Tebuthiuron was applied at 1 kg ai/ha to wet and dry Hathaway gravelly, sandy loam soil in the spring and fall to determine the amount of tebuthiuron removed in runoff water and the depth to which it would move within the soil profile by simulated rainfall. When pellets containing 20% ai of tebuthiuron were broadcast onto dry soil in the spring, the first simulated rainfall event, 37 mm, removed 5% of the applied tebuthiuron in runoff water and sediment. The second and third simulated rainfall events, 22 and 21 mm, respectively, removed an additional 2%. When tebuthiuron was applied to wet soil in the spring, the initial simulated rainfall events, totaling 42 mm, removed 15% of the tebuthiuron. When tebuthiuron was applied to wet soil in the fall, the initial rainfall events, totaling 40 mm, removed a total of 48% of the tebuthiuron in runoff water and sediment. No significant differences were found in the total amount of tebuthiuron within the soil profile after application to dry and wet soils. More than half of the tebuthiuron had moved into the surface 7 cm 1 day after application. Tebuthiuron was not detected below 90 cm after 165 mm of simulated rainfall and 270 mm of natural rainfall.

Trifluralin was evaluated at 1.1, 2.2, and 4.5 kg/ha in 1983 and 1984 at two locations in Iowa for residue carryover injury to corn the following seasons. Three methods of seedbed preparation (no-till, moldboard, and chisel plowing) for corn planting were also examined. There was no effect on corn growth at the 1.1 kg/ha rate of trifluralin. Averaged over the four experiments, reductions in corn height of 8 and 24% were observed 5 weeks after planting at 2.2 and 4.5 kg/ha, respectively. The relative degree of stunting due to trifluralin decreased as the growing season progressed. Early-season carryover injury was more severe in reduced tillage than in moldboard plow treatments in the 1983-84 Nashua experiment. Moldboard and chisel plowing reduced the concentration of trifluralin in the 0- to 7.5-cm zone of the soil profile by 62 and 31%, respectively, when compared to no-till. No yield reductions were observed at the 1.1 or 2.2 kg/ha rate of trifluralin. In 1984, grain yields were reduced by 8 and 16% at Ames and Nashua, respectively, by the 4.5 kg/ha trifluralin rate.

In 1985, 1986, and 1987, pendimethalin at 1.7 kg ai/ha plus 2.2 kg ai/ha cyanazine, 2.2 kg ai/ha atrazine, or 1.1 kg/ha atrazine plus 1.1 kg/ha cyanazine was applied delayed preemergence, early postemergence, and postemergence with and without cultivation to evaluate woolly cupgrass control and corn injury. Results varied from year to year. Dry conditions in 1985 resulted in poorer woolly cupgrass control while cold and wet environments in 1987 resulted in corn injury and reduced yields from postemergence treatments containing cyanazine. Good early-season suppression of woolly cupgrass deteriorated to less than 75% control by the late-season evaluation in all experiments. The best woolly cupgrass control and highest corn yields were usually achieved when herbicide applications were followed by row cultivation. Corn yield increases averaged 28, 17, and 11% in 1985, 1986, and 1987, respectively, when a herbicide treatment was followed by row cultivation. Pendimethalin/triazine treatments followed with a row cultivation adequately suppressed woolly cupgrass in field corn, but adverse environmental conditions often reduced herbicide effectiveness or increased corn injury.

Substitutive (replacement) and partial additive experimental designs, with their underlying models, remain the two most popular techniques in weed-crop competition studies, despite considerable criticism of these approaches in the recent literature. We review standard designs for two-species competition experiments and demonstrate the advantages of a reciprocal yield model applied to data from an additive series experiment, using mixtures of Japanese millet and tomato. A traditional replacement series analysis failed to provide a general model of competition among these two species over several total plant densities, while an application of a reciprocal yield (inverse linear) model to the same data was successful. This technique allows evaluation of the influences of both weed on crop and crop on weed, as well as the partitioning of net competition effects into intra- and interspecific components. One Japanese millet plant was competitively equivalent to 3.7 tomato plants, as measured by effects on tomato biomass, while one tomato plant was equivalent to 0.14 Japanese millet plants, as measured by effects on millet biomass. Skewness of per plant biomass distribution is shown to be a result but not an unambiguous measure of competition. Expansion of a reciprocal yield model to mixtures of more than two species is illustrated using three species of duckweed. While some caution is recommended, the reciprocal yield analysis applied to data from appropriately designed experiments is a substantial improvement over more traditional methods.

Simulated sprayer tank residues of the broadleaf weed herbicides dicamba, chlorsulfuron, and clopyralid applied alone and with the grass weed herbicides sethoxydim and diclofop on sunflower, tame mustard, and lentil, respectively, caused visible crop injury and reduced dry weight and yield. Dry weight production in the greenhouse and crop tolerance ratings in the field indicated that the grass weed herbicides enhanced crop injury from dicamba, chlorsulfuron, and clopyralid. Yield reductions in field experiments were also greater when dicamba and clopyralid were mixed with grass weed herbicides and applied on sunflower and lentil, respectively. This did not occur with chlorsulfuron applied to mustard. When mixed with simulated broadleaf weed herbicide residues, diclofop enhanced dry weight reductions and crop injury and reduced yield to a greater extent than sethoxydim. Crop tolerance ratings differentiated treatments and rates but were not a good estimate of the extent of yield loss. When broadleaf weed herbicides were applied at rates simulating sprayer tank residues alone or combined with grass weed herbicides, yield losses ranged up to 40% in sunflower, 70% in mustard, and 95% in lentil, compared to the untreated check.

Growth chamber studies were conducted to examine the soil residual properties of DPX-A7881, a new sulfonylurea herbicide. The phytotoxic residue levels in the soil were determined by a lentil radicle bioassay. The duration of activity was prolonged in soil adjusted to pH 7.6 and 8.1 relative to more acidic levels. The rate of breakdown in the soil was enhanced with increased temperature and soil moisture content; a significant temperature by moisture interaction was noted over the duration of the incubation period. The dissipation of DPX-A7881 in soil obeyed first-order kinetics in both studies. An accelerated rate of breakdown in unsterilized versus sterilized soil (pH 7.6) indicated that microbial degradation was an important factor affecting the persistence in alkaline soils. Herbicide residues in the soil caused a reduction in taproot length and number of primary lateral roots of canola seedlings 15 days after planting but there were no other morphological effects observed on the root. The secondary laterals, however, had generally recovered by this time.

The fungus Scopulariopis brumptii Salvanet-Duval (MSU 42018) was isolated from a soil sample collected from the rhizosphere of a potted asparagus plant. When cultured 7 days in A-9 broth medium, the organism produced compounds phytotoxic to seeding dicotyledonous weeds (via foliar application) and to Chlamydomonas reinhardtii Dangeard. Three herbicidal metabolites, 3-hydroxybenzyl alcohol, 2-methylhydroquinone, and (+)-epiepoformin, were isolated by column and thin-layer chromatography (TLC) and structures confirmed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic reasonance spectroscopy (NMR), and mass spectrometry (MS). Of the three compounds, (+)-epiepoformin was the most active, providing complete control of redroot pigweed and 88% control of white mustard when applied at 4.4 kg/ha.

The phytotoxicity, adsorption, and mobility of metribuzin and ethyl-metribuzin3 were studied in five soils to determine if differential soil behavior could explain: a) the greater activity of both chemicals in western Oregon than in eastern Oregon, and b) the greater activity of metribuzin than ethyl-metribuzin under similar conditions. Metribuzin had higher activity on oats than ethyl-metribuzin in all soils and in quartz sand. Metribuzin was adsorbed less and moved more than ethyl-metribuzin in all soils. Activity of both herbicides decreased as sand content increased, and activity in quartz sand was lower than in soil. Activity of both herbicides increased in a bioassay in which leaching was prevented, indicating that leaching may be important in the loss of activity of metribuzin and ethyl-metribuzin. Lowest herbicide adsorption and greatest movement were observed in coarser textured soils.

Growth of an agricultural weed population and a coastal nonweed population of poorjoe was compared in a competition experiment with soybean and when grown alone. Mean aboveground biomass production of the weed population was roughly twice that of the nonweed population, whether grown alone or with soybean. A path-coefficient model of growth was fitted to the data in order to compare the populations with respect to five independent measurements of weed growth: establishment rate, early and later growth rates, growth form, and final growth rate. These measurements indicated plant growth over successive intervals of the growing season by statistically equating the size of all plants at the beginning of an interval of growth using analysis of covariance. In general, growth measurements were positively associated with aboveground biomass production. The weed population of poorjoe had a greater establishment rate and a greater early aboveground growth rate compared to the nonweed population; however, the final growth rates of the two populations were similar whether soybean was present or absent. Genetic changes leading to earlier establishment and faster early growth, but not increasing tolerance of soybean and intraspecific competition, may have occurred in the adaptation of poorjoe to an agricultural ecosystem.

Corn hybrids and inbreds were ranked for their relative tolerance to soil-incorporated clomazone, as assessed by the level of discoloration injury in the greenhouse. Inbred W117 was the most tolerant corn line tested. Some corn lines were affected similarly by clomazone. Inbred A619 was in the most susceptible group. Clomazone injury to A619 (susceptible) and W117 (tolerant) corn was similar when the clomazone rate was 10-fold greater on W117 than on A619. The distribution of corn lines on a sensitivity scale was of limited range; the distribution of hybrids on this scale was a single symmetrical peak. Changing the growth temperature or soil composition would change the absolute level of corn injury caused by a rate of clomazone but did not change the relative ranking of the corn lines in the test. A subset of the greenhouse-tested corn lines also was evaluated in several field locations. The tolerance of corn in a given field was highly (P<0.005) correlated with tolerance in the greenhouse; however, the absolute levels of injury differed among locations. The tolerance of- hybrids of known pedigree was highly (P<0.0002) correlated with the tolerance of the parent inbreds, indicating this trait was inherited.

Laboratory studies enumerated butylate-, EPTC-, vernolate-, pebulate-, and cycloate-utilizing bacteria and actinomycetes in a Dothan loamy sand following repeated use of butylate, EPTC, or vernolate. Herbicide utilizers were isolated on a defined medium composed of minimal salts plus 200 mg/L herbicide as the primary carbon source plus 25 mg/L TTC as an indicator of organisms capable of metabolizing the herbicide substrates. Media inoculated with a butylate-history soil developed more TTC-red actinomycetes on the butylate-indicator agar than did media inoculated with a non-herbicide-history soil. Similarly, media inoculated with a vernolate-history soil developed more TTC-red bacteria on the vernolate-indicator agar. Cross-adaptation from vernolate to butylate and cycloate was evident, with inoculant from the vernolate-history soil exhibiting an increase in bacterial counts when butylate or cycloate was present as the primary carbon source. Likewise, previous butylate exposure corresponded with an increase in the number of vernolate- and pebulate-utilizing actinomycetes. Previous exposure to EPTC resulted in an increased population of butylate- and pebulate-utilizing actinomycetes.