Growth chamber experiments were conducted to elucidate the morphological and physiological responses of rice to postemergence application of fenoxaprop. Two rice cultivars, ‘Newbonnet’ and ‘Mars’, and barnyardgrass were treated with 0.17 kg ai ha−1 fenoxaprop at the five-leaf stage. Within 2 days of fenoxaprop application, rice cultivars developed a white chlorotic band across leaves that were in direct contact with spray droplets. Leaf elongation rates for the two rice cultivars were inhibited by 40% after 4 days and by over 50% after 14 days. Inhibition of leaf elongation by fenoxaprop contributed to an overall decrease in leaf area and shoot dry weight. Net photosynthesis was reduced by 35% in fenoxaprop-treated plants 11 days after application, although stomatal conductance was not affected. Nitrogen fertilization prior to fenoxaprop application increased foliar injury of both rice cultivars. Visible injury symptoms showed the following order of susceptibility to foliar-applied fenoxaprop: barnyardgrass > Mars rice > Newbonnet rice.

The mechanism of glyphosate tolerance was investigated in nine birdsfoot trefoil selections that exhibited a threefold difference in glyphosate tolerance. Single-stemmed ramets (vegetative clones) established from the nine selections were used to evaluate tolerance to glyphosate, spray retention, and 14C-glyphosate absorption and translocation in growth chamber experiments. The tolerance of greenhouse-grown ramets correlated with the tolerance of field-grown plants, indicating that tolerance was not a function of plant size or affected by environment. The nine selections differed in spray retention and 14C-glyphosate translocation but not in glyphosate absorption. The differences in retention and translocation were not correlated with the level of glyphosate tolerance but could contribute to the tolerance of an individual plant. The specific activity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) (EC 2.5.1.19) ranged from 1.3 to 3.5 nmol min−1 mg−1 protein among the nine selections assayed and was positively correlated with plant tolerance level. These results indicate that the primary mechanism of glyphosate tolerance in birdsfoot trefoil is based on the level of EPSPS activity.

Glasshouse experiments were conducted to determine the effects of trifluralin on root growth and mineral relations of corn seedlings. Root weight to shoot weight ratios of corn seedlings were positively correlated to concentrations of trifluralin in soil. Root length to shoot weight ratios, however, were inversely related to trifluralin concentrations. Phosphorous and potassium concentrations in shoot tissue were reduced 60 and 35%, respectively, by 0.25 mg trifluralin kg−1 soil. Growth inhibition due to trifluralin was partially overcome by supplementing soil with nutrients.

Temperature changes prior to picloram application affects its activity in leafy spurge. Absorption of 14C picloram was directly correlated with temperature changes; each 1 C increase in air temperature 24 h before treatment resulted in a 1% increase of 14C-picloram absorption in leafy spurge and vice versa. The greatest 14C-picloram absorption averaged 47% of applied 14C when the temperature increased from 18 C to 24 or 30 C 24 h before treatment compared to 33% when temperatures were constant. Translocation of 14C picloram was more sensitive than absorption to temperature changes with 4.3 and 1% of applied 14C-picloram translocated to the roots when the plants were maintained at 12 and 30 C, respectively. Even though absorption increased directly with temperature, 14C-picloram translocation to the root system declined as temperature increased.

The differences in tolerance, morphology, and physiological response of diclofop-resistant and -susceptible wild oat biotypes collected from fields in Saskatchewan, Canada, were investigated under growth room and field conditions. Under herbicide-free conditions the resistant biotype had more upright leaves with about 12% less leaf area and 50% less leaf width than the susceptible biotype. A marked difference in the level of tolerance to diclofop was observed. Photosynthesis was initially significantly reduced in both biotypes after treatment with diclofop at the rate of 0.70 kg ai ha−1, but the resistant biotype was able to recover. Injury to the susceptible biotype was reduced by coating the seeds with 1,8-naphthalic anhydride. Differential foliar retention was not an important factor in selectivity of diclofop among the two biotypes.

Efficacy of glyphosate on bulbous oatgrass, a creeping perennial grass, was investigated in greenhouse and growth chamber studies. Glyphosate at 1.2 and 2.5 kg ae ha−1 significantly reduced corm formation and corm viability. Glyphosate controlled bulbous oatgrass when applied at the 4- to 5- or 6- to 7-leaf stage. With 2.5 kg ha−1, a 24-h period between glyphosate application and removal of the shoots was sufficient to cause maximum reduction in regrowth. Total amount of 14C translocated out of leaves treated with radiolabeled glyphosate (1.0 kg ha−1 total glyphosate) did not differ among growth stages, but more 14C accumulated in the dormant corms when glyphosate was applied at the 2- to 3-leaf stage than at the 4- to 5- or 6- to 7-leaf stages. Lack of correlation between maximum plant injury and glyphosate accumulation in corms is not understood but may be related to differences in application method in the two studies.

Suspension cultures of proso millet cells were treated with haloxyfop at different phases of growth. Treatment of 1-d cultures with 1 μM haloxyfop completely inhibited growth within 48 h. In contrast, 1 mM haloxyfop was required to elicit a similar response in 4-, 7-, or 10-d cultures. Calculated IC50 values indicated a 300-fold decrease in haloxyfop sensitivity during the period from 1 to 4 d. The observed changes in sensitivity to haloxyfop could not be attributed to changes in cell concentration during culture growth. In both 1-d and 4-d cultures, an initial rapid uptake of radiolabel was followed by a slow loss of radiolabel from cells. Almost all radioactivity extracted from 1-d and 4-d cells was present as the parent acid. Several radiolabeled compounds in addition to the parent acid were present in media. No major differences in the amounts of these materials were found between 1-d and 4-d media. Our results indicate that a special aspect of metabolism expressed during cell division is particularly sensitive to the herbicide.

The resistance of four populations of slender foxtail from England to chlorotoluron, pendimethalin, and the methyl ester of diclofop was studied in whole-plant spray assays. Two populations showed cross-resistance to all three herbicides while a third showed resistance to chlorotoluron only. Resistance, although not absolute, was sufficient to cause substantial reductions in herbicide performance at recommended field doses. Resistant (Peldon) and susceptible (Rothamsted) populations also were assessed for resistance to six dinitroaniline herbicides in a petri dish test in which the effects on shoot development were measured. The degree of resistance of the Peldon population varied considerably among herbicides, with greatest resistance to pendimethalin, whereas sensitivity to ethalfluralin, isopropalin, and trifluralin was similar to that of the Rothamsted standard. Differential response to butralin and oryzalin was smaller than to pendimethalin. The large difference between populations in their response to pendimethalin and trifluralin was confirmed in a pot experiment.

Three herbaceous regimes were established, using herbicides, to examine the effects of interference on growth and biomass partitioning in loblolly pine (Pinus taeda L.). Trees were sampled near Auburn and Tallassee, AL. Trees at the Auburn site grown with low weed interference (LWI) had 4, 10, 10, 8, and 4 times greater total aboveground biomass than did trees with high weed interference (HWI) for ages one through five, respectively. Medium weed interference (MWI, Auburn site only) resulted in three times greater biomass the first 4 yr and two times greater total biomass by the fifth year compared to trees grown with HWI. Trees growing with LWI were 5, 8, 10, and 6 times larger than those with HWI for ages one through four, respectively, at the Tallassee site. At all levels of interference, the percentage of total biomass in foliage decreased, and stem and branch components increased, with increasing tree size at both sites. Trees growing with HWI had a lower percentage of total biomass in foliage and a greater percentage of total biomass in stem than those growing with LWI when compared over a common size. Growth efficiency per tree, expressed as annual increase in stem biomass per unit leaf area (g m−2), was slightly greater for trees growing with LWI compared to HWI when leaf area index (LAI3, total surface) was less than 0.2. For LAI values greater than 0.2 the relationship was reversed. The latter contradicts the idea that growth efficiency can be used as a measure of vigor for young loblolly pine. Changes in carbon partitioning to the development of leaf area are suggested to be driving the accelerated growth responses associated with a reduction of weed interference.

Weed emergence and viable weed seed numbers were determined in field plots during a 5-yr period where reseeding was prevented. Known numbers of seeds of seven weed species were added to the native seed population at the beginning of the study. Plots were nontilled or tilled to depths of 0, 5, 10, and 15 cm early in the spring of each year. Velvetleaf, spurred anoda, hemp sesbania, morningglory species, and pigweed species emergence was significantly greater from the nontilled plots during the first year after seeds were added to the native seed population. Tillage, regardless of depth, reduced weed emergence during the first year where seeds were added to the plots but had no effect on emergence from plots where no seeds were added. Tillage during the second through the fifth year did not affect emergence regardless of the addition of seeds. of the 5-yr emergence totals, 61 to 88% of all weeds in all plots emerged during the first year and 9 to 23% emerged during the second year. The 5-yr decline rate for emergence of all weeds was exponential. Viable seeds of prickly sida, spurges, and pigweeds in the nontilled plots declined from 590, 1531, and 4346 m−2, respectively, to zero by the third year. Tillage did not affect the decline. However, weed emergence in the field indicated that a few (1.0 to 5.6 m−2) seeds of those weeds were still viable after 3 yr. In nontilled plots, many recently added seeds were on or near the soil surface and germinated during the first year. Tillage moved many seeds to sites that were unfavorable for germination and emergence during the first year.

Research was conducted to evaluate the effects of primary tillage (moldboard plowing and chisel plowing), secondary tillage (row cultivation), and herbicides on weed species changes in the soil seed bank in three irrigated row cropping sequences over a 3-yr period. The cropping sequences consisted of continuous corn for 3 yr (CN), continuous pinto beans for 3 yr (PB), and sugarbeets for 2 yr followed by corn in the third year (SB). A comparison between moldboard and chisel plowing indicated that weed seed were more prevalent near the soil surface after chisel plowing. The density of certain annual weed seed over the 3-yr period increased more rapidly in the seed bank after chisel plowing compared to moldboard plowing. Species exhibiting the most pronounced increase included hairy nightshade and stinkgrass in the PB cropping sequence and redroot pigweed and common lambsquarters in the SB sequence. Conversely, kochia seed density in the SB sequence decreased more rapidly in chisel-plowed plots. Row cultivation generally reduced seed bank densities of most species compared to uncultivated plots. Herbicide use in each cropping sequence produced a shift in the weed seed bank in favor of species less susceptible to applied herbicides. In particular, seed of hairy nightshade became prevalent in the PB cropping sequence, and seed of kochia, redroot pigweed, and common lambsquarters became prevalent in the SB sequence.

A study was conducted under controlled environmental conditions to determine the effect of soil temperature, soil moisture, and depth of seed burial on the emergence of round-leaved mallow. Emergence occurred from 5 to 30 C but was optimal at 15 to 20 C. Soil moisture had a greater effect than soil temperature on percentage emergence. Emergence progressively declined below a soil water content of −0.28 MPa, with less than 20% emergence attained at −1.03 to −1.53 MPa. In contrast, rate of emergence of round-leaved mallow was affected more by soil temperature than by moisture. A decrease in temperature from 30 to 5 C increased the time to reach 50% emergence by 10 to 12 days over the moisture regime of this study. Emergence was greatest at depths of 0.5 to 2 cm. No emergence occurred at 8 cm or below. The potential of using the findings of this study to develop cultural control strategies for round-leaved mallow is discussed.

Field studies using area-of-influence techniques were conducted in 1987 and 1988 to evaluate soybean growth and yield, and to predict soybean yield losses from photographs of jimsonweed and common cocklebur canopies. Differences in weed competitiveness within the 100-cm area of influence were induced by dates of soybean planting, locations, weed species, and years. Soybean yield losses within the first 20-cm interval from weeds correlated well with yield of all soybean plants within 100 cm of weeds (r2 = 0.86). Soybean growth responses as a function of distance from weeds were best described by complex polynomials, but simple linear functions, based on a data point from soybean plants nearest a weed and from the average of plants 60 to 100 cm from a weed, closely approximately actual yield losses (r2 = 0.96). Soybean yield losses were highly correlated (r2 = 0.84) with leaf area of weeds as viewed from directly above the weed-crop canopy. Weed canopy diameter, measured from overhead photographs 8 weeks after soybean emergence, also correlated well with soybean yield losses (r2= 0.82), but correlation with actual weed leaf area was not significant (r2 = 0.31).

‘Mars’, ‘Lemont’, and ‘Tebonnet’ rice were treated in 1987 and 1988 with fenoxaprop, sethoxydim, and haloxyfop at 3- to 4-leaf preflood and at panicle initiation postflood growth stages. Rice stunting and stand loss generally were greater from herbicides applied at postflood than at preflood with all cultivars. Mars yields were reduced from 11 to 23% by fenoxaprop at 168 and 336 g ha−1 applied preflood or postflood. Only postflood applications of fenoxaprop reduced Lemont and Tebonnet yields. Sethoxydim at 112 and 224 g ai ha−1 reduced yields of all cultivars when applied postflood, but yields were not reduced with preflood applications. Haloxyfop preflood at 140 g ai ha−1 and postflood at 70 and 140 g ai ha−1 reduced yields of Mars both years compared to the untreated control. Yields for Lemont and Tebonnet were reduced both years only when haloxyfop was applied postflood at 140 g ha−1.

Field experiments were conducted to determine the interaction of grazing and herbicide treatments on cheat control and biomass, wheat biomass, wheat grain yield, and wheat yield components. Ethyl-metribuzin at 1120 g ai ha−1 and metribuzin at 420 g ai ha−1 reduced cheat biomass 91 to 99 and 97 to 98%, respectively. Grazing had no effect on herbicide efficacy. Grazing increased cheat biomass in the check by 24% at only one location but did not affect total wheat plus cheat biomass. With one exception, controlled cheat was replaced by wheat on a 1:1 biomass basis when herbicides caused no crop injury. All herbicide treatments increased grain yield, but grazing did not alter yield. At two locations, increased heads m−2 and spikelets/head accounted for most of the grain yield increases, but at one location seeds/spikelet and weight/seed were also increased. Harvest index was unaffected.

Wild poinsettia control in peanut was evaluated following various preplant-incorporated and postemergence herbicide applications. The addition of vernolate or alachlor to a preplant-incorporated treatment of benefin improved wild poinsettia control obtained with treatments of lactofen or paraquat plus alachlor applied at the time of peanut emergence. Lactofen applied postemergence following lactofen plus alachlor applied at the time of peanut emergence provided ≥92% wild poinsettia control and peanut yields equivalent to the handweeded control. Treatments of paraquat plus alachlor applied at the time of peanut emergence followed by acifluorfen (applied once or twice) or paraquat followed by acifluorfen provided good (74 to 100%) wild poinsettia control for up to 12 weeks after planting and peanut yields not different from the handweeded plots. Treatments that did not provide at least 92% wild poinsettia control when evaluated 5 weeks after planting resulted in significant peanut yield reduction compared to the handweeded control.

The interaction of reduced rates of bentazon, chlorimuron, imazaquin, and imazethapyr with cultivation for broadleaf weed control in soybeans was investigated in field experiments conducted at three sites in Missouri in 1987 and 1988. Single reduced-rate herbicide applications provided soybean yields equal to full rates although visual weed control was slightly lower. Sequential applications of all four herbicides at reduced rates provided weed control and soybean yields equal to full-rate applications. The number of velvetleaf plants m−2 and seeds plant−1 were not influenced by herbicide, herbicide rate, or application timing. Cultivation improved weed control and soybean yield and decreased late-season weed populations and seed production.

Chlorsulfuron was preemergence applied in field trials at La Coruña, Spain, at rates from 5 to 30 g ai ha−1 (1987–88) and from 5 to 100 g ai ha−1 (1988–89) for weed control in winter and spring wheat. Excellent control of broadleaf weeds was obtained in both; however, narrowleaf control at the last evaluation (heading) was poor. Chlorsulfuron at 30 g ha−1 did not cause a residual problem in 1987–88, with only 0.24 μg kg−1 of dry soil recovered after both a winter and spring wheat cropping season. However, chlorsulfuron persistence at the same rate was very high in the dry year 1988–89, with 0.43 μg kg−1 and 0.53 μg kg−1 recovered in winter and spring wheat, respectively. ‘Cardeno’ spring wheat showed no phytotoxic symptom at any rate of chlorsulfuron from 5 to 100 g ha−1. Its yield and yield components (spikes m−2, grain number spike−1, grain weight) were greater with all chlorsulfuron rates, but not in ‘Fiel’ winter wheat for rates of 30 g ha−1 or higher.

The influence of various factors on the tolerance of corn and selected weed species to thifensulfuron was studied in greenhouse experiments. Corn fresh weight was reduced by postemergence application of thifensulfuron when applied at 180 g ai ha−1 without surfactant or at 18 g ha−1 with a nonionic surfactant. Corn was more susceptible to root exposure whereas velvetleaf was more susceptible to foliar exposure. Velvetleaf was most susceptible when plants were young, when a nonionic surfactant was added, and at 20 rather than 30 C. Simulated rainfall 8 h after application reduced velvetleaf injury by thifensulfuron at 18 g ha−1 without surfactant; however, with addition of a nonionic surfactant, velvetleaf injury was reduced only if rainfall occurred less than 2 h after application. Soil moisture level did not affect velvetleaf susceptibility.

Common ragweed was the most difficult to control of the species present and its control appeared to have the greatest effect on peanut yield. Preplant-incorporated (PPI) treatments provided no control of common ragweed but ethalfluralin plus vernolate PPI provided better than 90% control of yellow nutsedge and nearly 50% control of morningglory species. Single applications of paraquat at 0.14 kg ai ha−1 following PPI applications of ethalfluralin or ethalfluralin plus vernolate provided less than 75% common ragweed control. Sequential applications of paraquat applied 1 and 3 weeks after peanut emergence (1 + 3 WAE) provided at least 81% common ragweed control. Peanut yield with ethalfluralin plus vernolate PPI followed by paraquat 1 WAE (4400 kg ha−1) was equivalent to the handweeded yield (4470 kg ha−1). Yields were not significantly less with the same PPI application followed by paraquat 1 + 3 WAE (3730 kg ha−1) or by acifluorfen plus bentazon 3 WAE (3730 kg ha−1), and ethalfluralin PPI followed by paraquat 1 + 3 WAE (3740 kg ha−1). Ethalfluralin plus vernolate PPI and paraquat 1 WAE provided the highest net returns of $1370 ha−1.