14C-labeled 4-amino-6-(1,1-dimethylethyl)-3-(ethylthio)-1,2,4-triazin-5(4H)-one (ethiozin)3 was metabolized more rapidly in tolerant than in sensitive wheat, Triticum aestivum L., cultivars. After a 6-h herbicidal pulse, the main metabolites were conjugates at all incubation times up to 48 h. The levels of deaminated and dethioethylated metabolites never exceeded 4% of the extractable radioactivity and also did not differ between tolerant and sensitive plants. On the contrary, 92% of the extractable radioactivity was in conjugates after 24 h in the leaves of tolerant plants compared to 25% in the leaves of sensitive plants. The differently sensitive wheat cultivars conjugated metribuzin, 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one, at about half the rate that was observed with ethiozin. This finding may explain the fact that most wheat cultivars are more sensitive to metribuzin compared with ethiozin.

A growth cabinet study was conducted to evaluate the effect of temperature on the distribution of several herbicides in quackgrass. Uniformly labeled 14C-sucrose and the radiolabeled herbicides glyphosate, sethoxydim, cloproxydim, the butyl ester of fluazifop, the methyl ester of haloxyfop, and the ethyl ester of quizalofop were applied to quackgrass grown at three day / night temperature regimes (10/5, 20/15, and 30/25 C). Seven days after treatment the plants were harvested, lyophilized, and later sectioned, mapped, and oxidized in preparation for 14C quantification. Quackgrass rhizome growth was more vigorous at 20/15 than 30/25 or 10/5 C. of the herbicides tested, haloxyfop was the most inhibitory to rhizome growth. Temperature increases from 10/5 to 20/15 or from 20/15 to 30/25 C resulted in more translocation to shoots. Increasing temperature had various effects on translocation to rhizomes depending on the chemical applied. At all three temperature regimes, more 14C was recovered from distal than basal buds in plants treated with 14C-sethoxydim. In contrast, at all three temperature regimes, similar amounts of 14C were recovered from the distal and basal buds of plants treated with 14-C-sucrose.

Two lettuce cultivars exhibiting differential levels of tolerance to thiobencarb in soil and nutrient solution assays were examined. Seedlings of ‘Dark Green Boston’ (BOS), a susceptible cultivar, were found to show significant inhibitions in foliar growth compared to the tolerant ‘Great Lakes 366’ (GLA). Reductions of 57% occurred in BOS leaf dry weights at rates of 3 μM thiobencarb as soon as 4 days after treatment. In addition, growth abnormalities including fused leaves were observed in the BOS cultivar, indicating inhibition early in leaf development at the meristem. Twenty-nine and 22% of applied 14C-thiobencarb was absorbed from nutrient solution by BOS and GLA, respectively. This difference is probably due to BOS having a 50% greater root system at the time of treatment. Greater absorption and accumulation of radioactivity in the leaves, as well as significantly greater amounts of parent 14C-thiobencarb in the foliage of BOS compared to GLA (30 and 19%, respectively) may account for the selectivity observed. Metabolism of 14C-thiobencarb occurred within 1 day in both cultivars, with the apparent production of herbicide conjugates accounting for more than 90% of the extracted radiolabel 12 days after treatment. A thiobencarb sulfoxide metabolite was not identified in these studies, indicating sulfoxide production is not a mechanism of selectivity in lettuce.

Metabolism and phytotoxicity of bentazon by suspension-cultured cells of soybean and velvetleaf were compared. Growth of suspension cells of both species was reduced when the cells were exposed to increasing concentrations of bentazon. However, soybean plants were tolerant and velvetleaf plants were susceptible to postemergence applications of bentazon. After incubation with 1 μM 14C-bentazon for 6 h, soybean and velvetleaf cells in the log phase of the culture growth cycle contained similar levels of 14C (6 nmol/g fresh weight). Of the total 14C in the soybean cells, 57 to 92% was present as the glucosyl conjugates of 6-OH- and 8-OH-bentazon with the remainder present as bentazon; the percentage depended on the phase of the culture growth cycle. Bentazon metabolism was greatest in the stationary phase of growth. Thin, transverse sections of soybean hypocotyl metabolized bentazon to the same two metabolites as soybean suspension cells did. The ratio of 6-O-glucosyl-bentazon to 8-O-glucosyl-bentazon was always greater than 1:1 for both the hypocotyl sections and the suspension cells. Bentazon metabolites were not detected in the velvetleaf cells, the velvetleaf hypocotyl sections, or the media of either species. Soybean suspension-cultured cells appear to be a valid and advantageous system for studying the hydroxylation and glucosylation of bentazon, the primary reactions believed to be responsible for detoxication of the herbicide in tolerant plants.

Ragweed parthenium (Parthenium hysterophorus L. #3 PTNHY) growing at six localities between 28°46′N and 13°O'N latitudes in India produced polymorphic ray achenes that varied in size and weight. Frequency of small achenes (1.9 to 2.2 mm) was higher at lower latitude (13°O'N) and that of large achenes (2.5 to 2.8 mm) at higher latitude (28°46′N). Achene weight was positively correlated with its germination percentage. Large-heavy achenes showed better germination than small-light ones.

The freezing resistance of overwintering buds of four perennial weed species was evaluated in artificial freezing experiments conducted in the winters of 1986 and 1987. Survival, dry weight of surviving shoots, and number of shoots produced/surviving section were determined at −4, −8, −12, −16, and −20 C for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes. The temperature required to reduce survival (LT50) and total dry weight (GR50) by 50% was determined. The response to freezing temperatures varied among species and in some cases between years. The LT50 was −7, −13, colder than −20, −17, and colder than −20, and the GR50 was −5, −11, −14, −15, and −13 for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes, respectively. The relative susceptibility to freezing among species appears to be directly related to the depth at which the overwintering buds generally are located.

Common ragweed (2n=36), giant ragweed (2n = 24), and their reciprocal F1 hybrids (2n = 30) were analyzed for a number of traits. Hybrids of the type giant ragweed♀ × common ragweed♂ were obtained via immature embryo culture. The analyses revealed a surprising contribution of the pollen parent for features such as biomass, leaf area, and specific leaf area. Leaf morphology, however, was similar to the maternal parent. One plant, originating from crosses of the type common ragweed♀ × giant ragweed♂, developed abnormally and cytological analysis revealed the presence of 24 chromosomes in its somatic cells. Probably this plant had an androgenetic origin.

Greenhouse, growth chamber, and laboratory studies were conducted to determine anatomical and morphological characteristics and cultural practices limiting the distribution of cogongrass, torpedograss, and johnsongrass in the United States. Cogongrass did not produce axillary buds along most of the rhizome nor regenerate when apical six-node-long rhizome segments were buried deeper than 8 cm. Both torpedograss and johnsongrass produced axillary buds along the entire lengths of their rhizomes. Torpedograss shoot emergence decreased at burial depths between 8 and 16 cm. Shoot emergence from johnsongrass rhizomes was not affected by burial as deep as 16 cm. Rhizomes of all three species were tolerant of desiccation. Cogongrass grew better in soil at pH 4.7 than in soil at pH 6.7, whereas torpedograss and johnsongrass grew equally well in either pH. It is postulated that cogongrass spread is limited by lack of axillary bud formation on most of the rhizome and the inability of rhizomes to send up new shoots if buried deeper than 8 cm. These factors could account for the intolerance of cogongrass to cultivation. Torpedograss appears to spread only vegetatively due to the lack of viable seed production.

Barnyardgrass [Echinochloa crus-galli (L.) Beauv. #4 ECHCG] and pigweeds (mixture of Amaranthus retroflexus L. # AMARE and A. powellii S. Wats. # AMAPO) seeded separately with alfalfa (Medicago sativa L.) in mid-August suppressed alfalfa severely before frost killed them in October and November. Some alfalfa was killed, and yield of alfalfa forage was reduced in each of three harvests the following year. These weeds did not harm alfalfa seeded in mid-September. Downy brome (Bromus tectorum L. # BROTE) and tumble mustard (Sisymbrium altissimum L. # SSYAL) suppressed alfalfa seeded in August and September. They reduced alfalfa stands and reduced yield of alfalfa forage in each of three harvests the following year. Alfalfa seeded August 27 and allowed to compete with a mixture of these species for various periods was injured most by weeds that emerged with the alfalfa and remained uncontrolled until forage harvest in May. These weeds did not reduce alfalfa yields if removed by 36 days after alfalfa emergence. Thereafter, yield decreased as the period of weed interference increased. Interference was most damaging in early spring, when growth of winter annual weeds was rapid and vigorous. Weeds seeded 65 or more days after alfalfa emergence did not reduce alfalfa yields but sometimes produced enough biomass to reduce the quality of the first-cutting alfalfa hay.

Field experiments were conducted in 1984 and 1985 at Stuttgart, AR, to investigate the interspecific and intraspecific interference of broadleaf signalgrass densities of 0, 10, 50, 100, and 150 plants/m2 with rice. In 1984, significant reductions in rice leaf area index (LAI) occurred 6 weeks after emergence with all broadleaf signalgrass densities. The first reduction in LAI occurred 8 weeks after emergence at the density of 150 plants/m2 in 1985. Densities of 50 plants/m2 or greater reduced rice dry weight 6 weeks after emergence in 1984, and the highest density of 150 plants/m2 reduced rice dry weight 12 weeks after emergence in 1985. Height of rice was reduced by densities of 100 and 150 plants/m2. Linear regression equations indicated that each broadleaf signalgrass plant/m2 reduced rough rice yield 18 kg/ha both years. Growth of broadleaf signalgrass was reduced by interspecific and intraspecific interference. The dry weight of broadleaf signalgrass increased at a decreasing rate at plant densities of 100 to 150/m2 when grown alone in 1984 and 1985, when a quadratic equation best described the response. Regression equations indicated interspecific interference from rice reduced broadleaf signalgrass dry weight an average of 48 and 81% in 1984 and 1985, respectively. The height of broadleaf signalgrass was greater when grown with rice than when grown alone.

The effect of Russian thistle density and duration of interference on spring wheat was investigated in two 3-yr field studies. In the density study, the best fit regression equation predicted a loss in spring wheat yield of 0.5, 0.5, and 0.6% in 1983, 1984, and 1985, respectively, for each percent of the total biomass contributed by Russian thistle. In 1985, when rainfall was 46% below normal, the highest weed density produced greater than 70% of the total plant biomass and reduced yields more than 50%. In contrast, in 1984 when rainfall was 65% above normal, the highest weed density produced less than 20% of the total plant biomass and reduced yields 11%. In the duration study, the predicted yield loss was 2.3 and 0.7% for 1983 and 1984, respectively, for each week of interference. In 1985, yield was not significantly reduced until after 6 weeks of interference. In both studies, spikes/m of row had the highest correlation with yield and were affected the most by weed interference.

Using an accession of velvetleaf having a high degree of purple pigmentation as the male parent made possible the identification of seedlings resulting from controlled or natural crosses. Cross-pollination, even after flowers had opened and anthers had shed pollen, was demonstrated by transmitting pollen manually. Natural outcrossing was demonstrated in the field by growing plants of the purplepigmented accession in close proximity to locally occurring velvetleaf. Insect transmission of pollen was suspected.

Patterns of weed emergence in upland and lowland soils were observed for several months after tillage by counting all emerged species. Forty to 50% of weed emergence occurred within 6 weeks of tillage on both sites. Significant emergence occurred within 3 weeks on both sites, but very little emergence occurred within 1 week in lowland soil. The data confirm the strong influence of soil tillage on weed emergence. They also suggest an influence of radiant energy on weed emergence. Peaks of weed emergence 6 or more weeks after tillage often occurred coincident with or soon after peaks of radiant energy.

Monoculture velvetleaf had greater dry weight, growth rate, leaf area index, and height than velvetleaf grown in association with conventional or no-tillage corn. Velvetleaf planted 5 weeks after corn had significantly lower dry weight, leaf area index, and height compared to velvedeaf planted at the same time as corn. The combination of interference from corn and delayed planting caused a significant reduction in velvetleaf population at the end of the season, delayed the date of 50% velvetleaf flowering, increased the number of days required for 50% flowering, and reduced the number of capsules per plant. Velvetleaf dry weight/hectare and leaf area index increased as plant population increased. Velvetleaf dry weight, leaf area, capsules, and seed/plant decreased as plant density increased. There were no differences in vegetative or reproductive growth between velvetleaf grown in conventional or no-tillage areas.

Research was conducted from 1983 through 1986 to compare the competitiveness of three cotton cultivars of different height with johnsongrass. Cultivars evaluated were: ‘1209-619-7’, ‘TAMCOT SP37-H’, and 'Stoneville 213’, which had an average height at maturity of 66, 122, and 168 cm, respectively. Each cultivar was grown with johnsongrass densities of 0, 4, and 6 plants/6 m of row with a 1-m row spacing. Johnsongrass biomass data confirmed that all three cultivars did compete with johnsongrass, but regression analyses using cultivar as a covariate revealed no difference in the competitiveness of the three cultivars.

Experiments conducted for two consecutive years indicated measurable interference of barnyardgrass, large crabgrass, and giant foxtail with container-grown Bailey's redosier dogwood as early as 21 days after transplanting of grass seedlings into the containers. Few differences in interference were seen among the three grass weeds. Grass weed densities ranging from one to six weeds per container reduced the dogwood growth. By the termination of the studies, dogwood shoot dry weight, averaged over all grass weeds, was decreased as much as 72% by five weeds per container. Eight weeks after recommended rates of fenoxaprop, haloxyfop, and ethyl ester of quizalofop were applied to dogwood, no phytotoxicity was apparent. These compounds exhibited selectivity for postemergence use in Bailey's redosier dogwood.

Corn cultivars differed in their response to imazaquin applied from 35 to 280 g ai/ha, as measured by shoot length. No cultivar was tolerant to all imazaquin application rates. There was less corn injury from imazaquin in studies conducted during the second and third years of research than in the first year. Preplant-incorporated applications caused significantly more injury than preemergence applications in two out of three studies. Lack of rainfall in 1985 and 1987 may have limited movement of imazaquin in the soil profile, which resulted in preemergence applications of imazaquin, causing very little corn injury. In all years of research, 35 g/ha of imazaquin incorporated in the top 6 cm of the soil profile resulted in 17 to 33% reduction in corn height 28 days after planting, when averaged across all the corn cultivars. Significant yield reductions (LSD = 0.10) of 45, 19, and 18% occurred in 1987 from preplantincorporated applications of 140, 70, and 35 g/ha, respectively, which had reduced corn height 46, 23, and 19%, respectively, when measured 28 days after planting.

Clomazone at 0.7 kg ai/ha or more, applied early preplant, completely controlled weeds before planting of no-till soybeans. Under low weed density (57 plants/m2 in untreated control) in 1985, grass weed control was nearly complete and not affected by clomazone application time. Late-season broadleaf weed control was less with preemergence application of clomazone at 1.1 or 1.4 kg/ha than with an early preplant or early preplant-preemergence split application of the same clomazone rate. Addition of metribuzin at 0.2 kg ai/ha overcame this control deficiency. Under greater weed densities (330 plants/m2 in untreated control) during 1986 and 1987, early preplant-preemergence split applications gave the greatest control of both grass and broadleaf weeds throughout the growing season. Preemergence application of clomazone failed to completely control common lambsquarters emerged at the time of application. Early preplant applications failed to maintain redroot pigweed control throughout the season. Differences in soybean yield were attributed to differences in weed control. No significant carryover of clomazone residue was detected through greenhouse or field bioassays.

Two field experiments were conducted in 1984 and 1985 to determine the effects of fluazifop, haloxyfop, and sethoxydim on the suppression of water uptake and competition of wheat to seedling alfalfa. In all cases, suppression of wheat growth and stand led to significant reduction in soil water use. The amounts of conserved soil water significantly correlated with increased numbers of alfalfa plants; the correlation coefficients, respectively, for mid- and late-summer plantings were 0.47 and 0.41 in 1984 and 0.90 and 0.86 in 1985. Conservation of soil water was attributed to growth reduction or cessation of wheat growth by the herbicides and, to some extent, suppression of wheat leaf transpiration. All of the three herbicides would be effective in controlling volunteer wheat in a new alfalfa planting.

Giant foxtail density in corn was greater under no-till and chisel plow tillage systems than conventional or till plant. Giant foxtail density in no-till was 1400 shoots/m2 56 days after corn planting compared to 170 under conventional tillage. Velvedeaf density was greater under conventional tillage than all other tillage systems. Velvetleaf density was 120 plants/m2 56 days after corn planting under conventional tillage compared to 20 in no-till. Control of giant foxtail was often less under no-till or chisel plow conditions than conventional or till plant with the same herbicide treatment. Giant foxtail control with metolachlor treatments was affected less by tillage than similar treatments containing alachlor. Velvedeaf control was less with conventional tillage than other tillage systems when less than 1.7 kg/ha of atrazine was applied. Corn injury was not influenced by tillage systems. Corn yield was not affected by tillage systems under weed-free conditions. Several herbicide treatments resulted in corn yield similar to the weed-free under conventional tillage, but no herbicide treatment produced corn yield similar to the weed-free control under no-till conditions.