Greenhouse and laboratory studies were conducted to determine the physiological basis for CGA-248757 and flumiclorac selectivity in five plant species. CGA-248757 and flumiclorac selectively control weeds postemergence (POST) by inhibiting protoporphyrinogen oxidase (Protox). Injury symptoms from CGA-248757 and flumiclorac include rapid desiccation and necrosis similar to injury from diphenyl ether and bipyridinium herbicides. Species sensitivity to CGA-248757 and flumiclorac was evaluated by comparing the dry weight reduction from POST applications. Abutilon theophrasti was sensitive to both herbicides, Amaranthus retroflexus was more sensitive to flumiclorac than CGA-248757, Brassica kaber was sensitive to CGA-248757 but tolerant of flumiclorac, and Zea mays and Glycine max were tolerant of both herbicides. Studies evaluated CGA-248757 and flumiclorac retention, absorption, translocation, and metabolism. Enhanced herbicide metabolism contributed to the tolerance of A. retroflexus to CGA-248757 and B. kaber to flumiclorac. Decreased herbicide retention, absorption, and translocation coupled with increased metabolism contributed to Z. mays tolerance of CGA-248757 and flumiclorac. Decreased herbicide retention and increased herbicide metabolism provided G. max tolerance of both herbicides.

Eighteen Lolium spp. (ryegrass) accessions collected in 1998 from several locations in Arkansas were tested for resistance (R) to diclofop in both seed and whole-plant response bioassays. Eleven accessions were L. temulentum and eight were L. perenne. Fourteen of eighteen accessions were confirmed resistant to diclofop in whole-plant assay. Three of the susceptible (S) accessions were L. temulentum. The GR50 (diclofop concentration that reduced shoot or root length by 50%) R/S ratios based on whole-plant response were greater than those of the seed bioassay in all test populations, indicating that the whole-plant bioassay was more sensitive than the seed bioassay for determining diclofop resistance in Lolium spp. The most resistant (#18) and most susceptible (#3) accessions of L. temulentum were used for multiple resistance and enzyme assay experiments. Based on whole-plant bioassay, accession #18 was 411 times more resistant to diclofop than the susceptible accession #3. Accession #18 exhibited cross-resistance to fenoxaprop and multiple resistance to chlorsulfuron applied preemergence or postemergence. Resistance to other herbicide families was not observed. Resistance to chlorsulfuron was not detected in the seed bioassay. Acetyl-CoA carboxylase (ACCase) from accession #18 was 833 times more resistant to diclofop and 10 times more resistant to sethoxydim than ACCase from accession #3. Cross-resistance to sethoxydim was not observed at the whole-plant level. Resistance to diclofop among Lolium spp. from Arkansas may be due to an alteration in the target enzyme, ACCase.

Studies were conducted to compare physiological characteristics of linuron-susceptible and -resistant Portulaca oleracea. The susceptible biotype had heavier seed and germinated more rapidly than the resistant biotype. Eight weeks after seeding, fresh and dry weights of susceptible P. oleracea plants were significantly greater than those of resistant plants. Susceptible P. oleracea had a significantly higher CO2 assimilation rate at 30 C, but was similar to that of resistant P. oleracea at 40 C. The susceptible biotype had a significantly higher CO2 assimilation rate at CO2 concentrations ≥ 600 ppm and at incident light levels ≥ 900 µmol m−2 s−1. The susceptible biotype had higher carboxylation and photochemical efficiencies.

The effect of two commonly used desiccant tank mixtures on Glycine max seed production and viability when applied at G. max growth stages R5 to R8 (R5 = beginning pod fill, R6 = full green pod, R7 = beginning maturity, R8 = full maturity) was evaluated. Two mid-Group IV G. max cultivars, one conventional (‘Northrup King 4260’) and one glyphosate-tolerant (‘Asgrow 4701RR’), were evaluated. Tank mixtures containing either glyphosate + sodium chlorate or paraquat + sodium chlorate + surfactant reduced yield, seed weight, and subsequent germination, emergence, and seedling growth when applied at R5 or R6 to either G. max cultivar. Glyphosate + sodium chlorate applied at R7 reduced germination of the glyphosate-tolerant cultivar. When applied at R6 or R7, glyphosate + sodium chlorate reduced the next generation's seedling length compared to paraquat + sodium chlorate. Glyphosate + sodium chlorate increased the number of abnormal seedlings produced in the conventional cultivar compared to paraquat + sodium chlorate. These results indicate that preharvest desiccant applications should not be made prior to G. max maturity (R7).

Group IV Glycine max cultivars, chlorimuron postemergence, and paraquat + sodium chlorate applied as preharvest desiccants were evaluated for their effect on weed seed production and viability. Early-maturing G. max cultivars reduced seed weight, seed production, and seedling growth in Ipomoea lacunosa in most instances and usually reduced seed weight, germination, emergence, and growth in Sesbania exaltata by allowing harvest prior to physiological maturity of these weeds. Tall, late-maturing G. max decreased weed seed production and seed weight of both species, presumably through increased competitiveness of G. max. The seeds produced by weeds that emerged after chlorimuron application were usually lower in weight, germination, and emergence, and growth of seedlings was reduced because most weeds present at the time of application were controlled. Thus, only late-emerging or stunted plants were present to produce seed. Preharvest desiccation of late-maturing S. exaltata parent plants with paraquat + sodium chlorate reduced height and fresh weight of the resulting S. exaltata seedlings 35 to 50% by reducing the amount of time for the S. exaltata seeds to mature.

Seed production of weedy species of Setaria in crops of Zea mays and Glycine max was studied for 2 yr in western Minnesota and eastern South Dakota. Viable seed production was curvilinearly related to panicle length. A 100-mm-long panicle of S. pumila, S. faberi, and S. viridis produced 129, 323, and 851 viable seeds, respectively. Values were consistent across years, crops, and herbicide treatments. Frequency distributions of panicle lengths of all panicles within a population closely followed nonlinear Weibull functions and were stable across years and crops but not species or herbicide treatment. Positive skewness of these distributions decreased, and median panicle size (mm) increased, in the following order: S. viridis (41), S. pumila (52), and S. faberi (78). Postemergence herbicides applied at full label rates increased skewness and reduced median panicle size (to 11 mm) and seed production of S. viridis. Skewness lessens the reliability of using average panicle size as a measure of seed production for the entire population. However, integration of panicle size–frequency and panicle size–fecundity relationships provided estimates of the number of seeds per panicle that were more representative of the population than the statistical average panicle. These estimates were 52, 242, and 246 seeds per panicle for S. pumila, S. viridis, and S. faberi, respectively. Multiplication of these values by panicle densities generated seed production estimates that were similar to actual counts of seeds. Setaria seed production tended to be higher in Z. mays than in G. max only because of higher plant and panicle densities. Early-maturing panicles tended to be larger than those maturing later, but seed viability generally was stable across maturity times.

Traditionally, herbicide efficacy has been evaluated by visual ratings, but these data provide little insight to the biological response of weeds to herbicides. Field studies were conducted in 1995 and 1996 to determine the rate response of Setaria faberi seedling survival, seed production, and biomass to postemergence herbicides in Zea mays and Glycine max. Nicosulfuron and sethoxydim were applied to Z. mays and G. max, respectively, at 1×, 12×, 14×, 18×, 116×, 132×, and 0× the label rate. Mature plant density of S. faberi was linearly related to seedling density, indicating that seedling survival was not density dependent. Based on a nonlinear dose–response analysis, maximum S. faberi survival was 55% in Z. mays across years and 60 and 45% in G. max in 1995 and 1996, respectively. Minimum survival was 0% except for Z. mays in 1996 when it was 13%. The minimum survival was greater in Z. mays in 1996 due to greater survival of late cohorts than in 1995. Setaria faberi seedling survival was greater in 12× than 1× herbicide treatments in Z. mays and G. max each year. Setaria faberi seed production was related to mature plant density with a negative exponential function. Seed production per plant was similar between 1× and 12× rates in Z. mays and among 1×, 12×, and 14× rates in G. max each year. However, seed production per square meter was greater in 12× than 1× treatments due to greater seedling survival. Regardless, seed production per square meter was 95% less in the 12× herbicide treatment compared to seed production by untreated plants in Z. mays and G. max.

The floristic diversity and the vertical distribution of the weed seedbank were studied in ridge tillage (RT) and conventional tillage (CT) systems in clay and clay loam soils. Viable seedbank populations were monitored during 3 yr using germination in a greenhouse. Ridge-tilled fields had a larger soil seedbank (2,992 seeds m−2) than moldboard-plowed fields (1,481 seeds m−2) in the top 15 cm. This result can be explained by the larger perennial seedbank of RT fields at both the 0- to 5-cm and 5- to 15-cm depths. Annual dicot seeds were more abundant in clay soils than in clay loams at the two soil depths. Annual grass seeds were more abundant under CT than under RT in clay soils at the two sampled depths. In clay loams, the density of annual grass seeds in RT fields was six times greater than in CT fields in the top 5 cm of soil and two times greater at the 5- to 15-cm depth. The vertical distribution of total seeds in soil did not differ between tillage systems. The top 5 cm of the 15-cm soil core contained 35 and 46% of all weed seed in CT and RT systems, respectively. However, the CT system had the highest concentration of annual dicot seeds 5 to 15 cm deep, whereas in the RT system, the same depth contained the highest concentration of perennial seeds. These results confirm that tillage systems and soil types can regulate seedbanks. Weed management programs must take this information into account.

Field studies were conducted to determine the effect of early-season and early- plus late-season acetolactate synthase–resistant Helianthus annuus interference on Glycine max and H. annuus growth and yield at two sites in Missouri. Helianthus annuus densities of 3 plants m−2 were established shortly after G. max emergence in all plots except the weed-free check. To study early-season interference, H. annuus were removed with postemergence glyphosate (0.84 kg ae ha−1) 2, 4, 6, and 8 wk after planting (WAP) and kept weed-free for the rest of the growing season. Glycine max yields were not different with 2, 4, 6, or 8 wk of early-season interference at either location. To study early- plus late-season interference, H. annuus densities were established at 3 plants m−2. They were then removed 2, 4, 6, or 8 WAP with glyphosate and subsequently reestablished at the same density within 2 wk after removal by newly emerging and transplanted H. annuus. These H. annuus were allowed to remain in the field for the remainder of the growing season. This provided a weed-free period of approximately 2 wk during the growing season beginning 2, 4, 6, or 8 WAP. Season-long interference and no-interference treatments were also included. Glycine max yields were reduced 47 to 72% with season-long interference. Helianthus annuus vegetative dry matter was approximately 56% lower at Columbia than at Miami. Glycine max yields tended to increase as the weed-free period was delayed into the growing season. Early-season weed-free periods (2 to 4 and 4 to 6 WAP) allowed H. annuus to become re-established before G. max formed a canopy and resulted in larger amounts of H. annuus biomass and seed production as well as G. max yield losses of 15 to 80%. Re-establishment of H. annuus in 6 to 8 WAP and 8 to 10 WAP weed-free treatments generally resulted in the plants surviving for only a few weeks after establishment and not producing seed or reducing G. max yield.

Experiments were conducted in controlled environments and in the field on winter-hardy Triticum aestivum and three weed species commonly found in cereal fields in the United Kingdom to examine whether overwinter shoot growth of individual plants could be described by accumulated thermal time calculated using base temperatures derived from growth cabinets. Individuals of each species were grown in a controlled environment at constant temperatures ranging from 5 to 20 C and harvested sequentially. Base temperatures for the increase of dry weight and green area were estimated by fitting a simple linear model describing growth in response to the thermal sum. The estimated base temperatures for shoot dry matter accumulation of Alopecurus myosuroides, Stellaria media, Galium aparine, and T. aestivum were −0.8, −3.3, −1.4, and 0.2 C, respectively, and estimated base temperatures for increase in green area were 0.4, −1.7, 1.9, and 2.2 C, respectively. Each species also was grown in monoculture in the field over 2 yr at a range of sites to examine whether the base temperatures estimated from the controlled environment studies could be used to model weed and crop growth in response to thermal time in the field. The field data were described well when an expolinear function was fitted to accumulated thermal time calculated using the base temperatures derived from the controlled environment studies.

Amaranthus retroflexus L. is a common annual weed worldwide. It can be found in a wide range of habitats and causes substantial yield reduction in many crops mainly through competition. Alternaria spp. are airborne molds that are considered to have potential for the biological control of weeds. The aim of this study was to assess the effect of spore concentration, host-plant growth stage, dew period, and temperature on the pathogenicity of three Alternaria alternata isolates against A. retroflexus. The pathogenicity of A. alternata increased with increasing spore concentration and length of dew period. A spore concentration of 107 spores ml−1 in a rapeseed oil emulsion and given a 24 h dew period caused 100% mortality of A. retroflexus plants at the four-leaf stage. Infection and mortality in older plants (>four-leaf stage) was lower. The highest levels of plant mortality were obtained at post-inoculum temperatures between 20 and 30 C. These experiments confirm the potential of A. alternata as a mycoherbicide under specific environmental conditions.

Seed production and seedling emergence of three broadleaf weed species were evaluated following glyphosate application at initial seed set, mid seed fill, and physiological maturity. In greenhouse experiments averaged across glyphosate rates of 0.42, 0.63, and 0.84 kg ae ha−1, Xanthium strumarium 100-bur weight and burs per plant were reduced at least 69 and 70%, respectively, for application at initial fruit set compared with later applications, and seedling emergence was 3% of the nontreated check. Glyphosate application at initial seed set reduced Sesbania exaltata 100-seed weight 73%, seed per plant 86%, and seedling emergence 94%. Senna obtusifolia 100-seed weight, seed per plant, and seedling emergence were reduced 46, 83, and 66%, respectively, when glyphosate was applied at initial seed set. In field experiments, X. strumarium and S. exaltata seed production were reduced only when glyphosate was applied at initial seed set. Compared with the nontreated check, seedling emergence following initial seed set application was reduced 82% for X. strumarium and 94% for S. exaltata. S. obtusifolia response in the field was inconsistent with no reductions in seed per plant or seedling emergence observed the first year. The second year, initial seed set application reduced seed per plant 88% and seedling emergence 72%.

Most farmers now rely on herbicides and, to a lesser extent, cultivation to control weeds in Glycine max in the Midwest. However, the general public is concerned that widely used herbicides will contaminate surface and groundwater. Alternative ways to control weeds in field crops are needed to reduce or prevent herbicide contamination of surface and groundwater. A new between-row-mowing weed management system that consists of band-applied herbicides over crop rows + two or more between-row mowings was tested in G. max over 6 yr in Missouri. Mowing weeds close to the soil surface two or more times between crop rows killed or suppressed annual grass and broadleaf weeds, chiefly Setaria faberi, Ambrosia artemisiifolia, and Amaranthus spp., if properly timed. Shading by crop canopy closure contributed to weed suppression in this weed management system. G. max yield also could not be distinguished from weed-free check plots and was greater than the weedy check plots. Herbicide use was reduced 50% by banding because only 50% of the field area was sprayed. The between-row-mowing weed management system may have use in environmentally sensitive areas to help reduce soil erosion or water contamination by herbicides.

Field studies were conducted in 1996 and 1997 to determine the response of Cyperus rotundus and Cyperus esculentus, Kyllinga brevifolia, and Kyllinga gracillima to mowing regimens common to recreational turfgrass. Treatments were selected to simulate Cynodon dactylon golf course management and included mowing at 1.3 and 3.8 cm with mowing frequencies of three times per week and once a week, respectively. A nonmowed check was included for comparison. Reductions in C. rotundus shoot number were observed beginning 6 wk after initial treatment (WAIT) in 1996 and 9 WAIT in 1997 for the 1.3-cm mowing regime. The 3.8-cm mowing regime did not reduce C. rotundus shoot number until the final evaluation of each year. Reductions in C. rotundus rhizome length, tuber number, and tuber size were observed for both mowing regimes in both years. Cyperus esculentus shoot number was reduced by the 1.3-cm treatment at each evaluation date in 1996 and 1997. Cyperus esculentus shoot number reductions in the 3.8-cm regime were first observed 4 and 6 WAIT in the 2 yr and continued until termination. The 1.3-cm regime reduced C. esculentus spread beginning 6 WAIT in 1996 and 3 WAIT in 1997. Cyperus esculentus spread was also reduced by the 3.8-cm treatment, but reduction began at later evaluations (8 and 9 WAIT). Tuber production by C. esculentus was completely inhibited by the two mowing regimes in both years. The only treatment effect observed in K. brevifolia and K. gracillima in 1996 was a reduction in internode length of K. gracillima by the 1.3-cm mowing regime. In 1997, the 1.3-cm regime reduced K. brevifolia shoot number at 15 and 18 WAIT and plant spread beginning 6 WAIT and continuing until termination. The 3.8-cm treatment did not affect K. brevifolia shoot number and reductions in spread were only observed at the final evaluation. Kyllinga gracillima shoot number and plant spread were reduced by the 1.3-cm mowing regime at each 1997 evaluation. Reductions in K. gracillima shoot number occurred at the final evaluation, and reductions in spread began 12 WAIT when subjected to the 3.8-cm treatment. Both mowing regimes reduced K. brevifolia and K. gracillima internode length. Kyllinga brevifolia total rhizome length and total node number were reduced by the 1.3-cm regime only. Kyllinga gracillima rhizome length, internode length, and node number were reduced by both regimes in 1997.

Field and greenhouse studies were conducted to evaluate the responses of several imidazolinone (IMI)-resistant Amaranthus hybridus populations to various acetolactate synthase (EC 4.1.3.18)-inhibiting herbicides. In 1996 field studies, IMI resistance was confirmed in one A. hybridus population (R4) that was not cross-resistant to the sulfonylurea herbicides CGA-277476, chlorimuron, or thifensulfuron. Amaranthus hybridus control with triazolopyrimidine herbicides was ≤ 35%, but control with cloransulam-methyl or flumetsulam plus cloransulam-methyl was higher than with IMI herbicides. Follow-up greenhouse studies were conducted in 1997 and 1998 to investigate the response of one IMI-susceptible (S) A. hybridus population collected near Painter, VA, and four IMI-resistant A. hybridus populations (R1, R2, R3, R4) collected from fields in Somerset County, MD, to postemergence imazethapyr, chlorimuron, thifensulfuron, pyrithiobac, and cloransulam-methyl applications. Resistance to imazethapyr was confirmed in all R populations, and no practical level of cross-resistance to chlorimuron, thifensulfuron, pyrithiobac, or cloransulam-methyl was detected. Based on resistance ratios, R populations were slightly more tolerant to chlorimuron and slightly more sensitive to pyrithiobac, thifensulfuron, and cloransulam-methyl than the S population.

To determine simazine movement and dissipation in a drip-irrigated Vitis vinifera vineyard under two irrigation schedules [ grower standard (GS) and current evaporation/transpiration (CET)], field experiments were conducted in a Hanford fine sandy loam, a soil type prone to leaching. In experiment 1, simazine was surface-applied in a 1.7-m swath down the vine row, and chloride was applied as a tracer. Total recovery of simazine was < 1.0% under the irrigation emitters 51 and 57 d after simazine application in 1997 and 1999, respectively. Simazine was not detected in the soil profile from 0 to 150 cm deep, 1.0 m from the emitters. A chloride tracer moved to a soil depth of 90 cm but not deeper. In experiment 2, simazine moved 75 cm under the emitters in 7 d but did not move deeper into the soil. Under the emitter, 28% of applied simazine was found 0 to 45 cm deep and 3% was > 45 cm deep. In experiment 3, which was conducted in the absence of irrigation, total recovery of simazine was 30% when sheltered from rain and 8% when exposed to rain. Rapid dissipation and proper irrigation management were key factors preventing deep percolation of simazine in these studies.