A four-year experiment was conducted near Urbana, IL to evaluate the effect of a rye cover crop on weed control, soybean yield and soil moisture. Soybeans were planted into either a rye mulch or corn stubble (with and without spring tillage). Giant foxtail, velvetleaf, smooth pigweed and common lambsquarters control in the rye mulch plots was generally greater than 90% and better than the corn residue treatments five weeks after planting. Weed control was generally better, except for lambsquarters, in the corn residue without spring tillage plots compared to the spring-tilled plots. Herbicides improved weed control in the corn residue plots but not in the no-till rye treatment, due to the excellent control by the rye mulch. Soil water content was lowest during June under the late-killed (killed at planting) rye during dry periods due to water depletion caused by the growing rye. During wet periods the rye mulch resulted in a wetter soil profile compared to the corn residue treatments. Soybean yields were reduced in late-killed rye compared to early-killed rye (killed 2 wk prior to planting) due to soybean stand reductions in the late killed rye. Yields in early-killed rye and spring-tilled treatments were similar to or better than soybeans planted in corn residue without spring tillage.

Adsorption of imazaquin and imazethapyr on soils, sediments, clays, and ion exchange resins was studied. Multivariate regression analysis of Freundlich K values and soil and sediment properties yielded a model for imazaquin adsorption which included soil pH, organic carbon, and clay as significant independent variables. Clay content and pH were significant independent variables for the imazethapyr adsorption model. Adsorption of imazaquin and imazethapyr was greater at pH below 6 than at a higher pH. Adsorption of both herbicides on Ca- and H/Al-kaolinite and Ca-montmorillonite was low, but increased on H/Al-montmorillonite. The results of adsorption studies with ion exchange resins indicated that binding of protonated herbicide at low pH could be due to physical forces, hydrogen bonding, or cationic binding. At soil pH levels above the pKa's of imazaquin and imazethapyr, ionic bonding of herbicide anions with positively charged soil components could also occur.

The availability and persistence of imazaquin, imazethapyr, and clomazone were studied in a Cisne silt loam (1.3% organic matter) and a Drummer silty clay loam (5.8% organic matter). Availability of all three herbicides to bioassay species was greater in the Cisne soil than in the Drummer soil. Corn root growth was more sensitive to imazaquin and imazethapyr than corn shoot growth. Shoot and root growth of wheat was inhbited by similar clomazone concentrations. In field experiments conducted in 1984, 1985, and 1986, all three herbicides were more persistent in the Drummer silty clay loam than in the Cisne silt loam. Clomazone and imazethapyr were detected by liquid or gas chromatographic analysis in the Drummer soil 3 yr following application. Crop injury occurred 5 months after application of imazaquin and clomazone to the Drummer soil. In the Cisne soil, only imazethapyr caused crop injury 5 months after application. Herbicide residues found below 7.5 cm were greater in the Drummer soil than in the Cisne soil.

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The extent of14C-imazaquin and14C-imazethapyr abiotic vs. biotic degradation in soil was investigated. Degradation was measured in an in vitro system which allowed 90% recovery of applied herbicide. Triallate biodegradation is well documented and therefore used as a standard. Herbicide degradation was compared in two soils, a Cisne silt loam and a Drummer silty clay loam. Herbicide degradation in gamma-irradiated soil was compared to fresh soil. Biomass quantities were measured for the duration of the experiments.14CO2evolution, extractable parent, metabolites, and unextractable residue were measured. After 12 weeks of incubation, 95% of the radioactivity could be extracted as parent from sterilized soil. In unsterilized soil, imazaquin and imazethapyr degraded at a similar rate which was dependent upon soil type. All herbicides degraded slower in the Drummer soil and triallate degraded two to three times faster than the imidazolinones in either soil.14C-imazaquin degradation products included14CO2and unextractable residues. The major product from14C-imazethapyr degradation was14CO2. Evolution of14CO2from an imazethapyr-treated Cisne soil, containing a serial dilution of activated charcoal, demonstrated that adsorption of herbicide was negatively correlated with degradation. Therefore imidazolinone microbial degradation is regulated by the amount of herbicide in soil solution as determined by soil characteristics.

As nitrogen management practices change to achieve economic and environmental goals, effects on weed-crop competition must be examined. Two greenhouse experiments investigated the influence of N amount and form on growth of maize and redroot pigweed (Amaranthus retroflexus L.).In Experiment 1, maize and pigweed were grown together in a replacement series (maize:pigweed ratios of 0 : 4, 1 : 3, 2 : 2, 3 : 1, 4:0) under three NHaNO3-N supplies (0, 110, and 220 mg N kg -~ soil). Maize was planted into established pigweed and plants were harvested 24 days after maize germination. Pigweed responded more to supplemental N than maize and accumulated 2.5 times as much N in shoots at the high N supply. Competition effects were not significant.Maize and pigweed were grown separately in Experiment 2 and supplied 220 mg N kg-l as either Ca(NO3) 2 or (NH4)2SO 4 plus a nitrification inhibitor (enhanced ammonium supply, EAS). In maize, EAS treatment did not affect shoot growth and reduced root growth 25% relative to the NO3-N treatment. In pigweed, shoot and root growth were restricted 23 and 86% by EAS treatment, respectively. Total plant N accumulation under EAS treatment was higher in maize, less in pigweed. Under EAS treatment, pigweed leaves were crinkled and chlorotic; leaf disks extracted in 70% ethanol, pH 3, contained less malate and oxalate but more NH4 compared to the NO3-N treatment. Maize leaf disk malate levels were generally higher compared to pigweed but were less due to EAS treatment. Ammonium level in maize leaf disks was unaffected by N form and EAS treatment increased oxalate levels. Final bulk soil pH was generally lower in pots where pigweed were grown and tended to be lower due to EAS. Leaf disk malate levels and soil pH were positively associated.Results indicate that pigweed is more likely to compete with maize when high levels of NO3-N are provided. Enhancing the proportion of N supplied as NHj-should restrict the growth of NH4-sensitive pigweed.

Imazethapyr at 0.05 to 0.14 kg ai/ha applied preplant incorporated, preemergence, and postemergence was evaluated alone and with complementary herbicides in the field for weed control in soybean. Imazethapyr controlled 90% or more smooth pigweed regardless of application method or herbicide rate. Imazethapyr at 0.05 kg/ha controlled jimsonweed 30% better postemergence compared to soil applications. Imazethapyr at 0.10 kg/ha controlled 90% or more velvetleaf regardless of application method. The addition of alachlor to soil-applied imazethapyr enhanced giant foxtail, jimsonweed, and velvetleaf control. Adding acifluorfen or bentazon to postemergence imazethapyr antagonized weed control. Adding sethoxydim to postemergence imazethapyr was not beneficial.

Effects of tillage and herbicide application method on the persistence and residual activity of clomazone, imazaquin, and imazethapyr were investigated in a 2-yr field study. Herbicides were applied preemergence and preplant incorporated to conventional- and reduced-tillage soybeans in 1987 and 1988. Herbicide dissipation was monitored using chromatographic and bioassay techniques. In 1987, dissipation rates for clomazone, imazaquin, and imazethapyr were similar, regardless of tillage system or application method. In 1988, all three herbicides applied preplant incorporated dissipated more slowly than in preemergence treatments. Corn planted in rotation in 1989 displayed greater levels of injury in the incorporated treatments for all three herbicides. Although herbicide concentrations were similar 322 d following application in both tillage treatments in 1989, corn injury was greater with imazaquin and imazethapyr and less with clomazone in the reduced-tillage plots than in the conventional-tillage treatments.