Field and laboratory studies were conducted in 1985 and 1986 to determine the effect of herbicide application timing and level of corn residue on metribuzin plus metolachlor for weed control in soybean. Herbicides were applied early preplant (35 days before planting), sequentially (early preplant and preemergence), or preemergence. Giant foxtail control was better with sequential application than early preplant herbicide application in both years. Control of velvetleaf, smooth pigweed, and jimsonweed was independent of application timing, except in 1986, when jimsonweed control was less from herbicides applied early preplant compared to sequentially. Metribuzin reaching the soil was less when applied to high levels of corn residue compared to low residue levels. However, corn residue level had no effect on weed control.

Terminal fluorescence (FT) was monitored following root or foliar uptake of metribuzin in soybean, velvetleaf, and smooth pigweed seedlings grown in solution culture. These species are known to be tolerant, moderately susceptible, and susceptible, respectively, to metribuzin under field conditions. The indirect monitoring of herbicide metabolism by fluorescence is complicated by several factors (particularly photoinhibition) and is difficult to interpret when plants are kept in the light. Rapid declines in FT at metribuzin concentrations causing high photosystem II reaction center inhibition were interpreted, in conjunction with fresh weight and injury data, as resulting from photoinhibition in velvetleaf and smooth pigweed, and from a combination of photoinhibition and herbicide metabolism in soybeans. At lower concentrations, FT declines were apparently not affected by photoinhibition but were probably representative of herbicide metabolism. These results suggested a significant rate of metribuzin metabolism in soybeans, a minor degree of metabolism in smooth pigweed beginning after 12 h, and no detectable metabolism in velvetleaf. FT monitored over a 24-h dark period following foliar absorption showed declines in soybeans indicating a modest degree of metribuzin metabolism (at intermediate herbicide doses) but no FT declines attributable to herbicide metabolism in velvetleaf or smooth pigweed. Results indicate that the inhibition of a high percentage of photosystem II reaction centers is required before photoinhibition and photooxidative leaf damage can result from photosystem II inhibitor herbicides such as metribuzin.

Field studies were conducted over a four-year period to determine the weed control efficacy of ICIA-0051 and SC-0774 in no-tillage and conventional corn. Treatments consisted of both pre- and postemergence applications of ICIA-0051, alone and in combination with atrazine, and preemergence applications in combination with metolachlor. SC-0774 was applied preemergence, alone and in combination with atrazine. Results indicated good control of triazine-resistant (TR) smooth pigweed and other annual broadleaf weeds present with no adverse effects on crop vigor and yield with ICIA-0051. Generally, the addition of atrazine to ICIA-0051 improved weed control. SC-0774 provided good control of fall panicum, but generally poorer and less consistent control of giant ragweed, common ragweed, common lambsquarters, and ivyleaf and tall morningglory species.

Competitive relationships between johnsongrass, smooth pigweed, and soybean were examined in 1986 and 1987, and the adequacy of an additive response model (ARM) and product response model (PRM) in predicting yield reductions due to multispecies weed populations was assessed. A severe drought reduced soybean yields statewide in 1986, whereas ideal conditions for soybean production were experienced in 1987. Averaged over monospecific weed densities of 1, 2, 4, and 8 plants 4.6 m−1 of row, smooth pigweed intercepted 2.5 and 1.8 times more light than johnsongrass in 1986 and 1987, respectively. In multispecies populations having either 4 or 8 smooth pigweed plants 4.6 m−1 of row, light interception by johnsongrass was negligible. Averaged over monospecific weed densities, smooth pigweed produced 5160 and 1760 kg ha−1, while johnsongrass produced 1530 and 450 kg ha−1 dry weight in 1986 and 1987, respectively. In multispecies populations, smooth pigweed contributed more than 80% of the total weed biomass. As monospecific johnsongrass density increased, reductions in soybean seed yield were linear, whereas exponential response patterns adequately characterized reductions in soybean seed yield due to smooth pigweed interference. Based on calibrated monospecific responses, the ARM and PRM generally projected higher soybean seed yield reductions for multispecies weed populations than were observed. When crop production conditions were favorable and competitive effects of weeds were low, both models adequately predicted soybean seed yield reductions. When dry conditions unfavorable for crop production existed, the PRM best accommodated the interactive effects between johnsongrass, smooth pigweed, and soybean.

Eight field studies were conducted in soybeans at seven locations in Kansas over a 3-yr period to examine the efficacy of using reduced rates of the herbicides acifluorfen, bentazon, chlorimuron, and tank mixes of acifluorfen and bentazon. POST applications of these herbicides at 1/2X rates at 2 wk after planting (WAP) resulted in broadleaf weed control similar to that obtained from standard treatments of 1X rates applied at 4 WAP at six of seven studies with acifluorfen, bentazon, and acifluorfen plus bentazon and at five of seven studies with chlorimuron. One-quarter rates applied 2 WAP were equivalent to standard treatments for broadleaf weed control in four of seven studies with acifluorfen and chlorimuron, five of seven studies with bentazon, and six of seven studies with acifluorfen plus bentazon. One cultivation at 4 WAP, increased the broadleaf weed control with all herbicide treatments.

Continuous corn was grown for 5 consecutive yr using conservation tillage methods in the Maryland Piedmont. Herbicide treatments were applied annually at common usage rates and were compared for their effects on weed cover, species stability, and grain production. Weed flora dominance shifted from giant foxtail the first year to smooth pigweed the following year. Smooth pigweed then dominated the flora for the duration of the study. Horsenettle was ubiquitous in all treatments but never became competitive. Canada thistle occurred in measurable densities in all plots that had not received atrazine applied preemergence. Grain yields were correlated negatively with densities of smooth pigweed in 1983 (r = −0.58, P = 0.01), and Canada thistle in 1983 and 1984 (r = −0.63 and −0.62, respectively; P = 0.05). Grain yields were correlated positively with midseason precipitation (r = 0.85, P = 0.01).

Based on chlorophyll content, hydroponically cultured soybean seedlings were 254, 66, and 13 times more tolerant to clomazone than velvetleaf, corn, and smooth pigweed, respectively. Clomazone, at concentrations that inhibited chlorophyll, did not affect fresh weight accumulations of any species except velvetleaf. However, in velvetleaf, fresh weight accumulation was only half as sensitive to clomazone as the leaf chlorophyll content. Uptake of 14C-clomazone from nutrient solution by 72 h after treatment (HAT) (pigweed > velvetleaf > soybean > corn) indicates that differential absorption cannot account for selectivity. Shoot:root ratios of 14C recovered from soybean, corn, velvetleaf, and pigweed by 72 HAT were 0.39, 0.84, 1.67, and 2.37, respectively. The limited acropetal clomazone translocation in soybean seedlings may account to a small degree for soybean tolerance to clomazone. Conversion of clomazone to more polar metabolites was rapid in all four species. There were no significant differences among species in the percentage of 14C activity recovered as clomazone from root tissue by 72 HAT. Of the 14C activity recovered from shoots of soybean, corn, pigweed, and velvetleaf seedlings by 72 HAT, 46, 59, 35, and 54%, respectively, was clomazone. Differences in clomazone uptake, distribution, and metabolism among the four species were either insignificant or poorly correlated to selectivity, and therefore cannot account for the tremendous differences in clomazone sensitivity among these species. These observations indicate, indirectly, that differences at the site of action may account for selectivity.

Seeds were collected from an imidazolinone-resistant (R) population of smooth pigweed near Marion, MD, and from an imidazolinone-susceptible (S) population near Painter, VA, and grown in the greenhouse. Acetolactate synthase (ALS) enzyme was extracted from both biotypes and assayed in the presence of CGA 152005, chlorimuron, halosulfuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, pyrithiobac, rimsulfuron, and thifensulfuron to determine if an altered ALS was the mechanism of resistance in the R biotype and to determine if this biotype was cross-resistant to other ALS inhibitor herbicides. The inhibitor concentration required to cause a 50% reduction in ALS activity (I50) was calculated for each herbicide. ALS from the R biotype was approximately 71-, 109,000-, and 9-fold more resistant to imazaquin, imazethapyr, and rimsulfuron, respectively, than that from the S biotype. ALS from the R biotype was approximately threefold more sensitive to pyrithiobac and thifensulfuron than that from the S biotype. R ALS was also slightly more tolerant to CGA 152005 and nicosulfuron and slightly more sensitive to primisulfuron and chlorimuron. ALS from both biotypes generally responded similarly to halosulfuron. Resistance in the R biotype was due to an altered form of ALS that is insensitive to the imidazolinone herbicides and rimsulfuron.

The absorption, translocation, and metabolism of the ethyl ester of chlorimuron and nicosulfuron in resistant (R) and susceptible (S) smooth pigweed (Amaranthus hybridus) were investigated. R and S smooth pigweed seedlings (5 to 10 cm tall) were treated with foliar-applied 14C-labeled chlorimuron and nicosulfuron and harvested at 3, 6, 24, and 72 h after application. Chlorimuron absorption increased with time in both populations of smooth pigweed and was higher in the R accession only at 6 h after application. Nicosulfuron absorption did not increase with time in the R accession, and after 6 h, there were no differences between the R and S accessions. Absorbed chlorimuron and nicosulfuron were translocated to shoots and leaves above and below the treated leaf, but not into the roots in both accessions. Translocation of chlorimuron and nicosulfuron out of the treated leaf was also similar in the two accessions. Metabolism of 14C-chlorimuron occurred more rapidly in the R accession with approximately 68% remaining as chlorimuron at 3 h after application. At the same time period, 81% of the absorbed radioactivity remained as chlorimuron in the S accession. Metabolism of 14C-chlorimuron at 24 and 72 h after application was greater in the S than the R accession of smooth pigweed. Metabolism of 14C-nicosulfuron was similar in the two accessions and did not increase with time. Rapid herbicide metabolism may explain the twofold level of crossresistance of the R accession of smooth pigweed to chlorimuron at the whole plant level.

Data from field studies at Fayetteville, AR, were used to predict the herbicide rate needed to provide 70, 80, or 90% control of a weed based upon weed age. Reduced herbicide rates generally needed to be applied within 6 to 12 d after emergence to control weeds 90%. Reduced rates (280 g ai ha–1 or less) of acifluorfen controlled hemp sesbania, smooth pigweed, Palmer amaranth, and pitted and entireleaf morningglory 90%. Bentazon at 350 to 650 g ai ha–1 controlled common cocklebur and prickly sida 90%. Common cocklebur, smooth pigweed, and pitted morningglory were controlled 90% with chlorimuron at 2 to 5 g ai ha–1 and imazaquin at 20 to 80 g ai ha–1. Prickly sida and hemp sesbania were controlled 90% with imazaquin at 70 g ha–1 and chlorimuron at 6 g ha–1, respectively. Barnyardgrass, large crabgrass, red rice, and sicklepod were not controlled with reduced herbicide rates.