Field studies were conducted in 1994 and 1995 at Dekalb and Urbana, IL, to evaluate preemergence broadleaf weed control and crop tolerance in imidazolinone resistant (IR) and susceptible (non-IR) corn using atrazine, imazethapyr, AC 263,222, CGA-152005, MON 12000 with and without MON 13900 (a safener), and flumetsulam + clopyralid. When sufficient rainfall occurred within 28 d of application to insure herbicide absorption, the IR corn variety was more tolerant than the susceptible variety to imazethapyr, AC 263,222, CGA-152005 at 40 and 80 g/ha, and MON 12000 with and without MON 13900. Overall crop tolerance of IR corn was equal to that of corn treated with atrazine for all herbicide treatments except CGA-152005, which injured IR corn. Control of velvetleaf, common lambsquarters, Pennsylvania smartweed, tall morningglory, and jimsonweed for all herbicide treatments was equal or superior to that of atrazine at 1.7 kg/ha. However, control of common cocklebur was significantly greater with atrazine compared to imazethapyr and the low rate of CGA-152005.

Field experiments showed that quackgrass could be controlled by glyphosate applied preharvest to barley. In separate experiments, field mint was controlled by glyphosate applied pre- and postharvest to barley and wheat. Suppression of quackgrass regrowth one year following treatment was 90% or greater when glyphosate was applied at 0.9 kg ai/ha alone or at 0.45 kg ai/ha in combination with both tallow amine ethoxylate at 0.5% of total spray volume and ammonium sulfate at 3 kg/ha. Field mint shoot counts were reduced 84% the year following treatment when glyphosate was applied at 0.25 kg ai/ha and 95% when applied at 1.0 kg ai/ha. Suppression of field mint shoot regrowth the following year was 84% when glyphosate was applied postharvest and 93% when applied preharvest. Addition of tallow amine ethoxylate did not improve field mint control. Glyphosate applied preharvest to barley and wheat at less than 30% grain moisture content had no effect on grain yield.

During the establishment of alfalfa, POST applications of imazethapyr at 50 g/ha, following PPI applications of trifluralin at 1,100 g/ha, consistently provided ≥ 80% control of wild oat, volunteer canola, and shepherd's-purse with no crop injury. POST applications of the same rate of imazethapyr without PPI applications of trifluralin provided similar volunteer canola and shepherd's-purse control, but ≥ 80% wild oat control was achieved only 25% of the time. Imazethapyr, alone or following trifluralin, did not control common groundsel. Broadleaf weed control from imazethapyr at 50 g/ha tank mixed with fluazifop-P at 125 g/ha or sethoxydim at 200 g/ha was not reduced compared to imazethapyr alone. However, the probability of controlling wild oat by ≥ 90% was reduced from 83% to 0 when the same rate of imazethapyr was tank mixed with fluazifop-P, compared to fluazifop-P alone. Tank mixtures of imazethapyr at 50 g/ha and sethoxydim controlled wild oat only slightly less than sethoxydim alone. First cut and total herbage yields, obtained 1 yr after alfalfa establishment, increased where treatments effectively controlled broadleaf weeds and wild oat, compared to check plots. Second cut herbage and seed yields, also obtained 1 yr after alfalfa establishment, were not affected by any of the treatments.

Field studies were conducted in 1994 and 1995 to determine the contribution of PRE applications of alachlor, metolachlor, acetochlor, SAN 582H, or pendimethalin on woolly cupgrass and wild-proso millet control when followed by POST nicosulfuron at 0, 0.018, 0.027, or 0.036 kg ai/ha. Sequential treatments controlled woolly cupgrass and wild-proso millet greater than single applications of PRE herbicides, which when applied alone resulted in the least wild-proso millet control and lowest corn grain yield. Lack of complete woolly cupgrass control with POST nicosulfuron alone resulted in corn grain yield that was less than with sequential treatments but was equal to PRE treatments. Wild-proso millet control with nicosulfuron at 0.027 kg/ha resulted in corn grain yield that was less than with sequential treatments, but greater than with all PRE treatments except for SAN 582H. All PRE herbicides, regardless of early season performance, when followed by nicosulfuron resulted in woolly cupgrass and wild-proso millet control that was similar. Woolly cupgrass seed production compared to the nontreated check was reduced 98% with acetochlor followed by nicosulfuron. Sequential treatments provided the most consistent woolly cupgrass and wild-proso millet control, the highest corn grain yield, and the greatest reduction in woolly cupgrass and wild-proso millet seed production.

Field experiments were conducted from 1993 to 1995 to compare weed control by the isopropylamine salt of glyphosate at 0.21, 0.42, 0.63, and 0.84 kg ae/ha applied at three stages of weed growth. Weed control by glyphosate applied at these rates alone or with ammonium sulfate at 2.8 kg/ha was also evaluated. In other experiments, potential interactions between glyphosate and acifluorfen, chlorimuron, and 2,4-DB were evaluated. Velvetleaf, prickly sida, sicklepod, pitted morningglory, entireleaf morningglory, palmleaf morningglory, and hemp sesbania were controlled more easily when weeds had one to three leaves compared with control when weeds had four or more leaves. Glyphosate controlled redroot pigweed, velvetleaf, prickly sida, sicklepod, and barnyardgrass more effectively than pitted morningglory, entireleaf morningglory, palmleaf morningglory, or hemp sesbania. Increasing the rate of glyphosate increased control, especially when glyphosate was applied to larger weeds. Greater variation in control was noted for pitted morningglory, palmleaf morningglory, prickly sida, and velvetleaf than for redroot pigweed, sicklepod, entireleaf morningglory, or hemp sesbania. Ammonium sulfate increased prickly sida and entireleaf morningglory control but did not influence sicklepod, hemp sesbania, or barnyardgrass control. Acifluorfen applied 3 d before glyphosate or in a mixture with glyphosate reduced barnyardgrass control compared with glyphosate applied alone. Chlorimuron did not reduce efficacy. Mixtures of glyphosate and 2,4-DB controlled sicklepod, entireleaf morningglory, and barnyardgrass similar to glyphosate alone.

Imidazolinone-resistant and -tolerant corn hybrids give growers a new weed control option. Field experiments were conducted in 1993 and 1994 to evaluate imazethapyr for controlling weeds in no-till corn planted into winter wheat stubble in a winter wheat–ecofallow corn rotation. Imidazolinone-tolerant (IT) and imidazolinone-resistant (IR) corn were protected genetically from injury by imazethapyr that was applied preplant, preemergence, or postemergence to ‘Pioneer Brand 3417,’ ‘Pioneer Brand 3417 IR,’ ‘ICI Seeds 8532,’ and ‘ICI Seeds 8532 IT’ hybrids. No difference in corn injury occurred between IR and IT corn. Imazethapyr applied to resistant or tolerant corn hybrids could be used to control weeds in ecofallow corn. Imazethapyr at 35 or 70 g ai/ha controlled triazine-resistant kochia better than the standard herbicide treatment of metolachlor plus cyanazine.

Eight cultivation programs with several equipment combinations were compared with each other and with an atrazine plus pendimethalin herbicide program with and without supplemental cultivation from 1992 to 1994. In two of the three years, cultivation with a rotary hoe or tine weeder reduced weed seedling density by 39 to 74%. Tine weeding was more effective than rotary hoeing in 1992. Rotary hoeing or tine weeding reduced corn populations by an average of 6%. Weed control by different types of inter-row cultivators varied little, except that an in-row cultivator provided better control than a rolling cultivator in two years and better control than a shovel cultivator in one year. Weeds establishing from seeds were better controlled by herbicides in all three years, but weeds establishing from roots, rhizomes, and tubers were controlled as well or better by cultivation. Weed control was sometimes better using herbicides plus cultivation than with herbicides alone, but the combination damaged the crop in two of the three years. Cost of mechanical treatments which combined inter-row cultivation with rotary hoeing or tine weeding differed from that for the herbicide treatment by less than 2%. Yields of the best mechanical treatment and the herbicide treatment were nearly equal in all years, but the best mechanical regime varied between years. Consequently, mean net return was moderately higher for the herbicide treatment.

A field study was conducted in 1994 and 1995 to determine the effect of MON 12037, linuron, and ametryn on purple nutsedge density in field corn in a 2-yr rotation with cotton. Each corn treatment included cultivation and noncultivation. A standard treatment for purple nutsedge control was applied to all cotton plots in the second season of the corn–cotton rotation. Purple nutsedge shoot densities were reduced in cultivated MON 12037, noncultivated MON 12037, and cultivated ametryn in the corn–corn and corn–cotton rotations. Purple nutsedge tuber population was reduced in the corn–corn rotation by MON 12037 with and without cultivation, cultivated linuron, and cultivated ametryn treatments relative to the noncultivated check. In the corn–cotton rotation, purple nutsedge tuber population was reduced only by the cultivated MON 12037 and cultivated ametryn treatments relative to the noncultivated check.

Competition between bluebunch wheatgrass and spotted knapweed was quantified using three addition series experiments in an environmental chamber. Using two density matrices, spotted knapweed plants were thinned by either 45 or 90% of their original densities 40 d after emergence. Another matrix of density combinations was not thinned. All plant material was harvested 90 d after thinning. Spotted knapweed was about four times more competitive than bluebunch wheatgrass. Reducing spotted knapweed by 45% did not alter the competitive relationship between the two species. We believe remaining knapweed individuals captured the majority of the newly available resources. Ninety percent reduction was necessary to shift the competitive relationship in favor of bluebunch wheatgrass. Successful integrated spotted knapweed management must exploit key mechanisms and processes directing plant community dynamics, in conjunction with weed density reduction, if communities are to be shifted toward those that are desired.

Over two years, acetolactate synthase (ALS)-inhibiting herbicides and herbicides with different mechanisms of action were tested individually and in combination for control of ALS-resistant Palmer amaranth in soybean. As expected, ALS-inhibiting herbicides did not control the resistant Palmer amaranth. Lactofen at 210 g/ha and acifluorfen at 560 g/ha gave the best postemergence control. Tank mixes of lactofen with either imazethapyr or chlorimuron plus thifensulfuron did not significantly increase control over lactofen alone. Sequential treatment with a soil-applied herbicide, either SAN 582 or pendimethalin, followed by lactofen postemergence, controlled weeds best (greater than 85%). The extent of cross-resistance of Palmer amaranth to ALS-inhibiting herbicides was determined in the greenhouse. Sixteen POST ALS-inhibiting herbicides were sprayed at ½, 1, 2, 4, and 8X field use rates on resistant and susceptible biotypes. All ALS-inhibiting herbicides at all rates controlled the susceptible biotype. The resistant biotype was cross-resistant to all ALS-inhibiting herbicides. Metsulfuron and imazapyr at 8X rates were the only treatments that provided 80% or greater control of the ALS-resistant Palmer amaranth. Chlorsulfuron and tribenuron were the only other herbicides that gave partial control at the highest rate. Control with these four herbicides decreased as the rate was decreased. Experimental results suggest that cultural practices and herbicides with different mechanisms of action will be required to manage this ALS-resistant Palmer amaranth biotype.

Field studies were conducted to evaluate preemergence weed control in irrigated potatoes with rimsulfuron plus metribuzin or three-way mixtures of rimsulfuron plus metribuzin plus EPTC, metolachlor, or pendimethalin. Redroot pigweed control was ≥ 96% with rimsulfuron plus metribuzin at 18 g ai/ha + 140 g/ha, and common lambsquarters control was ≥ 97% with rimsulfuron plus metribuzin at 26 g/ha + 210 g/ha. Hairy nightshade control with rimsulfuron plus metribuzin ranged from 58 to 99%, and green foxtail control ranged from 50 to 98%. EPTC, metolachlor, or pendimethalin rates required for acceptable green foxtail and hairy nightshade control in combination with a given rate of rimsulfuron plus metribuzin varied with environmental conditions. Potato tuber yield generally improved with rimsulfuron plus metribuzin application.

The effects of herbicide–adjuvant combinations and times of application on weed density and corn yield were determined in a 3-yr field study. At a given rate and time of application of bentazon/atrazine, Assist and Merge were equally effective adjuvants. Assist would be the adjuvant of choice for bentazon/atrazine rather than Merge because of its currently lower cost. Weed density depended on the rate and time of application of bentazon/atrazine. In general, bentazon/atrazine applied at early stages (7 d after emergence) of the development of corn seedlings or at high or intermediate rate (1.6 or 0.8 kg ai/ha) maintained low weed densities, with a relatively small range of variation over years. Delay in time (14 or 21 d after emergence) or reduction in herbicide rate (0.4 kg ai/ha) increased the risk of high weed pressure, although it was not always associated with yield loss. The best time for the application of postemergence herbicide appears to be a few days prior to the onset of the critical period of weed control.

Experiments were conducted to compare barnyardgrass control and rice injury and yield with emulsifiable concentrate and dry flowable formulations of propanil as single or repeat applications with crop oil concentrate, methylated seed oil, a blend of organosilicone surfactant and methylated seed oil or conventional nonionic surfactant, and organosilicone surfactant. Two applications of propanil were more effective in controlling barnyardgrass than a single application. The emulsifiable concentrate formulation of propanil controlled barnyardgrass more effectively than the dry flowable formulation in some but not all experiments. Differences in barnyardgrass control with propanil as influenced by adjuvants were minor and inconsistent. The most consistent barnyardgrass control and the highest rice yields were obtained with repeat applications of the emulsifiable concentrate formulation of propanil.

Component yields and forage quality were evaluated in established stands of alfalfa seeded alone and in binary mixtures with smooth bromegrass, orchardgrass, or timothy. Forage yields and quality were determined at three annual harvests in the third and fourth years after seeding. Weed yields in alfalfa seeded alone were significantly greater than those in the alfalfa–orchardgrass mixture. Average weed content was 24, 17, 2, and 15% for the alfalfa seeded alone, alfalfa–bromegrass, alfalfa–orchardgrass, and alfalfa–timothy, respectively. Seeding alfalfa with orchardgrass increased first harvest and annual forage yields while reducing alfalfa yield. Yields of orchardgrass often exceeded those of bromegrass or timothy. In the first harvest each year, crude protein was reduced and acid detergent fiber and neutral detergent fiber increased by including orchardgrass. Neutral detergent fiber was greater in alfalfa–orchardgrass than in the alfalfa seeded alone at most harvests. There were no consistent differences among the alfalfa seeded alone and the other mixtures in yield or quality.

In the summer of 1995, green foxtail seed samples were collected from 17 fields in southwestern Manitoba where trifluralin resistance was first identified in 1988. Since 1988, six of these fields had not been treated with dinitroaniline herbicides (DNAHs), four had been treated once, and seven had been treated two or more times. The proportions of resistant (R) and susceptible (S) seeds in each sample were determined using a petri-dish seed bioassay. Among the samples collected from fields that had not been treated between 1988 and 1995, two contained > 99% R seeds. The others ranged from 40 to 83%. Among the four samples collected from fields that were treated once, two contained > 99% R seeds and the other two contained 59 and 67% R seeds. Five of the seven samples collected from fields treated two or more times contained > 97% R seeds and the other two approximately 89 and 68% resistant seeds. There was no statistical correlation between the proportion of R seeds and the frequency of DNAH use since 1988. The proportion of R seeds present in 1988 and 1995 could not be compared because of the way in which fields were treated prior to sampling in the 2 yr; however, the results clearly indicate that trifluralin resistance can persist in green foxtail populations for at least 7 yr. By inference, the results indicate that trifluralin-resistant green foxtail plants are almost, if not equally, as fit as susceptible plants. Consequently, once resistance is present in a green foxtail population, DNAHs cannot be used for its control, even in the absence of additional selection pressure, for at least 7 yr and probably much longer.

Proso millet is a short-season summer annual grass that is well adapted to the central Great Plains. Proso millet is commonly planted as a summer crop when winter wheat stands are lost due to adverse conditions. Sulfonylurea herbicides labeled for use in winter wheat prohibit planting proso millet for intervals up to 10 mo following application. A series of greenhouse and field studies determined proso millet tolerance to CGA-152005, metsulfuron, and triasulfuron soil residue. In the greenhouse, proso millet was not affected by soil-applied CGA-152005 at doses up to 160 g ai/ha, while metsulfuron and triasulfuron doses of 4 and 15 g ai/ha, respectively, inhibited proso millet biomass accumulation. In the field, metsulfuron and triasulfuron caused early season stunting and chlorosis at doses two to four times those recommended; however, grain yields were not affected. Organic matter and clay content were highly correlated with proso millet growth response to the herbicides under greenhouse conditions, but in the field, soil pH may have influenced herbicide bioavailability.

Shredded newspaper (2.5, 7.6, 12.7, and 17.8 cm depth), chopped newspaper (2.5 and 7.6 cm), wheat straw (15.2 cm), black plastic, and plastic landscape fabric were evaluated during 1993 and 1994 in West Virginia for their effect on soil temperature, soil moisture, weed control, and yield in tomato. Shredded newspaper and wheat straw applied at 0, 2, 4, or 6 weeks after transplanting (WAT) and napropamide (2.1 kg ai/ha) plus metribuzin (0.28 kg ai/ha) applied at transplanting were evaluated during 1992 and 1993 in North Carolina for effect on weed control and tomato yield. Results from West Virginia indicated that shredded (7.6 cm) and chopped (7.6 cm) newspaper conserved moisture similar to higher application rates of the shredded material. Higher newspaper mulching rates reduced soil temperature compared to black plastic and bare ground. Chopped newspaper controlled weeds more consistently than other treatments. At least 7.6 cm of chopped newspaper mulch was required to control weeds at least 90%. Wheat straw was not as effective in controlling weeds as 7.6 cm or greater of newspaper mulch. Chopped newspaper provided higher tomato yields than shredded newspaper applied at the same rates. Mulches applied at 0, 2, or 4 WAT resulted in weed control similar to the chemical treatment. In North Carolina, mulches applied 2 or 4 WAT resulted in tomato yields similar to the chemical treatment. Shredded and chopped newspaper have potential as a mulching material but may vary in effect in different environments and vegetable crops.

Two experiments were conducted over a two-year period for large crabgrass control in centipedegrass. The experiments were: (a) sequential applications of PRE and POST herbicides, and (b) POST herbicides applied as sequential or tank-mixed applications. Large crabgrass control was greater than 80% throughout the spring and summer when dithiopyr was applied PRE at 0.14 kg/ha late in February or when tank mixed at the same rate with sethoxydim at 0.11 kg/ha and applied POST in late May. Control was similar from a sequential program of dithiopyr applied PRE at 0.14 kg/ha in February and followed either by dithiopyr at 0.14 kg/ha or sethoxydim at 0.11 kg/ha as POST in early to mid-summer. Dithiopyr at 0.28 kg/ha as a POST application in late May did not control large crabgrass consistently. Control was > 80% for 6 to 8 wk when the POST dithiopyr treatment was repeated at the same rate 1 mo later. Sethoxydim applied POST controlled large crabgrass greater than 80% throughout the spring and summer when applied once at 0.22 kg/ha or twice at 0.11 kg/ha in each of two applications. Dithiopyr applied alone or with sethoxydim as sequential or tank-mixed treatments caused slight to moderate centipedegrass injury but was commercially acceptable (< 30%). However, injury from tank-mix herbicides was similar to either applied alone. Quinclorac severely injured (> 30%) centipedegrass, and 8 to 10 wk were required for recovery.

Quackgrass is a serious weed problem in the northern United States and southern Canada. Field experiments were conducted in 1990 and 1991 to examine the effect of herbicide rate, application timing, quackgrass growth stage, and cultivation on quackgrass control with nicosulfuron and primisulfuron in corn. Nicosulfuron (35 g ai/ha) and primisulfuron (40 g ai/ha) each provided 84% or greater control of two-leaf plants and 93% or greater control of four-leaf quackgrass plants. Sequential applications of both nicosulfuron and primisulfuron provided greater season-long control than single applications of the same rate on two-leaf but not four-leaf quackgrass. Cultivation 10 d after nicosulfuron or primisulfuron application often increased early-season control, but by corn harvest few differences were observed. Postemergence (POST) applications of nicosulfuron and primisulfuron provided at least 88% control, compared with 66 to 86% from an early preplant application of glyphosate. Corn yields were similar regardless of the quackgrass control program.

Cyanobacteria (blue-green algae) are undesirable in ponds used to raise fish for human food. Management of cyanobacterial communities in aquaculture ponds has been hindered by the small number of herbicides approved for use in food-fish ponds and by the lack of selectivity toward cyanobacteria for those herbicides that are approved for that use. To facilitate development of additional management options, a rapid bioassay utilizing 96-well cell culture plates was developed for screening herbicides and other phytotoxins for selective toxicity toward cyanobacteria. Oscillatoria cf. chalybea and Selenastrum capricornutum were chosen as representatives of cyanobacteria (Cyanophyta) and green algae (Chlorophyta), respectively. In the bioassay, wells of the cell culture plates were inoculated with cyanobacterial or unialgal culture. One of five herbicides (atrazine, diuron, endothall, fluridone, or paraquat) was then added to the wells at various concentrations, and absorbance (650 nm) was measured at 24-h intervals. Growth of treated cultures relative to untreated cultures was used to determine relative toxicity of the herbicide to the two organisms. Paraquat was the most selective of the herbicides tested and was over 10-fold more toxic to O. cf. chalybea than to S. capricornutum. This method was demonstrated to be a rapid, effective, and highly reproducible bioassay to identify selective algicides.