A three-year experiment was conducted near Scottsbluff, NE, to evaluate the selectivity of POST-applied imazethapyr for weed control in alfalfa, birdsfoot trefoil, cicer milkvetch, red clover, sainfoin, and yellow sweetclover. Imazethapyr injured all legumes 15 DAT. Imazethapyr also reduced the height of birdsfoot trefoil, cicer milkvetch, red clover, and yellow sweeclover 28 DAT. Legume first cutting yields were not reduced by imazethapyr and with alfalfa, birdsfoot trefoil, cicer milkvetch, and sainfoin, first cutting yields were increased by imazethapyr. Imazethapyr reduced weed biomass in all legume seedings. Weed biomass in new seedings of alfalfa was reduced more than that of the other legumes.

A two-year herbicide residue study was conducted at a commercial nursery to determine the presence of herbicide residues in pond water and sediment following normal production cycles. The runoff water from irrigation and rainfall was contained on the nursery and reused for irrigation. The nursery site was approximately 60 ha with two interconnected ponds, one for containment of runoff water and one serving as a source of irrigation water. The most commonly used herbicides were oxyfluorfen plus pendimethalin (granular formulation-Ornamental Herbicide 2); approximately twice as much herbicide was applied during the second year of the study. Herbicide levels found in pond water and sediment were approximately two-fold greater during the second year corresponding to the increase in active ingredients applied. The highest concentration of oxyfluorfen found in water and sediment was 0.04 μg/ml and 4.0 μg/g, respectively. The highest concentration of pendimethalin found in water and sediment was 0.008 μg/ml and 14.3 μg/g, respectively. In irrigation water samples, the highest concentration of oxyfluorfen and pendimethalin detected was 0.005 μg/ml and 0.002 μg/ml, respectively. The herbicides did not accumulate in water or sediment over the two-year period. No landscape plant phytotoxicity problems are likely at the herbicide levels detected in irrigation water samples.

Influences of a hairy vetch cover crop and residual herbicides were examined in field corn in 1991 and 1992. Hairy vetch was seeded in mid-August and killed the following May with tillage, mowing, or glyphosate plus 2,4-D (no-till). These cover crop management systems were compared with a no-cover treatment. Residual herbicides including atrazine plus metolachlor applied PRE at three rates and nicosulfuron plus thifensulfuron applied POST at a single rate were compared within cover crop management systems. All cover crop management systems effectively controlled hairy vetch except mowing in 1992. The corn population was reduced in mow treatments containing uncontrolled vetch. Hairy vetch mulch suppressed some weeds in the no-till treatments in 1991, but more annual grass was noted late in the season with no-till into hairy vetch than with the no-cover treatments in 1992. Residual herbicide performance was similar across cover crop management systems, except for fall panicum control which decreased in some no-till systems. Unlike soil-applied herbicides, performance of POST herbicides was unaffected by cover crop management systems.

Six corn hybrids were studied in field experiments in 1989 and 1990 to identify hybrids and corn plant characteristics that may be valuable in systems using reduced levels of weed control. Four weed control treatments (no-control, cultivation-only, band herbicide-plus-cultivation, and broadcast herbicide) represented main plots and maize hybrids were subplots. Medium-season hybrids with differences in height, early-season vigor, and leafiness were used. There were significant differences among hybrids in leaf angle, leaf width, leaf number, plant height, leaf area index (LAI), plant dry matter (DM) and grain and stover yields. Lower yielding hybrids had grain yields that ranged from 87 to 91% of the highest yielding hybrid. Aboveground corn characteristics were not correlated with weed numbers, weed cover, or weed biomass. A significant interaction between hybrid and weed control for grain yields was observed in 1989, suggesting that some hybrids are more competitive when weed pressure is high.

Manure applied to cropland may serve as a source of weed introduction and dispersal. In 36 manure samples from 20 farms, apparently-viable seeds from 13 grasses and 35 broadleaf plants were found. Common lambsquarters was on more than half the farms, yellow foxtail on 35%, common chickweed and dandelion on 30%, and wild mustard, redroot pigweed, and barnyardgrass on 25%. Four farms had manure with no seeds at all; the remainder averaged 75 100 per 1000 kg manure. Twelve milking-cow and heifer barns on six farms with large velvetleaf infestations also were sampled with an average of 133 000 seeds per 1000 kg manure. Only one barn contained velvetleaf seeds. When compared to soil seedbank numbers, manure is not an important seed source for New York farms. However, problems may arise with imported feeds heavily infested with weed seed or which contain even small numbers of noxious weeds.

Field experiments were conducted in 1992 and 1993 in southern Manitoba to investigate the tolerance of buckwheat, canola, field pea, lentil, and sunflower to simulated drift of 2:1 mixtures of thifensulfuron:tribenuron. Thifensulfuron:tribenuron at 0.23 to 3.6 g ai/ha was highly phytotoxic to all crops examined. Leaf area index 2 and 4 wk after treatment, seed yield, and seed weight were reduced for all crops at the rates examined. Predicted yield losses at 0.23 g/ha, averaged over two years, were 16% for sunflower, 20% for buckwheat, 22% for canola, 26% for lentil, and 37% for field pea. For each of the five crops, there was a strong linear relationship between percentage visual injury 2 wk after exposure and final yield. Simulated thifensulfuron:tribenuron drift reduced seed oil content of canola and sunflower. In one year, canola, field pea and lentil seed germination was reduced following exposure to simulated drift.

Greenhouse experiments were established to investigate the effects of tank-mixing glyphosate, paraquat, or glufosinate with metribuzin plus chlorimuron, imazaquin, or metribuzin on entireleaf morningglory, sicklepod, and johnsongrass control. Antagonism was the most frequent interaction, and usually occurred when the lower rates of non-selective foliar-active herbicides were used in tank mixtures with selective soil-active herbicides. Antagonism occurred on all species when 180 g ai/ha paraquat was tank-mixed with 90 g ai/ha metribuzin plus 15 g ai/ha chlorimuron. When the rates of non-selective herbicide were increased, antagonism was usually overcome. Antagonism also occurred on entireleaf morningglory control when 210 g ai/ha glyphosate was tank-mixed with 90 g/ha metribuzin plus 15 g/ha chlorimuron or 36 g ai/ha imazaquin. When lower rates of paraquat or glufosinate were tank-mixed with 210 g/ha metribuzin, antagonism also occurred. Less antagonism was noted with glufosinate.

Field experiments were conducted in 1988, 1989, and 1990 in Greece to investigate the combination of cultural and chemical methods for weed control in soybean. The selectivity of herbicides or the combination of herbicides used was dependent on application rate. Imazaquin applied PRE was selective on soybean even at 0.18 kg/ha, whereas imazethapyr was safer on soybean than imazaquin at 0.22 kg/ha. Metribuzin at 0.37 kg/ha tank-mixed with alachlor, metolachlor, imazaquin, and SAN 582H were not phytotoxic at their recommended rates. All herbicides satisfactorily controlled the weeds in the experiments. The early-season period threshold level for a mixed population of Amaranthus spp. occurred 3.0 to 3.5 wk after weed emergence. Density of Amaranthus spp. of about 270 to 470 plants/m2 caused soybean yield reduction equal to 11 to 35%, respectively. The treatments did not influence the oil content of the soybean seed but in one experiment the protein content was influenced slightly by some of the treatments. The results emphasize the importance of using chemical, cultural, and competition threshold levels of cultural practices for an integrated approach for weed control in soybean.

Field experiments were conducted at Vegreville, Alberta in 1984, 1985, 1986, and 1988 to determine the effects of green foxtail and pale smartweed on yield of wheat, barley, and canola. There was considerable variation among years in the response of crop yield to both weeds and in the relationship between weed dry weight and weed density. Mostly relationships between crop yield and either weed density or dry weight were poor, suggesting that the weeds competed weakly with the crops. Thus density or dry weight may be poor predictors of crop yield losses due to green foxtail or pale smartweed. Where the crops emerged ahead of these weeds, and where soil moisture was not a limiting factor, crop yield losses were minimal and control with herbicides probably uneconomical. In some instances, growth and development of the weeds was suppressed by the crops to the extent that little or no weed dry matter was present at crop maturity. This was most evident with barley, and where the crops emerged ahead of the weeds.

Mean Idaho suction trap collections of Myzus lythri, an aphid which infests purple loosestrife, reflected weed distribution and were used successfully to predict undocumented weed populations. To our knowledge, this is the first time insect survey records have been used to document the range of an introduced weed.

Canada thistle control in no-tillage corn was studied in Western Maryland from 1990 to 1992. The best treatment at planting for controlling Canada thistle was a tank mixture of 2240 g/ha glyphosate plus 560 g/ha 2,4-D. Clopyralid applied at 210 and 280 g/ha alone and 106 g/ha tank mixed with 560 g/ha 2,4-D effectively controlled Canada thistle in no-tillage corn (85 to 96%). Nicosulfuron applied at 35 g/ha and 20 or 40 g/ha primisulfuron suppressed Canada thistle (59 to 75%). Tank mixtures of nicosulfuron or primisulfuron with 2,4-D or dicamba generally increased Canada thistle control (75 to 87%) compared with control by these herbicides applied alone. Treatments that controlled Canada thistle generally increased corn yields compared with the weedy controls. Yield increases were most dramatic in 1991 when precipitation was low.

Normal field applications of both terbufos and nicosulfuron can cause decreased corn tolerance to nicosulfuron because terbufos inhibits nicosulfuron metabolism. In greenhouse studies, applications of 2,4-D at 0.07, 0.14, and 0.28 kg/ha with nicosulfuron reduced terbufos induced foliar injury by more than 50%. Plants treated with nicosulfuron, terbufos, and 2,4-D had significantly higher shoot dry weights than those treated with nicosulfuron and terbufos. When 2,4-D was applied sooner than 24 h before nicosulfuron application or later than 24 h after nicosulfuron application, it did not prevent the interaction. Uptake of 14C-nicosulfuron was increased by the presence of terbufos but was unaffected by 2,4-D. Translocation of 14C-nicosulfuron was low with the highest amounts occurring in the nicosulfuron plus terbufos treatment. Terbufos reduced metabolism of nicosulfuron at 4 and 24 h after treatment compared to plants without terbufos. Metabolism of nicosulfuron in plants treated with terbufos was increased by 2,4-D to a rate equivalent to that in plants not treated with terbufos.

In field experiments at three locations, wheat row spacing, seeding rate, and herbicide treatment affected Italian ryegrass control, wheat yield, dockage in the grain, and net returns. Diclofop at 560 or 840 g ai/ha controlled Italian ryegrass better than chlorsulfuron at 18 or 26 g ai/ha. Net returns were increased at all locations by diclofop POST at either rate and at two locations by chlorsulfuron PRE at either rate. Although increasing the wheat seeding rate reduced dockage at two of three locations, net returns were maximized by herbicide application alone without increased seeding rates or reduced row spacing.

Fringed sagebrush is a native, drought-resistant, increaser species in Saskatchewan pastures and rangelands. Conventional control by cultivating and reseeding is neither highly effective nor sustainable. Two field experiments were conducted in 1988 and 1989 through 1991 to develop a management plan for fringed sagebrush. Comparison of fringed sagebrush control using various applications of clopyralid, dicamba, dichlorprop, fluroxypyr, picloram, triclopyr and 2,4-D, showed control was sufficient using the low-cost herbicide 2,4-D isooctyl ester at 1.5 kg ai/ha. Total yield of crested wheatgrass plus smooth brome was compared and the plots subjected to two applications of 2,4-D yielded the greatest accumulation of grass over the length of the experiments. The economic threshold of fringed sagebrush is the density at which yield is reduced by approximately 290 kg/ha; above this density it is economically viable to use 2,4-D at 1.5 kg/ha to control fringed sagebrush. Grazing practices can aid greatly in management of fringed sagebrush by allowing the forage to out-compete fringed sagebrush.

Field experiments were conducted near Scottsbluff, NE, to evaluate the selectivity of sequential POST applications of ethofumesate and triflusulfuron for weed control in sugarbeet. When applied alone ethofumesate and triflusulfuron reduced common lambsquarters, hairy nightshade, and redroot pigweed density while triflusulfuron also reduced kochia density compared with an untreated control. Combining ethofumesate with desmedipham plus phenmedipham increased sugarbeet injury compared with desmedipham plus phenmedipham. Combining triflusulfuron with desmedipham plus phenmedipham increased kochia control but did not increase sugarbeet injury over that obtained with desmedipham plus phenmedipham. Weed control and sugarbeet injury from herbicides generally were unaffected by application in 97 or 182 L/ha of water carrier. However, herbicide control of hairy nightshade in 1993 was greater with 97 than 182 L/ha of water carrier.

Flower production and pollen dispersal patterns of diclofop-methyl-susceptible and -resistant Italian ryegrass were examined to determine whether evolution of herbicide resistance could be controlled by cross-fertilization between biotypes. The susceptible and resistant ryegrass biotypes differed in timing and abundance of both ovule production and pollen release such that pollen from the susceptible plants had a much greater chance of fertilizing the resistant plant population than vice versa. Susceptible ryegrass, growing with or without wheat competition, produced more than 60% of its seeds before any pollen from resistant plants was released. In contrast, throughout the course of resistant plant seed generation, pollen from susceptible plants composed at least 30% of the total pollen load. These phenological differences, in conjunction with herbicide treatment results for third-generation ryegrass seedlings, suggest that gene flow can be used to reduce development of diclofop-methyl resistance in ryegrass populations within wheat cropping systems. In a wheat field infested with resistant ryegrass, cessation of herbicide application and sowing of a susceptible ryegrass biotype can be expected to reduce the evolution of diclofop-methyl resistance by at least 6% per year.

Seed response to grinding in a hammer mill or extrusion through a pellet die was determined. Less than 1% of spotted knapweed, sulfur cinquefoil, timothy, and alfalfa seeds were intact after grinding to pass through a 1-mm screen. Intact seeds of all tested species except sulfur cinquefoil were able to germinate after grinding. Extruding unground alfalfa seed through a die to make hay pellets decreased viability 60 to 63%. After grinding and extrusion through a grain pellet die, 53% of intact alfalfa seeds in the pellets were viable. These data demonstrate that feed pellets can contain viable weed seeds.

Field experiments were established at Columbus and near South Charleston, OH to determine the effects of giant ragweed population density on soybean yield and to characterize the development of giant ragweed grown in 76-cm soybean rows. An economic threshold was calculated for Ohio using a common treatment for giant ragweed control in soybean. A cost of $41/ha was estimated for a farmer to apply 0.56 kg/ha bentazon plus 0.28 kg/ha fomesafen plus COC (1.25% v/v). Assuming a soybean value of $0.22/kg, the cost of control was equivalent to 5.4 and 7.1% of the soybean yield in 1991 and 1992, respectively, which corresponded to the yield loss caused by 0.08 and 0.03 giant ragweed plants/m2. The competitiveness of giant ragweed can be at least partly attributed to its ability to initiate and maintain axillary leaves and branches within the shaded confines of the soybean canopy.

In separate studies in west-central Kansas, a prepackaged mixture of chlorsulfuron + metsulfuron (5:1 w/w) and triasulfuron alone or in combination with metribuzin as a sequential or tank mix treatment were evaluated two years for downy brome control in winter wheat. Downy brome and wheat responses to each herbicide alone PRE or plus metribuzin POST varied between years. Visually, downy brome biomass was reduced 32 to 79% by chlorsulfuron + metsulfuron at 16 to 26 g ai/ha and 58 to 76% by triasulfuron at 30 g ai/ha applied PRE. Metribuzin POST at 158 g ai/ha following chlorsulfuron + metsulfuron PRE at 26 g/ha and metribuzin POST at 140 g/ha or more following triasulfuron PRE at 30 g/ha reduced downy brome biomass in both years more than PRE treatments alone. Chlorsulfuron + metsulfuron at 11 or 16 g/ha plus metribuzin at 105 or 158 g/ha applied POST affected downy brome less in 1992 than chlorsulfuron + metsulfuron applied PRE at 21 or 26 g/ha. Conversely in 1993, POST treatments except for chlorsulfuron + metsulfuron at 11 or 16 g/ha plus metribuzin at 105 g/ha affected downy brome more than PRE treatments. Triasulfuron plus metribuzin POST in late fall controlled downy brome better than a sequential application in one of two years; late-fall POST was better than early-spring POST applications in both years. Despite some treatments retarding growth, no treatment in either study reduced wheat yield in 1992. However in 1993, chlorsulfuron + metsulfuron PRE at 26 g/ha, chlorsulfuron + metsulfuron at 16 g/ha plus metribuzin at 105 or 158 g/ha applied POST, and metribuzin POST at 315 g/ha reduced wheat yields. Triasulfuron at 30 g/ha plus metribuzin at 140 g/ha or higher applied PRE, sequentially, or POST in late fall and metribuzin applied POST in early spring at 280 g/ha also reduced wheat yields.

Postemergence applications of nicosulfuron and primisulfuron were compared to preplant glyphosate and atrazine plus simazine for quackgrass control in reduced tillage and no-till corn. The level of quackgrass control was reduced by no-till practices. At 6 wk after planting, glyphosate and atrazine plus simazine were most effective in controlling quackgrass. Quackgrass biomass 12 wk after planting indicated that the performance of the herbicides were generally similar, although primisulfuron was less effective in no-till. One year after corn planting, levels of quackgrass control in the tilled plots were the same as or better than in the no-till treatments. Atrazine plus simazine was the most effective herbicide treatment over tillage systems, while primisulfuron was the least effective.