Field experiments were conducted from 2004 through 2006 to evaluate cressleaf groundsel control following fall or early-spring preplant herbicide applications. Glyphosate, glyphosate + imazethapyr, glyphosate + 2,4-D ester, paraquat, paraquat + simazine, chlorimuron + tribenuron, and pendimethalin + glyphosate + 2,4-D ester applied in the fall controlled at least 93% of the cressleaf groundsel. Glyphosate, glyphosate + imazethapyr, glyphosate + 2,4-D ester, chlorimuron + tribenuron, and pendimethalin + glyphosate + 2,4-D ester applied in the spring provided at least 94% control of the cressleaf groundsel. Chlorimuron + tribenuron provided at least 98% control, regardless of application timing. These results indicate that herbicides applied in the fall or early spring can control cressleaf groundsel. However, certain herbicides provide greater control when applied in the fall compared with spring.

Field studies were conducted during 1999 and 2000 to compare weed control after fall and early-preplant (EPP) herbicide applications in no-till soybean. Three residual treatments (chlorimuron plus metribuzin, chlorimuron plus sulfentrazone, and metribuzin) were applied at two rates and timings (fall and 30 d EPP) either alone or in combination with glyphosate and 2,4-D. The addition of glyphosate and 2,4-D to fall-applied residual herbicides significantly increased control of common chickweed, annual bluegrass, cressleaf groundsel, and shepherd's-purse. The effect of application rate on weed control was species dependent. Fall-applied residual herbicides were comparable with EPP treatments with respect to winter annual weed control; however, at planting control of summer annual weed species with fall treatments was less consistent compared with EPP residual herbicides.

Minimizing inputs such as fertilizers, herbicides, or tillage may be sought by producers to satisfy economic as well as environmental goals. One of the challenges in reducing inputs, whether synthetic fertilizers or herbicides, or substituting a synthetic nutrient with an organic source, is to identify practices that will provide optimum growing conditions for the crop while maintaining an adequate level of weed control. Our objective was to measure the cumulative effects of 12 yr of nitrogen (N) and phosphorus (P) fertilization treatments applied to two tillage systems [conventional tillage (CT) vs. no tillage (NT)] in a corn–soybean rotation on weed communities and crop yields. Residual (postherbicide treatment) weed species assembly was determined by multivariate analysis and was influenced mainly by tillage, with weeds more strongly associated with NT than with CT. Diversity of weed communities as measured by richness, evenness (E), and a diversity index (H′), and total weed biomass were greater for NT than for CT. Nutrient treatments had little or no effect on these parameters. Corn yields were reduced by 70% in the absence of N and by 25% in NT compared to CT treatments. Soybean yields were reduced in NT with increasing P rates compared to other treatments, but reductions never exceeded 10%. Overall, corn and soybean had different responses to treatments, with corn yields being far more affected by fertilization and tillage than soybean yields. Conversely, the absence of tillage had a much greater effect than the absence of nutrient input on weed community assembly and biomass, suggesting the importance of a weed management program specifically tailored for NT systems.

The effect of planting system and cover crop residues on weed emergence in irrigated vegetable row crops was studied in field experiments from 1995 through 1997. Vegetable crops were either no-till planted (NTP) through cover crop residues or conventionally planted (CP) into soil with cover crop residues incorporated. NTP reduced emergence of hairy nightshade by 77 to 99% and Powell amaranth emergence by 50 to 87% compared with CP. Cover crop treatments were much less important than planting system in regulating weed emergence. Tillage in the spring did not increase the number of viable seeds near the soil surface. Hairy nightshade emergence ranged from 0.6 to 9.8% of the intact seeds in CP compared with 0 to 0.1% emergence of the seeds in the NTP plots. Powell amaranth emergence ranged from 4.9 to 6.5% of the intact seeds in CP contrasted with only 0.4 to 0.9% emergence of the seeds in NTP plots.

Field experiments were conducted to study the effects of various tillage and mulching practices on fruit maturity and weed suppression in pumpkins. Conventional tillage (CT), disking, no tillage with rye removed (RR), no tillage with standing rye (SR), and strip tillage (ST) were evaluated with and without ethalfluralin plus halosulfuron (1.5 plus 0.036 kg ai/ha, respectively) applied preemergence. In 2001, when heavy rain after herbicide application caused significant crop injury, the herbicides delayed maturity and significantly reduced yields of mature pumpkins within each herbicide treatment, total yields did not differ with tillage. In 2002, weed populations were significantly greater than those in 2001, and in 2002, regardless of herbicides, yields of mature fruit were greater in tillage treatments with higher rye residues (SR, ST). Although weed populations were less in one year than the other, herbicides provided effective control in both seasons, and RR, ST, and SR effectively suppressed weeds compared with CT. Averaged over treatments, greater yield losses were attributable to weed competition (42%) in 2002 than to herbicide injury (32%) in 2001.

Understanding the factors regulating recruitment in diverse tillage systems will improve the efficacy of weed-management strategies. Experiments measured the effect of hairy nightshade winter seed position (burial depth) on seedling recruitment and seed germination, dormancy, and mortality. Hairy nightshade seeds were placed in soil tubes at 1 cm below the soil line and buried in the fall so that seeds were positioned in the soil at 1, 6, 13, and 25 cm below the soil surface. Tubes with undisturbed soil and seeds were removed from the field in March, April, and May of 2004 and 2005 and placed in wells in a heated, aluminum block with a linear temperature gradient from 22.7 to 36.0 C at 2.7 C intervals, and recruitment was measured. Seeds were also recovered from the soil tubes to determine effects of seed position on germination rate, mortality, and seed dormancy. Hairy nightshade seedling recruitment was greater for seeds positioned at 6, 13, and 25 cm during the winter than at 1 cm. Seed dormancy and mortality were greatest for seeds positioned at 1 cm, but that did not adequately account for the significant decrease in recruitment at 1 cm compared to seeds buried at 6, 13, or 25 cm. Protecting the seeds buried at 1 cm from rainfall during the winter increased seedling recruitment from 0 to 20% of buried seeds but had a negligible effect on seed mortality and dormancy. Soil density was negatively correlated with recruitment.