Experiments were conducted to examine the effects of low soil moisture on yellow nutsedge emergence in relation to thermal and chronological time and test the ability of thermal models generated previously to predict emergence under water stress. Unsprouted tubers from California and Arizona were planted in pots, buried at field sites in California and Arizona, and subjected to wet or dry treatments. Pots were monitored weekly to determine date of emergence and number of emerged shoots. The California genotype emerged 47 to 61 days after planting (DAP) in the dry treatment and 33 to 49 DAP in the wet treatment, depending on planting site. The range for the Arizona genotype was 51 to 76 DAP in the dry treatment and 43 to 61 DAP in the wet treatment. Days and degree-day intervals to first emergence differed between irrigation treatments and planting sites but interactions were not significant. All models were accurate in predicting emergence dates for genotypes in the wet treatment at the California site, while emergence in Arizona was underestimated by 9 d. Tubers subjected to the dry treatment needed a higher number of accumulated degree-day units before emergence occurred and had fewer emerged shoots as compared to the wet treatment. Degree-day models generated for yellow nutsedge under optimal conditions lack sufficient robustness to be predictive under water stress conditions.

Eastern black nightshade is a problem weed in soybeans primarily due to interference with harvesting operations and reductions in crop quality. Field and greenhouse studies were conducted in 1990 and 1991 at Harrow, Ontario, to determine whether sublethal rates of imazethapyr and metolachlor could suppress growth and berry production by eastern black nightshade. In the greenhouse, plants treated with metolachlor PRE at 0.8 kg ai ha−1 or imazethapyr POST at 30 g ai ha−1 (1/3 to 1/2 the labelled rates) produced significantly fewer berries in the first flower cluster, with fewer seeds per berry, than untreated plants. When a surfactant was added to the imazethapyr, almost all flower buds in the first cluster were aborted. Treated plants recovered from the damage, however, and did not differ significantly from control plants in dry weight or berry production 110 DAP. In the field, application of 50 g ai ha−1 imazethapyr POST (2/3 of the lowest labelled rate), with or without a surfactant, resulted in delayed flowering, abortion of most of the flower buds in the first cluster, and greatly reduced growth and total berry production between soybean leaf abscision and harvest. Application of metolachlor + metribuzin PPI in the field at 2/3 of the labelled rate (1.6 + 0.25 kg ai ha−1) was not sufficient to reduce nightshade growth and berry production after soybean leaf abscision. Sublethal rates of imazethapyr and metolachlor did not reduce seed viability in either the greenhouse or the field.

Field studies were conducted to determine the influence of annual herbicide treatments plus cultivation on weed populations and corn yields in ridge-till corn during a 3-yr period at Mitchell, NE, and a 7-yr period at North Platte, NE. When the experiment was initiated at North Platte, no weeds were present before corn planting. It took 4 yr before triazine-resistant kochia became a problem before corn planting in plots treated with atrazine, but these were controlled by other operations prior to corn harvest. In the cultivated check, green foxtail densities before harvest increased from 0 in 1985 to 32 plants 100 m−2 in 1991. Annual applications of dicamba plus 2,4-D 10 d early preplant followed by cultivation controlled triazine-resistant kochia and velvetleaf, but common lambsquarters, nightshade species, and green foxtail increased. Volunteer corn was controlled with cultivation. After 3 yr at Mitchell, the annual weed population increased 10-fold in the cultivated check. Thus, corn yields were reduced 64% with two cultivations compared with an annual early preplant application of dicamba plus 2,4-D followed by alachlor plus cyanazine PRE and two cultivations. With two cultivations under low annual weed populations at North Platte, grain yield from the cultivated check treatment was not different from annual treatments of herbicides after 7 yr. Metolachlor plus atrazine occasionally caused a reduction in corn grain yields.

Field trials were conducted from 1991 to 1993 in the northern cotton-producing area of Alabama to evaluate the interaction of various production inputs for pest management and cotton development, maturity, and yield. Two levels of tillage (conventional and no-till), herbicide (1.12 and 2.24 kg ai ha−1 fluometuron, preemergence, with post-directed herbicides), insecticide (0.5 and 0.85 kg ai ha−1 aldicarb, in-furrow), and fungicide (0.9 kg ai ha−1 quintozene plus ethridazole, in-furrow or 0.14 kg ai ha−1 metalaxyl, hopper-box) programs were evaluated. The inputs investigated did not interact significantly to change the overall production strategy. Prickly sida required a higher level of herbicide input than did the entireleaf/ivyleaf morningglory complex. Both species were sufficiently controlled using reduced levels of fluometuron without sacrificing yield; however, cultivation was necessary in conventional tillage treatments to maintain control. A postemergence-directed herbicide treatment was also necessary for weed control, regardless of tillage. Decreasing the levels of each input simultaneously did not interact to affect cotton stand, height, early-season thrips counts, cotton maturity, or yield. Cotton was shorter in no-till than conventional plots.

A bioeconomic weed management model was tested as a decision aid for weed control in corn at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, corn yield, herbicide active ingredient applied, and economic return with model-generated treatments were compared to standard herbicide and mechanical control treatments. Effects of these treatments on weed populations and soybean yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as the standard herbicide treatment. The quantity of herbicide active ingredient applied decreased 27% with the seed bank model and 68% with the seedling model relative to the standard herbicide treatment. However, the frequency of herbicide application was not reduced. In 1 yr, seed bank model treatments did not control weeds as well as the standard herbicide or seedling model treatments. Corn yields reflected differences in weed control. Net economic return to weed control was not increased by using model-generated control recommendations. Weed control treatments the previous year affected weed density in the following soybean crop. In 2 of 3 yr, these differences did not after weed control or soybean yield. Although tactics differed, the bioeconomic model generally resulted in weed control and corn yield similar to the standard herbicide. The model was responsive to differing weed populations, but did not greatly after economic returns under the weed species and densities in this research.

Field research was conducted from 1990 through 1992 to evaluate the effect of the fungal pathogen, Colletotrichum coccodes, on velvetleaf intra- and interspecific (with soybean) competition across a range of monoculture and 1:1 mixture densities. In pure stand, application of this velvetleaf foliar pathogen had little impact on seed yield of the weed. In these plots, velvetleaf intraspecific competition stimulated vertical growth and favored the rapid replacement of diseased leaf tissue that had prematurely senesced. In mixtures, however, C. coccodes inoculation differentially influenced the yield of both species. In two of three years, C. coccodes inoculation reduced velvetleaf seed yields by, an average, 60% compared with yields for control (uninoculated) plants. Velvetleaf suffered greater yield losses from soybean interspecific competition in the presence of C. coccodes, especially at the lower planting densities. The decline in velvetleaf yield was primarily attributed to the stunting effect of the pathogen, which allowed soybean plants to grow above the weed. Consequently, soybean yield losses within inoculated mixture plots were generally lower than for control plots, although significant increases (23%) in soybean yield were recorded only in 1992. The inoculation treatment had relatively little impact on the number of seeds produced per fruit or seed unit weight in both species regardless of whether plants were grown in monocultures or in mixtures. The finding that C. coccodes has only a limited effect on velvetleaf performance in pure stand, while having a significantly greater effect in a competitive environment with a soybean crop, has important ramifications as to the value and accuracy of initial efficacy testing that rates potential biocontrol agents based solely on their effect within pure stands of the target weed.

The interaction of cultivation and photoperiod on the initiation of Canada thistle rosettes, and the effect of growth stage on control of Canada thistle with glyphosate were determined in a series of field experiments. Under the natural photoperiod occurring in the southern Canadian Prairies, rosettes of Canada thistle were initiated by cultivation to remove the shoot growth during the last week of July. Regrowth remained as rosettes without any stem elongation and formed dense clusters of leaves. Application at the “August rosette stage” improved the effectiveness of the glyphosate treatment and resulted in consistent control of Canada thistle with less than half as much herbicide as recommended for control when applied at the bud-stage. The number of shoots of Canada thistle on plots treated with glyphosate was less than on the intensively summer-fallowed check plot. Check plots received 5 cultivations during the summer-fallow season. One year after application of glyphosate at the rosette stage, the reduction in shoot density was 99% compared to the summer-fallowed check. By year three, without glyphosate applications in years two and three, the benefits of applying glyphosate at the rosette stage rather than the bud-stage were very evident. When applied at the bud-stage the shoot density on plots treated with glyphosate at 2.25 kg ha−1 was 24 m−2 compared to only 10 shoots m−2 when applied at the rosette stage, (35% vs 72% control). Glyphosate at 0.9 kg ha−1 or less than half of the rate recommended for application at the bud-stage, applied to Canada thistle in the rosette stage, resulted in consistently fewer shoots 2 and 3 yr after treatment. Physical removal of shoot top growth, simulating an in-crop herbicide treatment, improved the consistency of control. Two years after application of 0.9 kg ha−1 of glyphosate at the rosette stage, the control of Canada thistle was 98% compared to the summer-fallowed checks.

Experiments were conducted from 1992 through 1994 to determine the effect of 0 to 12 Mg ha−1 of surface wheat residues (SWR) on giant foxtail density and crown node length, and soybean yield. Giant foxtail density decreased as SWR increased from 0 to 12 Mg ha−1. SWR of 6 to 12 Mg ha−1 reduced giant foxtail density by 2 to 50 % compared to bare soil. The crown node of giant foxtail was 2 cm above the soil surface with 12 Mg ha−1 of SWR. Frost in 1992 injured soybean more than weeds in plots with SWR while soybean in soil with no SWR was not injured. In absence of frost in 1993 and 1994, yield of weedy soybean increased 20 to 29%, respectively, with the increase of SWR from 0 to 6 Mg ha−1. In weed-free plots, soybean yield was similar across all SWR levels. These results confirm the hypothesis that high levels of SWR increased soybean yield in weedy plots because of decreased giant foxtail infestation.

Experiments were conducted from 1989 to 1991 to determine the critical period of johnsongrass control in field corn. Maximum weed-infested and weed-free periods of 0 to 20 wk after corn emergence were maintained by either hand weeding or nicosulfuron application. Interference duration effects on corn grain yield were not affected by johnsongrass control methods. The critical period for johnsongrass control was determined to be between 3 and 6.5 wk after corn emergence to avoid losses above 5% of yield produced by full-season weed-free corn.

The response of pepper varieties to Egyptian broomrape and nodding broomrape was characterized in comparison with tomatoes grown in pots and in aqueous solution, according to two criteria: 1. germination of the parasite's seeds in the presence of the host roots; and 2. the number of parasites per host plant. More than 50% of Egyptian broomrape seeds germinated in the presence of all pepper variety roots except the sweet pepper Maor. However, only a few attached and developed. The two paprika varieties Lehava and Shani were hosts to more parasites than the sweet pepper varieties, and showed as many as 15 parasites per host plant. In contrast, tomato roots induced less then 10% seed germination but were highly susceptible to the parasite (30 parasites per host plant). Damage caused to pepper foliage and fruit was not significant, compared to the heavy damage caused to tomato. Pepper roots stimulated germination of 22 to 26% of nodding broomrape seeds but without the formation of parasitic attachments, whereas tomato roots stimulated germination of less than 10% of the seeds but the plants were then heavily damaged by the parasite. Interplanting of tomato with pepper in nutrient solution did not change the amount of Egyptian broomrape parasites on pepper, hut a fourfold increase in the number of nodding broomrape was observed on tomato roots, compared to the number of parasites on tomato roots when planted without pepper.

Weed control in cole crops and onion by postemergence spraying with liquid ammonium nitrate was studied to determine its feasibility under Fraser Valley, British Columbia conditions. Ammonium nitrate solutions (7.5, 10, 15 and 20% N) were applied at 800 L ha−1 on warm, sunny days. Shepherd's-purse, low cudweed, redroot pigweed and ladysthumb were susceptible to this treatment; common lambsquarters, common purslane and annual bluegrass were tolerant. In order to determine the effect of weed density on weed control, several shepherd's-purse densities were established in broccoli and sprayed with ammonium nitrate solution (20% N; 800 L ha−1). Two days after treatment, weed control was 87.2% for an initial density of 500 shepherd's-purse plants m−2 (1987) and 76.1% for an initial density of 988 plants m−2 (1988). The seedlings that survived the treatment were sufficiently set back that they did not cause any adverse effect on crop yield. In a separate study, cabbage and onion were found to be tolerant to the ammonium nitrate treatment. Initially, cauliflower, broccoli and Brussels sprouts growth was slightly inhibited by the ammonium nitrate treatment but the plants recovered and crop yield was not affected.

Clomazone dissipation in soil was examined in field and laboratory experiments. Field studies suggested a potential for injury to rotational crops such as wheat. Field half-lives were 5 to 29 d (average of 9 field studies was 19 d) for the Etowah clay loam (fine-loamy, siliceous, thermic typic Paleudult) and Lily loam (fine-loamy, siliceous, thermic typic Hapludult) soils. Clomazone had an average half-life of 34 d under lab conditions, indicating slower dissipation in the lab than in the field. Clomazone was moderately adsorbed to soil as indicated by a Kd of about 1, and a Kom of 52 in the two soils.