Seven corn inbred lines previously shown to differ in response to soil residues of chlorsulfuron were characterized as to the target-enzyme acetolactate synthase (ALS) specific activity and to its susceptibility to the herbicide. ALS from plantlets at the five-leaf stage of growth was similarly susceptible to chlorsulfuron in all lines and its specific activity in the shoots was not significantly correlated with in vivo tolerance to the herbicide. By contrast, differences in ALS specific activity in roots of plants both at the five- and three-leaf stages of growth were significantly correlated (r = 0.96∗∗ and r = 0.93∗∗, respectively) with in vivo tolerance. Correlation was also noted in extracts from cultured excised root tips (r = 0.94∗∗). Callus tissue of a chlorsulfuron-tolerant line was less affected by the herbicide and had a significantly higher ALS specific activity than callus from a chlorsulfuron susceptible line, whereas inhibition of ALS by the herbicide was similar in both lines. These results indicate that the naturally occurring differences in ALS levels in the roots of the investigated inbred lines contribute largely to the differential in vivo response observed to chlorsulfuron.

Optimum temperature for germination and emergence of redvine seed in either petri dishes or soil was 35 C. Reductions in germination and emergence occurred at constant temperatures above or below this temperature. No germination or emergence occurred at temperatures above 40 or below 25 C. Germination in petri dishes first occurred on day 8, and emergence was first observed 12 days after planting in a soil medium. Maximum redvine seedling emergence of 74% occurred in soil when seed were planted 0.5 cm deep, with decreases in emergence noted at depths below 5 cm. No emergence occurred over the 35-day period evalμated when redvine seed were planted on the soil surface or at a depth of 10 cm in soil. Redvine germination was not affected by solution pH, whereas germination did not occur when osmotic stress of as little as −200 kPa was applied to seed.

Effects of several inorganic ions on short-term glyphosate absorption by cultured plant cells were determined by measuring the glyphosate content of cells exposed to inorganic salts in a 0.01-mM glyphosate solution buffered at pH 5.7. Within 10 min, glyphosate absorption by potato and velvetleaf cells was stimulated about equally by 1.5 mM CaCl2 or 1.5 mM MgCl2 but was not affected by 1.5 mM KCl. Glyphosate absorption by both species increased linearly with log10 CaCl2 concentration; for velvetleaf cells, the response was greatest for CaCl2 concentrations greater than 0.2 mM. Over time of absorption, glyphosate concentration within velvetleaf cells approached an asymptote. Addition of 0.1 mM KH2PO4 inhibited glyphosate absorption by potato cells in the presence or absence of Ca2+ and Mg2+ salts; all other K+ salts tested had no effect. Glyphosate absorption by potato, velvetleaf, soybean, corn, and rice cells was stimulated by 5.0 mM CaCl2 but was inhibited by 0.1 mM KH2PO4 in the presence of 0.05 or 5.0 mM CaCl2.

Density and spatial interference of barnyardgrass with ‘Lemont’ and ‘Newbonnet’ rice cultivars was investigated in 1987 and 1988. Barnyardgrass reduced grain yield of Lemont, a semidwarf cultivar, more than Newbonnet, a short-statured cultivar. Linear regression indicated that season-long interference of barnyardgrass at 1 to 20 plants m–2 reduced grain yields of Lemont and Newbonnet by 301 and 257 kg ha–1 per barnyardgrass plant, respectively. Grain yields of Lemont and Newbonnet were reduced by barnyardgrass densities to 40 plants m–2 but were not affected further at densities ≥40 plants m–2. Barnyardgrass reduced straw yield of Lemont more than of Newbonnet. Although barnyardgrass culms m–2 and straw yield increased as weed density increased, the weed produced more culms per plant at 2 and 5 m–2 than at 20 to 80 m–2. In a spatial interference experiment, Lemont grain yield reduction averaged 21% when plants were grown within 0 to 25 cm of a barnyardgrass plant group (group = 4 plants per 140 cm2). However, rice yields were not reduced when barnyardgrass plant groups were 25 to 50 or 50 to 100 cm away. Additional treatments included two barnyardgrass groups spaced 20, 40, 80, or 100 cm apart When Lemont grew between barnyardgrass groups spaced 20 or 40 cm apart, grain yields were reduced 40 and 27%, respectively, compared with weed-free rice. However, Lemont yield was not reduced when rice plants grew between weed plant groups spaced 100 cm apart.

Virginia buttonweed seeds germinated at temperatures between 15 and 40 C, with optimal germination occurring at 25 or 30 C in 12 h light alternated with 15 to 25 or 20 to 30 C in darkness, respectively. Germination rate was lower in continuous darkness than in light. Germination was reduced under simulated water stress conditions with less than 10% of seeds germinating at osmotic potentials below −0.3 MPa. Overall seed germination was reduced under simulated flooding conditions; greater germination occurred in aerated water than in unaerated water. Maximum germination occurred at pH 6. Seedlings emerged from depths as great as 8 cm, with maximum emergence occurring from 2 cm.

Field experiments were conducted at Stuttgart, AR, from 1986 to 1988 to determine effects of season-long interference of red rice densities of 1, 2, 5, 10, 20, and 40 plants m–2 on ‘Lemont’ and ‘Newbonnet’ rice cultivars. This interference reduced straw dry weights of Newbonnet and Lemont by 100 and 130 kg ha–1 per red rice plant, and grain yields by 178 and 272 kg ha–1 per red rice plant, respectively. Grain yield reductions were due to decreases in panicle number and length, and in number of grains per panicle. Ten plants m–2 or more reduced height of Lemont, while 40 plants m−2 were required to reduce height of Newbonnet. Red rice at 10 plants m−2 or more reduced total milled and head rice yields of Lemont, but red rice did not affect these components for Newbonnet. Red rice interfered with rice, even at densities as low as two plants m−2, and interference was greater in Lemont, a semidwarf cultivar with mature plants 92 cm tall, than in Newbonnet, a conventional cultivar with mature plants 115 cm tall. Red rice produced more panicles m−2 and 31 to 64% greater straw dry weight when grown with Lemont than when grown with Newbonnet because red rice shaded Lemont more than Newbonnet. Red rice plants grew taller as red rice densities increased and also were taller when grown in Newbonnet than when grown in Lemont.

Replacement series and growth analysis experiments under greenhouse and field conditions quantified and explained the competitive ability of wild oats and barley. Barley was a stronger competitor than wild oats under greenhouse and field conditions. The reciprocal yield approach showed that barley's intraspecific competition was 7.3 times greater than its interspecific competition with wild oats when calculated on a dry weight per plant basis. When leaf area per plant was the yield variable, barley's intraspecific competition was only 2.4 times greater than its interspecific competition. The difference was explained by wild oats' higher leaf area ratio. Barley had a greater leaf area, root and shoot biomass, absolute growth rate, and shoot-root ratio than wild oats, but wild oats' leaf area ratio was always higher. No differences were detected in relative growth rate and net assimilation rate.

Rhizome johnsongrass grown in the greenhouse and treated with glyphosate at 1680 g ai ha−1 at an early (3- to 4-leaf) or late (6- to 8-leaf) growth stage displayed injury within a week. Plants treated with CGA-136872 or DPX-V9360 at 40 g ai ha−1 at both growth stages displayed injury 1 to 2 weeks later. CGA-136872 did not prevent regrowth at either growth stage. No regrowth occurred from DPX-V9360 or glyphosate-treated plants. Foliar absorption by greenhouse-grown plants within 24 h of application was greater with 14C-glyphosate than with 14C-DPX-V9360 or 14C-CGA-136872. More 14C-DPX-V9360 was absorbed than 14C-CGA-136872. Growth stage influenced glyphosate absorption (more by younger plants) but not CGA-136872 or DPX-V9360 absorption. Translocation of the 14C-CGA-136872 and 14C-DPX-V9360 out of the treated leaf was less than 20% of the absorbed label and was less than glyphosate translocation. Growth stage of rhizome johnsongrass at the time of treatment had no effect on the distribution of radiolabeled herbicides within 24 h.

Four years of field experiments were used to assess the competitive ability of red rice, a weedy variety of rice, and ‘Mars’, a cultivar of rice. Red rice was the dominant competitor each of the 4 yr, and an average of one red rice plant reduced Mars grain yield per plant equal to the effect of four Mars plants. Intervarietal competition was more important than intravarietal competition for Mars, whereas the reverse was true for red rice. The product of the coefficients for intravarietal competition did not significantly exceed the product of the coefficients for intervarietal competition, indicating that the two varieties were competing for the same resources. Using the reciprocal yield model coefficients from 1989, grain yield losses of Mars, due to red rice densities of 4, 16, 25, and 300 plants m−2, were predicted to approximate 13, 37, 48, and 92%, respectively, at a fixed cultivar density of 100 plants m−2.

Field experiments were established near Portageville, MO, to determine the effects of giant ragweed interference in soybeans. Threshold densities of giant ragweed that reduced soybean yield were less than two plants 9 m−1 of soybean row. This density reduced soybean seed yield 46 and 50% in 1988 and 1989, respectively, after full-season interference. The critical duration of giant ragweed interference in soybeans was between 4 and 6 weeks after emergence (WAE) in 1988 and between 2 and 4 WAE in 1989. Full-season giant ragweed interference at densities of 220 000 and 360 000 plants ha−1 in 1988 and 1989, respectively, resulted in almost complete soybean yield loss. Eight to 10 weeks after emergence of giant ragweed-free conditions were required to prevent soybean yield reductions.

Wild oat reduced light penetration and growth of dwarf hard red spring wheat in field experiments performed under nonlimiting nitrogen and moisture conditions. Wild oat grew taller than wheat and had a greater portion of its canopy above 60 cm at maturity. Light penetration in a mixed canopy was similar to that in a monoculture wheat canopy when wild oat was clipped to the height of the wheat. A mathematical model was developed which accurately predicted the reduction in the growth rate of wheat from wild oat interference. The model also predicted that interference from wild oat was due to reduced leaf area of wheat at early growth stages and low wild oat densities, and reduced light penetration to wheat leaves at later growth stages and higher densities of wild oat.

Field studies were conducted in 1986, 1987, 1988, and 1989 to determine the stability of crop loss functions across site by year environments. Environment was a significant source of variation for the soybean crop loss function as influenced by velvetleaf, but not as influenced by tall waterhemp and common sunflower. Weed density was a highly significant source of variation for all weed species studied. Regressions between percent soybean seed yield reductions and weed populations were linear. The velvetleaf interference regression equations were divided into two groups, those with high soybean-yielding intercepts and those with low-yielding intercepts, to explain the variance observed across environments. The regression equation for the high-yielding intercept group was Ŷ = 4.24X while the low-yielding group was Ŷ = 2.14X, where Y is percent soybean yield reduction and X is weed density per 10.7 m of soybean row. Tall waterhemp and common sunflower regression equations were determined to be Ŷ = 1.37X and Ŷ = 6.52X, respectively. Confidence intervals were used to account for the variance associated with the mean regression equation for each model and to develop economic threshold models that include risk aversion principles.

A weed survey of 115 Nova Scotia lowbush blueberry fields was conducted during the summers of 1984 and 1985 to determine the most common and prevalent weeds associated with blueberry production. A total of 119 weed species were observed within the surveyed blueberry fields. The most common weed species were bunchberry, colonial bentgrass, poverty oatgrass, sheep sorrel, and false lily-of-the-valley. Weeds with low relative abundance in this study, but which may become more widespread, include three-toothed cinquefoil, Kentucky bluegrass, hair fescue, canker-root, and Canadian St. Johnswort.

Several weed-free and bermudagrass competition periods were established and maintained in cotton to evaluate their influence on cotton yields and reproduction of bermudagrass. Cotton sprayed with fluazifop for bermudagrass control 4 or 8 weeks after emergence yielded 93 and 90% as much as weed-free cotton. This compared to losses of 16 and 26% when bermudagrass was permitted to compete with cotton for 12 and 25 weeks, respectively. Cotton handweeded for 8 to 12 weeks yielded the same as weed-free cotton and 9% more than cotton weed free for only 4 weeks. Bermudagrass seed production was prevented and rhizome production was negligible when weed competition did not exceed 8 weeks and weed-free periods exceeded 4 weeks. Bermudagrass produced significant amounts of seeds (45 to 88% of weedy checks) and rhizomes (9 to 31% of weedy checks) in plots weed free for 4 weeks and in plots where control was delayed for 12 weeks. Cotton grades were reduced because the cotton lint was contaminated with grass in plots weed free for only 4 weeks, and in plots where bermudagrass competed for 25 weeks.

Red rice at 20 plants m−2 was grown with two rice cultivars for 0, 40, 60, 80, 100, or 120 days after emergence. Red rice interference for 120 days after rice emergence reduced straw dry weights of Lemont and Newbonnet 58 and 34%, respectively. Grain yield of Lemont and Newbonnet was reduced 86 and 52%, respectively, by red rice interference for 120 days after emergence. Regression analyses indicated that red rice interference reduced straw dry weights of Newbonnet and Lemont 25 and 50 kg ha−1 day−1, respectively, for interference durations of 40 to 120 days after emergence. Grain yield of Newbonnet and Lemont was reduced 60 and 93 kg ha−1 day−1, respectively, for interference durations of 60 to 120 days. Negative linear relationships occurred between interference durations of red rice and plant height, panicles m−2, spikelets panicle−1, filled grains panicle−1, or panicle length of both cultivars. However, all parameters were reduced more for Lemont than for Newbonnet as interference duration increased. Head rice (whole kernels) and total milled rice yields of both cultivars were reduced by season-long red rice interference. Red rice straw dry weight and number of culms m−2 were greater when red rice was grown with Lemont than when grown with Newbonnet.

Field studies were conducted to determine the effect of varying densities of hairy nightshade and varying durations of interference on the biomass and seed yield of dry beans. As few as two hairy nightshade plants per meter of row reduced bean seed yield by an average of 13% over the 2 yr of the study. Increasing the density of hairy nightshade to 100 plants per meter resulted in bean yield losses of 77%. Hairy nightshade interference during the first 3 weeks after crop emergence was sufficient to reduce bean yields. Up to 9 weeks of hairy nightshade-free maintenance after crop emergence was required to prevent bean yield losses. Hairy nightshade was a prolific seed producer. At low infestation densities, over 45 000 seeds per plant were produced. Hairy nightshade seed production peaked at over 300 000 seeds m–1 at about 30 plants per m of row. Depending on the length of the growing season, 6 to 9 weeks of weed-free conditions after crop emergence were required to prevent hairy nightshade from producing viable seeds before the first killing frost in the fall. Results are discussed in terms of timing and longevity of control required to reduce hairy nightshade interference in dry beans.

Soils from long-term tillage plots at three locations in Ohio were sampled to determine composition and size of weed seed banks following 25 yr of continuous no-tillage, minimum-tillage, or conventional-tillage corn production. The same herbicide was applied across tillage treatments within each year and an untreated permanent grass sod was sampled for comparison. Seed numbers to a 15-cm depth were highest in the no-tillage treatment in the Crosby silt loam (77 800 m–2) and Wooster silt loam (8400 m–2) soils and in the grass sod (7400 m–2) in a Hoytville silty clay loam soil. Lowest seed numbers were found in conventional-tillage plots in the Wooster soil (400 m–2) and in minimum-tillage plots in the Crosby (2200 m–2) and Hoytville (400 m–2) soils. Concentration of seeds decreased with depth but the effect of tillage on seed depth was not consistent among soil types. Number of weed species was highest in permanent grass sod (10 to 18) and decreased as soil disturbance increased; weed populations were lowest in conventional tillage in the Hoytville soil. Common lambsquarters, pigweeds, and fall panicum were the most commonly found seeds in all soils. Diversity indices indicated that increased soil disturbance resulted in a decrease in species diversity. Weed populations the summer following soil sampling included common lambsquarters, pigweeds, fall panicum, and several species not detected in the seed bank.

Experiments were conducted to evaluate the effects of preemergence herbicides on the growth characteristics and competitiveness of sicklepod and common cocklebur to soybean. Alachlor was applied at 1.1 or 2.2 kg ai ha–1 and metribuzin at 0.2 or 0.4 kg ai ha–1 in glasshouse and field experiments. Alachlor at 2.2 kg ha–1 reduced sicklepod dry weight and leaf area 4 weeks after planting. Sicklepod water use was initially lowered by both rates of alachlor while soybean and common cocklebur water use was initially lowered by metribuzin at 0.4 kg ha–1. Sicklepod and common cocklebur fresh weights were reduced at soybean harvest by the high rates of alachlor and metribuzin. Soybean competing with sicklepod injured by 1.1 or 2.2 kg ha–1 of alachlor produced yields greater than soybean competing with untreated sicklepod. Soybean yields were not improved when metribuzin was used on common cocklebur.

Growth characteristics and competitive relationships of cotton and three perennial weeds (johnsongrass, purple nutsedge, and yellow nutsedge) were investigated at the USDA Cotton Research Station in Shafter, CA. In growth analysis experiments, yellow nutsedge and johnsongrass had highest values for height, biomass, leaf area production, growth rate, and photosynthetic efficiency. The weeds exhibited greater overall resource use and production efficiency than cotton, while cotton attained greater leafiness and canopy closure than weeds over the 10-wk experiment. Linear correlation indicated that most growth variables were significantly correlated with aggressivity (competitiveness); however, all correlation coefficients were less than 0.80. Stepwise multiple regression, using aggressivity as the dependent variable, defined 4 growth variables out of 12 that best described competitiveness. These were unit leaf rate (ULR), height, relative growth rate (RGR), and initial propagule weight (PWT). Thus, parameters of light utilization (ULR, height) and early establishment (RGR, PWT) were best predictors of competitive success in this system.

Several weed-free and barnyardgrass competition periods were established and maintained in cotton in field plots to evaluate their influence on cotton yield and seed production of barnyardgrass. When barnyardgrass was sprayed with fluazifop 3, 6, 9, or 12 weeks after cotton emergence, only plots sprayed at 3 weeks yielded as much cotton as plots kept weed free. Barnyardgrass permitted to compete for 6, 9, 12, and 25 weeks reduced cotton yields an average of 21, 59, 90, and 97%, respectively. When barnyardgrass was seeded into weed-free plots at 3, 6, 9, and 12 weeks after cotton emergence, a weed-free period of 9 weeks was required to prevent significant cotton yield reduction. Cotton in plots weed free for 3 and 6 weeks yielded an average of 13 and 87% as much seed cotton, respectively, as weed-free controls. Barnyardgrass produced 15 to 85% as many seeds as weedy check plots in plots weed free for less than 9 weeks and in plots where control was delayed for 9 and 12 weeks. Cotton grades were reduced because of grass in plots weed free for 3 and 6 weeks, and in plots where barnyardgrass competed for 9 and 12 weeks.