Haloxyfop-methyl toxicity to johnsongrass was reduced when sprayed in combination with 2,4-D in greenhouse studies. The antagonism was not seen when 2,4-D application followed haloxyfop-methyl treatment by 72 h or preceded it by 24 h. The antagonism of haloxyfopmethyl activity was evident regardless of chemical form of 2,4-D used (DMA, BOE ester, IPA, or acid). Absorption of 14C-haloxyfop-methyl into johnsongrass leaves was slowed by the presence of 2,4-D in the treatment solution but total absorption after 72 h was not affected. Translocation of 14C was decreased when the 14C-haloxyfop-methyl was applied in combination with 2,4-D. More haloxyfop-methyl, less haloxyfop, and more ether-soluble haloxyfop-methyl metabolites were found in treated leaves when haloxyfop-methyl was applied with 2,4-D compared to haloxyfop-methyl applied alone. The changes in haloxyfop-methyl translocation and metabolism caused by 2,4-D can quantitatively account for the antagonism observed.

Glasshouse experiments were conducted to evaluate the effects of preemergence applications of metribuzin, pendimethalin, alachlor, and imazaquin on the competitive relationship between soybean and sicklepod, tall morningglory, or common cocklebur. An equal-ratio series design was employed where the proportion of species in a mixture remained constant and total plant density was varied. The ratio of soybean:sicklepod fresh weights was altered by alachlor and imazaquin while the ratio of soybean:tall morningglory fresh weights was altered only by imazaquin. The ratio of soybean:common cocklebur fresh weights was altered by metribuzin and imazaquin. Increasing herbicide rates resulted in higher soybean:weed fresh weight ratios and higher herbicide response coefficients.

Multiple regression and response surface plots were used to analyze the effects of common lambsquarters population density and interference duration on weed growth and soybean seed yield. Under favorable growing conditions in 1986, weed biomass production at all population densities and interference durations was four to five times that produced in 1987, under less favorable conditions. However, there was no significant treatment by year interaction for soybean seed yield reduction by common lambsquarters, and production of each kg/ha weed biomass resulted in an average soybean yield reduction of 0.26 kg/ha. Utilizing 5% yield loss as an arbitrary threshold level, the regression equation predicted a common lambsquarters density threshold of 2 plants/m of row for 5 weeks of interference after crop emergence and 1 plant/m of row for 7 weeks. Seed production by individual common lambsquarters plants was highly correlated (r=0.92) with weed dry weight, and seed production ranged from 30 000 to 176 000 seeds/plant.

Field surveys were conducted during 1978 and 1979 to determine the abundance and distribution of weeds in fields seeded to barley, oats, spring wheat, and mixtures of barley and oats in various proportions in the province of Prince Edward Island. Using a stratified random sampling procedure, weeds were counted in 536 fields during the 2-yr survey period. The weed flora had a large number of species that occurred at high densities, probably due to the limited herbicide use on Prince Edward Island. The average total number of species (64), number of species per field (20), and weed density (253 plants/m2) were similar among the four crop types and the five Extension Districts of the province. Only 49 of the 77 species encountered during the survey were found in 5% or more of the fields. Low cudweed, corn spurry, and common lambsquarters were the most abundant species, occurring in more than 80% of the fields at a mean density higher than 33.4 plants/m2. Red sorrel, smartweed, common hempnettle, broadleaf plantain, and quackgrass were also found in 80% or more of the fields, but at a mean density from 14.4 to 16.5 plants/m2. The perennials accounted for 51% of the commonly occurring species, annual and biennial broadleaf species accounted for 43%, and annual grasses were a minor group with only 6% of the species.

Changes in dormancy and viability of freshly harvested seed of yellow nutsedge buried in the field, at depths of 5, 25, and 50 mm, were studied over a 2-yr period. The number of dormant, viable seed decreased most rapidly at 5 mm because more seed lost their dormancy and germinated (up to 30%) than at lower depths. Loss of viability through decay of seed appeared similar at all depths. No seedlings emerged from any depth. In a laboratory experiment, conducted in optimal soil moisture conditions, seedlings emerged from all seed that germinated, demonstrating that adequate soil moisture was critical for seedling establishment. This was substantiated in a second field experiment in which irrigation and mulching greatly increased seedling survival. The number of seedlings surviving in irrigated plots was 0.78% of seed sown and 0.03% in the rain-fed plots. The role of seed in the establishment of yellow nutsedge infestations is probably of little importance in dryland cropping areas despite the longevity and viability of the seed.

Field experiments were conducted in 1985 and 1986 under eight environments to evaluate the population dynamics of a range of silverleaf nightshade densities and to measure the effects of those populations on cotton lint yield. Dry weed weights of silverleaf nightshade stands were influenced by growing conditions among years, but were positively related to initial densities as long as 2 yr after establishment. Stem numbers increased as initial densities and stand age increased. A negative linear relationship existed between cotton lint yield and weed biomass and between cotton lint yield and stem number from both 1- and 2-yr-old weed stands. For each 1 kg/10 m of row increase in dry weed weight from 1- and 2-yr-old stands, a 9 and 21% lint yield loss/ha was predicted, respectively. For each stem/10 m of row, a 0.35 and 0.31% yield loss was predicted, respectively. Late-planted cotton was less vulnerable to yield reductions by silverleaf nightshade; however, its yield potential was also less.

Seed of 17 weed species that were buried at 2 and 15 cm at Fairbanks, AK, in 1984 were exhumed at 1-yr intervals and tested for germination and viability. Burial depth had a significant effect on seed viability of wild oats, bluejoint reedgrass, quackgrass, prostrate knotweed, and corn spurry with viability remaining higher at 15 than at 2 cm. The percent of viable foxtail barley and quackgrass seed that were dormant was greater at 15 than at 2 cm. Eleven of 17 species had greater than 6% seed viability 4.7 yr after burial. American dragonhead seed viability in 1989 was not significantly lower than in 1984. In contrast, viability of common hempnettle and quackgrass had dropped to zero by 2.7 and 3.7 years after burial, respectively. Foxtail barley and wild oats had less than 1% viable seed at the end of 4.7 yr. For most of the 17 species, a rectangular hyperbola model accounted for more of the variability in seed viability decline with time than did a negative logarithmic model.

Field experiments were conducted to evaluate competitive effects of johnsongrass and sicklepod on growth and yield of irrigated and nonirrigated soybeans. Johnsongrass reduced soybean growth early in the growing season. Sicklepod was competitive all season, but competitiveness was greatest during the soybean reproductive stage. Soybean yields were reduced 31% by sicklepod, 14% by johnsongrass, and 36% by both weeds growing together. Nonirrigated soybeans yielded less than irrigated, but percent yield reductions were similar. Johnsongrass dry matter production and seed yield were reduced more than sicklepod by soybean interference. Soybean and sicklepod interference reduced johnsongrass seed production 73 to 95%. Johnsongrass produced 245 to 1238 seeds per plant when interfering with soybeans and sicklepod. Sicklepod seeds per plant were reduced 6 to 31% by johnsongrass interference and 47 to 75% by soybeans and soybeans plus johnsongrass.

A tandem mass spectrometric procedure was developed to detect sesbanimide A and its isomers in plant materials. Analysis of seed extracts from 10 Sesbania species using this procedure readily detected sesbanimides in Drummond rattlebush, rattlebush, and bagpod sesbania. The toxic sesbanimides were not detected in hemp sesbania or in other species with no known history of animal toxicity.

Several herbicides were evaluated for their ability to control duncecap larkspur, a serious poisonous plant on mountain rangelands in the western U.S. Duncecap larkspur density was reduced from 33 to 93% by triclopyr applied at 2.2, 4.5, and 9.0 kg ae/ha. Picloram applied at 2.2 and 4.5 kg ae/ha reduced density from 33 to 99%. Metsulfuron applied at 86 and 138 g ai/ha reduced density from 50 to 98%. Glyphosate was the most effective herbicide, reducing density by 90 to 100% when applied at 2.2 kg ai/ha. Glyphosate reduced the cover of grasses and perennial forbs but increased cover of annual forbs. All rates of picloram and metsulfuron reduced forb cover. Grass cover increased in most plots where duncecap larkspur and forbs were reduced.

Field research was conducted at Arlington, WI, and Janesville, WI, in 1986 and 1987 to evaluate the effect of conventional-tillage, chisel plow, and no-till systems on the density and control of annual weed species in solid-seeded soybean. Common lambsquarters densities were not greatly influenced by tillage systems, but redroot pigweed densities were generally highest in the chisel plow system. Conventional tillage always had greater velvetleaf densities than no-till and no-till always had greater giant foxtail densities than conventional tillage. Giant foxtail and redroot pigweed became more difficult to control when tillage was reduced, while velvetleaf became less of a problem. This response was not observed with all herbicide treatments evaluated and several herbicide treatments provided excellent weed control. Soybean yield was not affected by tillage systems under weed-free conditions and differences in soybean yield appeared to be due to differences in weed control.

The establishment and management of nine cover crops in Kentucky production systems were evaluated in field experiments over a 2-yr period. ‘Wheeler’ rye, ‘Barsoy’ barley, and ‘Tyler’ wheat cereal grains produced greater biomass (180 to 260 g/m2) than the pasture species tall fescue, creeping red fescue, and white clover (55 to 110 g/m2). ‘Kentucky 31’ tall fescue, creeping red fescue, and white clover proved most difficult to control, and significant regrowth occurred regardless of herbicide or rate applied. HOE-39866 (1.7 kg ai/ha) was effective in rapidly controlling all cover crops except tall fescue by 30 days after application. Sethoxydim and fluazifop (0.4 and 0.3 kg ai/ha, respectively) effectively controlled the cereals and two ryegrass species. Glyphosate applied at 1.1 and 2.2 kg ai/ha was also effective, while 0.6 kg ai/ha controlled only cereal grain growth adequately. After chemical control, pasture grass plots contained fewest weeds/m2 with some reductions likely due to density and regrowth of the sods. Cover crops were effective in suppressing weed growth at 45 days after chemical control. However, significant weed growth existed in all cover crop plots by 60 days after kill. Row crop establishment increased linearly with increasing glyphosate rate. Cereal grain covers provided the most compatible planting situations for greatest seedling establishment, with rye and wheat providing greatest weed suppression. Generally, increased weed suppression provided by a cover crop was accompanied by reduced row crop establishment, with greatest reductions observed in pasture grass plots. Cucumber was most easily established while snap pea was most difficult.

Effects of a 28% N solution containing urea and ammonium nitrate (28% UAN) on field efficacy of the methyl ester of thifensulfuron and combinations of thifensulfuron and the methyl ester of chlorimuron were investigated. Velvetleaf control and soybean injury increased as rates of thifensulfuron increased (0, 2.2, 4.4, and 5.8 g ai/ha) and as rates of chlorimuron increased (0, 4.4, and 8.8 g ai/ha). Treatments that included 28% UAN at 9 L/ha controlled velvetleaf 9% better, averaged across herbicides and rates 21 days after treatment (DAT), but also increased soybean injury. Thifensulfuron applications controlled common lambsquarters well, but control was not affected by additions of 28% UAN. In studies where 14C-thifensulfuron was applied without an additive to the second true leaf of growth-chamber-grown velvetleaf, less than 4% of that applied was absorbed 84 h after treatment (HAT), whereas treatments that included nonionic surfactant or 28% UAN absorbed 33 and 45%, respectively. When both nonionic surfactant and 28% UAN were added, plants absorbed 76% 84 HAT. Translocation 84 HAT was also greatest with both additives and the majority moved to tissues above treated leaves. The 28% UAN increased uptake of thiameturon regardless of soil fertility level.

Field and laboratory studies were conducted to examine the influence of continuous use and rotation of extenders on EPTC persistence in soils from Clay Center and Scottsbluff, NE. Rotation of EPTC + dietholate and EPTC + fonofos in soils with three prior annual treatments of each combination did not improve weed control compared to continuous use. SC-0058 was generally effective in slowing EPTC biodegradation in soils previously treated with EPTC, EPTC + dietholate, EPTC + fonofos, or EPTC + SC-0058. Dietholate was effective in slowing EPTC biodegradation in soil previously treated with EPTC or EPTC + SC-0058. SC-0058 appeared to have an inhibitory influence on the initial development of soil-enhanced EPTC biodegradation. Any enhanced biodegradation of dietholate or SC-0058 that may occur after repeated use was not a factor in enhanced EPTC degradation in EPTC + extender history soils.

Degradation of diphenamid in soil with accelerated degradation and in nontreated control soil and the involvement of soil microorganisms in these processes were investigated. Soil with accelerated degradation and mixed bacterial cultures originating from the same soil degraded diphenamid and its monodemethylated metabolite (diphen M-1) much faster than the control. The bidemethylated derivative (diphen M-2) was degraded much more slowly than diphenamid or diphen M-1. The abundance of fungi capable of degrading diphenamid was similar in the soils with and without accelerated degradation. Degradation of diphenamid by mixed bacterial cultures from a soil with accelerated degradation was much faster than by a culture from nontreated soil and was suppressed by the fungicides thiram and fentin acetate. These two fungicides, as well as the bactericide chloramphenicol, suppressed diphenamid degradation in mixed bacterial cultures. The antifungal cycloheximide and the actinomycete suppressor PCNB did not affect the degradation of diphenamid. Metabolism of 14C-diphenamid in a soil with accelerated degradation and in mixed bacterial cultures originating from the same soil showed similarities with regard to evolution of 14CO2 and production of certain metabolites, while the metabolism of diphenamid by Fusarium, which rapidly degrades diphenamid in cultures, was different with regard to the above parameters. This study suggests that development of accelerated degradation in a soil involves a shift in population and/or in activity of microbial degraders in favor of bacteria. It appears that the accelerated degradation occurred by inducing oxidative reaction which involves demethylation of diphenamid.

Field and laboratory studies were conducted to examine effects of prior butylate use on biodegradation of subsequent applications of butylate and four other carbamothioate herbicides. Bioassays were used to demonstrate reductions of butylate and EPTC activity in four soils preconditioned by annual butylate applications. Combining these herbicides with dietholate, an enzyme inhibitor, prolonged persistence and restored normal herbicidal activity. Expected herbicidal efficacy occurred in adjacent plots with no history of carbamothioate use. Prior applications of butylate resulted in cross-adapted degradation of EPTC but not vernolate, pebulate, or cycloate. When samples of Cecil soil were treated in the laboratory with 14C-butylate, evolution of 14CO2 was significantly higher from soils that had received repeated field applications of butylate. The use of 14C-herbkides could be used to predict problem soils and potential herbicide failure.

Ceutorhynchus litura dispersed 9 km in 15 yr from its point of release in 1972 with incident of infestation of Canada thistle plants at one study site reaching 92% in 1987. Plants generally survived larval feeding in the spring, but holes created by departing larvae provided entrances for arthropods, nematodes, and diseases. Subsequent mortality rates were high for belowground Canada thistle shoots. However, the number of new shoots from underground roots the following spring usually offset the mortality that had been indirectly caused by C. litura.