Evaluations of the effects of minimum tillage vs. conventional tillage and the effects of organic amendments (cover crops and compost) vs. no organic amendments were conducted in a California vegetable field. Weed densities were monitored, and soil samples were taken to measure the effects of the treatments on weed seedbanks and microbial biomass over a 24-mo period. Reduced tillage increased the density of shepherd's-purse in the upper soil layer (0 to 15 cm) of the soil seedbank compared with conventional tillage. Evidence is presented that suggests relationships between organic amendments, weed population reductions, and increases in soil microbial biomass: (1) shepherd's-purse emergence and seedbank densities were lower in the organic amendment plots, (2) microbial biomass was nearly always higher in the organic amendment plots, and (3) significant negative correlations between microbial biomass and burning nettle and shepherd's-purse emergence densities were found. These results suggest that organic matter addition may lead to reduced weed emergence.

Field studies were conducted to evaluate corn vigor reduction, weed control, corn yield, and economic returns in a no-till system with various herbicide strategies using full and reduced rates of acetochlor and atrazine with glyphosate, glufosinate, or imazethapyr + imazapyr in their respective type of herbicide-resistant, no-tillage corn. Crop vigor reduction due to herbicide injury was 10% or less with all treatments. A burndown plus a full label rate of a residual herbicide applied early preplant (EPP) generally provided less than 80% control of giant foxtail, common waterhemp, and common cocklebur but usually greater than 85% control of common ragweed and common lambsquarters. Two-pass strategies generally provided greater than 85% control of all species evaluated. Early postemergence, mid-postemergence (MPOST), and late postemergence strategies generally provided inconsistent and poor overall weed control. EPP–MPOST strategies generally provided lower weed control than strategies using acetochlor or atrazine EPP followed by a postemergence application. Corn yield and net economic returns followed a similar trend as weed control, with strategies that provided greater than 80% weed control showing minimal crop vigor reduction and high grain yields. Two-pass strategies with residual herbicides generally provided the highest yields, economic returns, and low coefficients of variation (CV) of net income. Although EPP strategies provided similar economic returns as some of the two-pass strategies, they had higher CVs, implying greater risk to economic return.

A survey of 174 fields was conducted during August and September of 1998 to investigate effects of cultural and herbicide practices on postharvest weed control in winter wheat stubble fields across western and southern Nebraska. Seventy-four percent of the fields were seeded at rates of 67 to 100 kg/ha, with 60% of the wheat seeded in rows spaced 25 cm apart. Wheat seeded in east–west rows contained 98% more stinkgrass and 82% more tumble pigweed than wheat seeded in north–south rows. Sixty-nine percent of winter wheat stubble fields were rated excellent for weed control. Postharvest weed control with herbicides was not affected by row spacing. In western Nebraska, density of tumble pigweed and Russian thistle was greater when wheat seeding rate was 50 kg/ha than at higher seeding rates. Short-stature winter wheat cultivars had greater densities of Pennsylvania smartweed and toothed spurge than taller cultivars. The most common winter wheat cultivars were ‘Arapahoe’ (34%) and ‘Alliance’ (17%). Weed control was positively correlated with number of winter wheat stems per square meter (r = 0.22∗∗). Density of several weed species was greater in fields seeded with a disk than with a hoe drill. The most common crop rotations sampled were winter wheat–corn–fallow (50%), winter wheat–fallow (18%), and winter wheat–corn–soybean (13%). Winter wheat yields and wheat stem densities were greater and weed density was less when winter wheat was seeded after an 11- to 14-mo fallow period rather than a 0- to 5-mo period.

Studies were initiated at two different planting dates and conducted at two different locations in 2001 to determine the critical weed-free period for certain populations of weeds in organically produced ‘Beauregard’ sweetpotato. Naturally occurring weed populations were used, and they included sicklepod, redroot pigweed, and yellow nutsedge. Treatments included allowing weeds to grow for 2, 4, 6, or 8 wk after transplanting (WAT) sweetpotato before weed removal and maintaining the sweetpotato weed-free for 2, 4, 6, or 8 WAT. Weedy and weed-free checks were also included in the study. These treatments were used to determine the length of time weeds can compete with sweetpotato without reducing yield and the length of time sweetpotato must grow before yield is no longer affected by newly emerging weeds. Yield of number one grade sweetpotato roots best fit a quadratic plateau curve for the grow-back treatments and a logistic curve for the removal treatments. Yields in weed-free plots of sweetpotato were higher at the early planting date, whereas yields in plots of weedy sweetpotato were higher at the late planting date. Weed biomass was lower in the grow-back treatments at the late planting date. Data indicate that sweetpotato may gain a competitive advantage over weeds when planted at a later date. At both planting dates, a critical weed-free period of 2 to 6 WAT was observed.

A survey of 174 fields was conducted to investigate performance of herbicides applied after winter wheat harvest on weeds across western and southern Nebraska during August and September 1998. Glyphosate plus 2,4-D plus atrazine was applied on 32%, glyphosate plus 2,4-D or dicamba on 24%, paraquat plus atrazine on 23%, glyphosate on 8%, ICIA0224 plus 2,4-D or atrazine on 10%, and atrazine plus 2,4-D on 3% of the fields. These treatments controlled 85 to 100% of the weeds except atrazine plus 2,4-D, which controlled 30%. The frequency of occurrence of the most prevalent summer annual grasses was as follows: green foxtail, 65%; barnyardgrass, 46%; stinkgrass, 41%; witchgrass, 39%; and longspine sandbur, 36%. The most common broadleaf weeds and their frequency were redroot pigweed, 32%; tumble pigweed, 30%; tall waterhemp, 28%; and kochia, 25%. Virginia groundcherry, 22%; common milkweed, 11%; yellow woodsorrel, 9%; and field bindweed, 6% were the most common perennial weeds. The five most difficult weeds to control were yellow nutsedge, spotted spurge, Virginia groundcherry, common milkweed, and toothed spurge, with control ratings of 0, 3, 17, 26, and 33%, respectively. These weeds were not controlled with glyphosate or mixtures containing glyphosate. Only 35% of the fields were treated before summer annual grasses had headed. Late applications required higher herbicides rates for effective control.

Field experiments were conducted in 1999 at Stoneville, MS, to determine the potential of multispectral imagery for late-season discrimination of weed-infested and weed-free soybean. Plant canopy composition for soybean and weeds was estimated after soybean or weed canopy closure. Weed canopy estimates ranged from 30 to 36% for all weed-infested soybean plots, and weeds present were browntop millet, barnyardgrass, and large crabgrass. In each experiment, data were collected for the green, red, and near-infrared (NIR) spectrums four times after canopy closure. The red and NIR bands were used to develop a normalized difference vegetation index (NDVI) for each plot, and all spectral bands and NDVI were used as classification features to discriminate between weed-infested and weed-free soybean. Spectral response for all bands and NDVI were often higher in weed-infested soybean than in weed-free soybean. Weed infestations were discriminated from weed-free soybean with at least 90% accuracy. Discriminant analysis models formed from one image were 78 to 90% accurate in discriminating weed infestations for other images obtained from the same and other experiments. Multispectral imagery has the potential for discriminating late-season weed infestations across a range of crop growth stages by using discriminant models developed from other imagery data sets.

Field studies were conducted from 1998 to 2000 at Belleville, IL, to evaluate tolerance and weed control in glyphosate-resistant soybean with sulfentrazone application. Sulfentrazone alone caused 14 to 16% height reduction 14 d after treatment (DAT), and sulfentrazone plus chlorimuron caused 26% height reduction 14 DAT. Minimal height reduction (0 to 11%) was observed 56 DAT. Sulfentrazone alone controlled giant foxtail 97 to 100%, yellow nutsedge 96 to 98%, common waterhemp 97 to 98%, common cocklebur 91 to 94%, and ivyleaf morningglory 100%. Sulfentrazone alone controlled common ragweed 63 to 89% and giant ragweed 50 to 72%. Sulfentrazone plus chlorimuron or cloransulam increased control of common and giant ragweed to 95% or greater. Sulfentrazone followed by glyphosate increased control of yellow nutsedge, common waterhemp, and ivyleaf morningglory compared with a single application of glyphosate. Sequential applications of glyphosate controlled weeds 93 to 100%. Sulfentrazone plus chlorimuron or cloransulam postponed the application of glyphosate at the 10-cm weed height by 12 d. Despite the injury, sulfentrazone did not reduce grain yield. Inadequate giant ragweed control reduced grain yield by approximately 48%.

Seeds were collected from field-grown glufosinate-resistant rice treated at various growth stages from the one-leaf through the boot stage with glufosinate at 0.84 kg ai/ha. The collected seeds were evaluated for 100-count seed weights, germination, and seedling vigor. Germination of the long-grain transformant CPRS PB-13 at five temperatures was unaffected by glufosinate treatment or temperature. At 14 d after initiation, germination of the medium-grain transformant BNGL HC-11/62 was reduced at 22 C when treated at the preboot timing. One hundred–count seed weights and seedling vigor were not affected for either glufosinate-resistant line.

Field experiments were conducted in north-central Oklahoma to evaluate the effects of MON 37500 at 35, 70, and 140 g ai/ha applied to wheat on rotational crops seeded no-till 16 to 29 mo after treatment (MAT). Wheat yields were not reduced by MON 37500 at site 1 but at site 2 were decreased 6, 11, and 24% by 35, 70, and 140 g/ha, respectively. Wheat yield reductions at site 2 were attributed to late seeding, small wheat growth stage, and cool, wet weather during the month after application. Corn and soybean seeded approximately 16 MAT were not visibly injured by residual MON 37500 at any rate at either site. At site 1 grain sorghum seeded 17 MAT was visibly injured by residual MON 37500 applied at 70 and 140 g/ha (soil pH value 6.4), and grain yield was reduced 58% by residual MON 37500 applied at 140 g/ha. Grain sorghum seeded 17 MAT was not visibly injured at site 2 (soil pH value 5.0), and that seeded 29 MAT was not injured at either site. Sunflower seeded 17 MAT at site 1 was visibly injured by residual MON 37500 at 140 g/ha, and yield was reduced 17% but at site 2 was not affected by MON 37500. Sunflower seeded 29 MAT was not injured at either site.

Dioscorea oppositifolia (Chinese yam) is an exotic perennial vine invading natural areas in the temperate regions of the eastern United States. Rapid early-season growth of D. oppositifolia is facilitated by an extensive tuber system. Plants can reach heights greater than 370 cm, as the plants climb trees and other vegetation. Shoot length increased 3.6 cm/d from late May to mid-August under field conditions, and primary and secondary tuber length increased 0.28 and 0.2 cm/d, respectively. This indicated rapid vegetative growth and substantial food reserves to form new plants in subsequent years. Dioscorea oppositifolia plants also formed aerial bulbils of 0.8- to 1.2-cm diameter, which are important in dissemination of the species over geographical areas. A field study indicated incomplete control from manual removal, clipping by hand, or glyphosate (2% v/v) application to control D. oppositifolia, although glyphosate was the most effective. Additionally, the use of herbicides was more efficient from a time-utilization perspective than either manual removal or clipping. In a separate study, glyphosate application at flowering was more effective in reducing D. oppositifolia growth the year after application as compared with glyphosate applications soon after emergence. Under greenhouse conditions, however, glyphosate at 0.84 kg ae/ha provided <15% control. The ester formulation of triclopyr at 2.5 kg ai/ha provided >90% D. oppositifolia control. Metsulfuron provided 31% control, and mesotrione provided 36% control and at higher rates may reduce D. oppositifolia growth. Several other herbicides having diverse modes of action provided minimal control of D. oppositifolia.

Field and greenhouse studies were conducted from 1997 to 2001 to determine cabbage response to posttransplant applications of pendimethalin (0.56 to 2.24 kg ai/ha). Differential variety response was minimal, and applications greater than 0.56 kg caused severe and persistent crop injury and reduced head number and yield in ‘Azan’, ‘Storage 4’, ‘Super Elite’, and ‘Super Red 90’. Pendimethalin (1.7 kg) applied posttransplant reduced cabbage yield weights 23, 30, and 87% with bare root, large, and small transplants, respectively. Application (0.84 kg) to soil, foliage, or soil and foliage caused 0, 81, and 82% dry weight reduction by 21 d after treatment, respectively. Anatomical analysis of two-leaf seedlings collected 3 wk after pendimethalin treatment (1.12 kg ai/ha) showed stunting of the shoot apical meristem and its emerging leaves, disorganization of apical structure with disruption of normal cell division and cell expansion, and abnormal differentiation of the vasculature in leaves and hypocotyls.

Five field studies on sandy soils with ≤ 1% organic matter in south Texas showed that halosulfuron at 0.066 kg/ha preemergence (PRE) controlled ≥ 92% purple nutsedge and at 0.066 kg/ha postemergence (POST) controlled purple nutsedge 77 to 95%. Sulfentrazone at 0.11 to 0.28 kg/ha PRE or POST controlled purple nutsedge < 65% at one location but > 75% at two other locations. Poor control at the one location may have been due to a lack of early-season rainfall or irrigation. Potatoes were stunted 5 to 26% with halosulfuron PRE, whereas POST treatments caused 7 to 40% stunting. Sulfentrazone at 0.11 to 0.28 kg/ha applied PRE or POST caused 2 to 38% stunting. ‘Atlantic’ potato stunting with sulfentrazone POST at 0.14 to 0.28 kg/ha was ≥ 20%, whereas ‘Snowden’ and ‘1625’ potatoes were stunted ≤ 20%. Potato yields were reduced 65 and 39% with sulfentrazone and halosulfuron POST, respectively, at the high rates, but yield reductions occurred with all POST treatments on Atlantic potatoes 10- to 20-cm tall. Halosulfuron PRE at 0.033 kg/ha and sulfentrazone PRE at 0.14 kg/ha did not reduce yields; however, all other treatments of halosulfuron and sulfentrazone reduced potato yields.

Field experiments were conducted in 1996 and 1997 to evaluate the tolerance of imidazolinone-resistant (IR) and non-IR corn cultivars to preemergence (PRE) and postemergence (POST) treatments of diclosulam. Crop injury was evaluated early- (5 to 6 wk after planting [WAP]), mid- (10 to 11 WAP), and late-season (13 to 15 WAP). Early-season injury of IR corn was no more than 12% in systems that included diclosulam PRE or POST at 18, 27, or 36 g ai/ha. Early-season injury of non-IR corn ranged from 85 to 89% in systems that included diclosulam PRE at any rate. At the mid-season evaluation, crop injury to IR corn was 1% or less. Non-IR corn was injured 73 to 94% in systems that included diclosulam PRE, while systems that included diclosulam POST caused 45 to 58% injury at mid-season. At the late-season evaluation, non-IR corn was injured 56, 88, and 96% with diclosulam PRE at 18, 27, and 36 g/ha, respectively, whereas systems that included diclosulam POST had 11 to 14% injury. Injury to IR corn from diclosulam PRE or POST was not apparent at the late-season evaluation. Weed-free yield of IR corn treated with diclosulam was 6,490 to 6,850 kg/ha and was equivalent to or better than yield from IR corn treated only with atrazine plus metolachlor PRE. Yield from non-IR corn treated with any diclosulam-containing system did not exceed 3,770 kg/ha.

Field experiments were conducted at four locations in Kansas in 1999 and 2000 to evaluate grain sorghum response to simulated drift rates of four herbicides. Imazethapyr, glufosinate, glyphosate, and sethoxydim were applied at 1/3, 1/10, 1/33, and 1/100 of the use rate when plants were 10 to 20 cm tall. Visible crop injury increased as rates of each herbicide increased. Glyphosate and imazethapyr caused the most injury and glufosinate the least. Data show that some plants that were significantly injured 2 wk after treatment (WAT) recovered 8 WAT. However, some plants that received the highest rate of imazethapyr or glyphosate died. Grain sorghum yields were reduced only when injury was severe. This research showed that the potential for sorghum injury from off-target herbicide drift is greater from imazethapyr and glyphosate than from sethoxydim or glufosinate.

Site-specific weed management can increase crop production efficiency by minimizing herbicide input costs without compromising crop yields. A reduction in herbicide inputs resulting from site-specific weed management may also decrease the probability level of nonpoint pollution compared with conventional herbicide applications. A 4.5-ha field was selected to compare site-specific and conventional weed management techniques in 1997 and 1998 at Knoxville, TN. Variable rate applications (VRAs) of atrazine preemergence (PRE) followed by dicamba postemergence (POST) were investigated for the reduction of herbicide inputs and their resulting impact on weed control and corn yield. VRAs of atrazine were on the basis of weed density data collected in 1996. VRAs of dicamba were according to common cocklebur density evaluations within the field. Compared with conventional applications, atrazine usage was decreased by 43 and 32% in the site-specific application treatments in 1997 and 1998, respectively. VRAs of dicamba reduced herbicide inputs by greater than 45% for 1997 and 1998. Corn yields were similar for the conventional and site-specific treatments in both years. On the basis of these data, site-specific herbicide applications have the greatest potential and least risk for managing weeds when POST or PRE + POST variable rate herbicide applications are used.

Preharvest applications of glyphosate can be useful in controlling perennial weeds. Experiments were conducted from 1996 to 1999 to determine whether preharvest glyphosate applications are affected by differences in the amount of soybean canopy present at the time of application by measuring spray deposition and subsequently horsenettle or Canada thistle control. Soybean leaf interference levels were achieved by use of three soybean cultivars with different maturity groups to achieve no leaf interference, moderate leaf interference, and maximum leaf interference, and soybean was planted in three row spacings ranging from 19 to 76 cm. As soybean leaf interference increased, spray coverage of spray deposition cards decreased. There was a similar trend for relative spray volume, determined by intensity of the color change with water-sensitive cards. Row spacing did not influence spray coverage or relative spray volume. Percent change in horsenettle or Canada thistle stems from fall to spring counts was inconsistent. Differences detected in spray coverage did not influence weed control or weed stem density the following spring.

Field and greenhouse studies were conducted near Painter, VA, in 1999, 2000, and 2001 to evaluate mesotrione postemergence for control of horsenettle in corn. Mesotrione at 105 g ai/ha controlled horsenettle at least 80% in all studies, and in 2001, after two consecutive annual applications, mesotrione controlled horsenettle up to 91%. Additions of primisulfuron, dicamba, and 2,4-D to mesotrione did not increase horsenettle control. Occasionally, combinations of dicamba with mesotrione controlled horsenettle less than did mesotrione alone, and primisulfuron combinations with mesotrione delayed or reduced development of bleaching symptoms associated with mesotrione. Initial horsenettle response to mesotrione was increased by addition of 280 g ai/ha atrazine; however, late-season horsenettle control was not improved by atrazine. Two consecutive annual applications of mesotrione alone decreased horsenettle biomass > 89%. Treatments of primisulfuron plus dicamba, primisulfuron plus CGA 152005 plus dicamba, and 2,4-D plus dicamba provided similar horsenettle control and biomass reductions as did 105 g/ha mesotrione alone.

Studies were conducted in 1999, 2000, and 2001 to evaluate broadleaf weed control in cotton with postemergence applications of bromoxynil and bromoxynil plus CGA 362622. Bromoxynil was applied at 280 and 560 g ai/ha, and CGA 362622 was applied at 0, 3.8, and 7.5 g ai/ha in a factorial treatment arrangement. Cotton injury 7 d after treatment (DAT) during the 3 yr was 11 to 35% with the herbicide mixtures, but injury did not exceed 2% 28 DAT when averaged over years. CGA 362622 plus bromoxynil controlled velvetleaf, smooth pigweed, common ragweed, and common cocklebur at least 77% 28 DAT. Control of morningglory species was at least 87% with herbicide combinations, except in 2001 when control was only 60% with 280 g/ha bromoxynil plus 3.8 g/ha CGA 362622. Bromoxynil also controlled jimsonweed, but CGA 362622 did not. Spurred anoda control pooled over years did not exceed 53% with bromoxynil or bromoxynil plus CGA 362622 mixtures. Cotton yields generally reflected the level of weed control from the herbicide treatments. In these studies, mixtures of CGA 362622 with bromoxynil controlled several broadleaf weeds better than did either herbicide alone.

Experiments were conducted from 1994 through 1996 to determine the effect timing of malathion applications have on cotton response relative to postemergence applications of pyrithiobac. Treatments were initiated approximately 1 mo after planting (four to six leaves) and included pyrithiobac at 70 g ae/ha applied alone or in combination or with malathion applied at 1 or 3 d before and at 1 or 3 d after pyrithiobac. At 7 d after treatment (DAT), pyrithiobac plus malathion applied in combination caused 34, 28, and 21% cotton injury in 1994, 1995, and 1996, respectively. Malathion applied 1 d before pyrithiobac resulted in 18% injury in 1994, 11% in 1995, and 15% injury in 1996, whereas malathion applied 1 d after pyrithiobac injured cotton 15 and 11% in 1994 and 1996, respectively. Injury of cotton subjected to malathion applied 3 d before pyrithiobac was not different from pyrithiobac applied alone. When malathion was applied 3 d after pyrithiobac, injury occurred only in 1995. At 14 and 21 DAT, the combination of pyrithiobac plus malathion caused the most cotton injury. Seed cotton yield was not adversely affected by any treatment when compared with pyrithiobac applied alone.

Field trials were conducted in Virginia during 2000 and 2001 to evaluate long-term trumpetcreeper control in corn with dicamba, BAS 654 plus dicamba, 2,4-D, CGA 152005 plus primisulfuron, halosulfuron, primisulfuron, and mesotrione. Each of these herbicides was applied alone as a single postemergence (POST) treatment or as a component of a POST herbicide combination. Trumpetcreeper suppression ratings 3 mo after treatment (MAT) revealed a general trend toward higher levels of suppression with combinations of dicamba, BAS 654 plus dicamba, or 2,4-D with any of the sulfonylurea herbicides and lower levels of suppression with applications of any of the sulfonylurea herbicides alone. Combinations of dicamba, BAS 654 plus dicamba, or 2,4-D with mesotrione also provided some of the highest levels of trumpetcreeper suppression 3 MAT in both years. At 1 yr after treatment (YAT), 2,4-D alone, BAS 654 plus dicamba, CGA 152005 plus primisulfuron plus 280 g ai/ha dicamba, primisulfuron plus 280 g/ha dicamba, primisulfuron plus 2,4-D, mesotrione plus BAS 654 plus dicamba, and mesotrione plus 2,4-D reduced trumpetcreeper stem density by at least 52% when compared with that of the nontreated control. These herbicide treatments were the only ones that provided reductions in trumpetcreeper stem density 1 YAT when compared with that of the nontreated control. In 2000 and 2001, there were few differences in corn yield among the treatments evaluated in these trials, and no treatment resulted in corn yields that were lower than the nontreated control. Acceptable trumpetcreeper suppression may be achieved during the season of treatment with any of these herbicide combinations, but only a few treatments will provide long-term trumpetcreeper control.