growth of dodder and did not injure alfalfa significantly. Treated dodder began to grow normally again 2 to 3 wk after treatment. This regrowth injured the alfalfa severely in spite of earlier control. Glyphosate at 75 or 150 g ha-1 applied two or three times controlled dodder effectively and protected the alfalfa. A rate of 40 g ha-1 was not effective.

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.

Five field experiments were conducted at two locations in Virginia during 1988–89 to evaluate DPX-V9360 and CGA-136872 for postemergence control of johnsongrass in field corn grown under no-till and conventional conditions. DPX-V9360 at rates of 35 (except in no-till plots), 52, and 69 g ai ha-1 applied to 8-leaf johnsongrass was 88 to 98% effective in 1988. CGA-136872 at 25, 49, and 99 g ha-1 was 80% effective at the highest rate. Applications of these herbicides to 6-leaf johnsongrass resulted in 69% or less control. DPX-V9360 at 35, 52, and 69 g ha-1 and CGA-136872 at 20 and 40 g ha-1 applied to johnsongrass up to 48 cm in height provided 77 to 97% control at 7 WAT3 in an excellent stand of conventionally planted corn during 1989. Crop yields were more than doubled with herbicide treatments relative to yields in control plots. At a second location where crop damage caused by other factors resulted in a weak stand, DPX-V9360 provided 61 to 96% control compared to 29 to 56% control with CGA-136872.

This review considers the development of herbicide-resistant weed biotypes in Australia. Biotypes of the important annual weed species, capeweed, wall barley, and hare barley are resistant to the bipyridylium herbicides paraquat and diquat. These resistant biotypes developed on a small number of alfalfa fields that have a long history of paraquat and diquat use within a distinct geographical area in central western Victoria. The resistant biotypes are controlled by alternative herbicides and pose little practical concern. Some populations of wild oat are resistant to the methyl ester of diclofop. Of greatest concern is the development of cross resistance in biotypes of rigid ryegrass to aryloxyphenoxypropionate, cyclohexanedione, sulfonylurea, and dinitroaniline herbicides. The cross-resistant rigid ryegrass infests crops and pastures at widely divergent locales throughout the cropping zones of southern Australia. The options for control of cross-resistant rigid ryegrass by herbicides are limited. A biotype of rigid ryegrass on railway tracks treated for 10 yr with amitrole plus atrazine has resistance to amitrole and atrazine and other triazine, triazinone, and phenylurea herbicides. Management tactics for cross resistance are discussed.

Wild-proso millet control in furrow-irrigated corn was evaluated in Colorado and Nebraska in 1986 and 1987. No single herbicide alone controlled wild-proso millet all season. In Colorado, EPTC applied preplant incorporated followed by cyanazine plus pendimethalin applied early postemergence controlled 94% of wild-proso millet. Acetochlor applied preemergence followed by cyanazine plus pendimethalin applied early postemergence controlled wild-proso millet better all season (93%) than alachlor (81%) or metolachlor (71%) followed by the same early postemergence strategy. Successful Colorado wild-proso millet management treatments (>85% season-long control) increased corn yields an average of 3260 kg ha-1 compared to the untreated control. To obtain wild-proso millet control of 90% or more in Nebraska in 1986, alachlor, cycloate, EPTC, and metolachlor applied had to be combined with cyanazine plus pendimethalin applied early postemergence. Average corn yields in herbicide-treated areas in Nebraska were 2980 kg ha-1 higher than those recorded in the untreated control.

Alachlor at 3.4 and 6.7 kg ha-1, metolachlor at 2.8 and 5.6 kg ha-1, and propachlor at 4.5 and 9.0 kg ha-1 were applied to grain sorghum in field experiments conducted in 1981 and 1982. ‘Dekalb E-59’ and ‘Pioneer 8311’ grain sorghum were tested with and without the seed protectants cyometrinil and flurazole in both experiments. ‘Funk's G-1350’ was tested with and without cyometrinil and CGA-92194 in one experiment. Alachlor and metolachlor caused serious injury (stunting and, in some instances, reductions in stand and yield) to grain sorghum without the seed protectants. Less severe injury to grain sorghum occurred with alachlor and metolachlor at high rates even in the presence of the seed protectants. The three seed protectants were equally effective in preventing crop injury. Only minor injury to grain sorghum, with or without the seed protectants, occurred with propachlor at both rates. Overall, early-season weed control was 81 to 100%; however, propachlor at the rates tested was noticeably weaker than alachlor and metolachlor against some annual grasses.

The effect of urea ammonium nitrate (UAN) on the antagonism of sethoxydim, haloxyfop, or the methyl ester of haloxyfop activity by bentazon was evaluated in greenhouse and field trials on yellow and giant foxtail. Including UAN in the spray solution in the absence of bentazon did not enhance the activity of any of the three grass herbicides. However, adding UAN to sethoxydim or haloxyfop in the presence of bentazon decreased the bentazon antagonism of grass activity. Conversely, UAN increased bentazon antagonism of the activity of haloxyfop methyl ester. The differential effect of UAN was not linked to effects on spray solution pH.

The effect of irrigation and weed control by various mechanical and chemical means was evaluated over a 4-yr period on newly planted pecans [Carya illinoensis (Wang.) K. Koch]. Weed control effect on tree growth became evident in the second growing season, and became more pronounced after the third and fourth season. After the fourth season, the maximum increase in tree diameter (384%) was achieved with comprehensive, herbicide-based weed control. Mowing provided a 224% increase, which was equivalent to the untreated (229%). Disking and selective chemical control of grass weeds resulted in 339 and 292% increase, respectively. Cumulative increase in tree diameter was 316 and 271% with and without irrigation, respectively.

Growth and development of three wild-proso millet biotypes (Cambridge, LeSueur, and Morris) and one cultivated proso millet (‘Crown’) were compared under noncompetitive conditions in field studies. LeSueur and Cambridge were taller than Crown and Morris at maturity. All wild types had greater leaf area and dry weight at maturity than Crown. Crown headed earlier than the wild types, and among the wild types, Cambridge was slowest to mature. Seed production was 1.4 to 2 times greater for wild than cultivated proso millet, and averaged 48 000, 69 000, 83 000, and 94 000 seeds/plant for Crown, Cambridge, LeSueur, and Morris, respectively. Mean seed weight was 4.0 and 3.8 mg/seed for LeSueur and Morris, respectively, and 5.8 and 5.9 mg/seed for Crown and Cambridge, respectively. LeSueur and Morris seed shattered readily, but Cambridge seed shattered relatively little. Seed dormancy was greater in LeSueur and Morris than in Cambridge. Based on reproductive potential, shattering, and dormancy characteristics, LeSueur and Morris appeared to have more weedy characteristics than Cambridge.

Herbicide-resistant populations have evolved only in monoculture and/or monoherbicide conditions at predictable rates for each compound and weed. No populations of triazine-resistant weeds have appeared in corn where rotations of crops and herbicides or herbicide mixtures were used. This is due to the greatly reduced competitive fitness of the resistant individuals, which could be expressed only during rotational cycles, and also to the greater sensitivity of resistant individuals to other herbicides, pests, and control practices (“negative cross-resistance”). The model presented here describes how an understanding of all of these factors can provide strategies to decrease the frequency of the resistant individuals during rotation. Rotations or mixtures may not delay the rate of appearance of resistance to inhibitors of acetolactate synthase (ALS), where the fitness of resistant biotypes is claimed to be near normal. The best way to delay resistance to ALS inhibitors is to use those compounds with less persistence so that the selection pressure will be lowered. Too little is known about the frequency of resistance to other herbicides with target-site resistance–to dinitroanilines, to acetyl CoA carboxylase inhibitors, or to those situations where a single enzyme system confers resistance to a broad spectrum of seemingly unrelated herbicides.