Competition of weeds was characterized by determining the distance down the soybean row that a weed affects soybean biomass and yield. Field studies were conducted for 2 yr to compare competitive effects of common cocklebur, johnsongrass, Palmer amaranth, sicklepod, and tall morningglory on ‘Forrest’ and ‘Centennial’ soybeans. The weeds did not significantly reduce soybean biomass for 6 weeks after emergence. Palmer amaranth, common cocklebur, and tall morningglory had the greatest biomass by 6 weeks after emergence. However, only competition from common cocklebur and Palmer amaranth measurably reduced soybean biomass during the growing season. Biomass of Forrest and Centennial soybeans was reduced when these cultivars were growing within 12.5 and 50 cm of common cocklebur, respectively. Johnsongrass, sicklepod, and tall morningglory grew more slowly than the other weeds and had no measurable competitive effects on soybean biomass. Soybean competition reduced biomass of all weeds 90 to 97%. Soybean cultivar influenced the level and duration of competitiveness depending on the weed species present. Biomass of both soybean cultivars was reduced when they were growing within 50 cm of Palmer amaranth. Soybean seed yield was reduced when soybeans were growing within 25 cm of common cocklebur and Palmer amaranth and also when they were growing within 12.5 cm of tall morningglory. Sicklepod had no effect on soybean seed yield.

Terbacil (3-tert-butyl-5-chloro-6-methyluracil), diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione], and metribuzin [4-amino-6-butyl-3-(methylthio)-as-triazin-5(4H)-one] controlled palmer amaranth (Amaranthus palmeri S. Wats. ♯ AMAPA) in established alfalfa (Medicago sativa L.) when applied after first harvest. However, these same herbicides did not control palmer amaranth when applied early-season in March. Alfalfa forage yields were increased when palmer amaranth was controlled. Residue levels of terbacil and its metabolites in alfalfa forage from plots treated postharvest with 1.1 kg/ha of terbacil were well below the 5-ppm tolerance level established for terbacil in alfalfa. There was some chlorosis of alfalfa foliage following the postharvest use of the 1.1 kg ai/ha of terbacil. There was also a yield reduction of alfalfa in two of the six experiments (one from a March and one from a June application) where 1.1 kg ai/ha of terbacil was evaluated.

Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) residue was incorporated into soil to determine its allelopathic effects on the seedling growth of grain sorghum (Sorghum bicolor L. Moench.), cabbage (Brassica oleracea, var. capitata L.), carrot (Daucus carota L.), and onion (Allium cepa L.). Root and shoot growths were equally sensitive to the toxic effects of soil-incorporated Palmer amaranth. Growth of ‘Grand Slam’ cultivar of cabbage was 17 to 30% more sensitive than the growth of 'Sanibel’ cabbage. Growth of onion and carrot seedlings was less inhibited than either cabbage or grain sorghum. Growth of grain sorghum root was severely inhibited by 8000 and 16 000 ppm of Palmer amaranth in soil and was not affected by oven dryings other than lyophilization. Seedling growth was more severely inhibited by thyrsus and leaf tissues than by stem and root tissues of Palmer amaranth.

An experiment was conducted at four locations in North Carolina during 1994 and 1995 to evaluate weed control, cotton yield, fiber quality, and net returns in no-tillage bromoxynil-tolerant cotton. The experiment focused on using bromoxynil or pyrithiobac sodium applied early POST over-the-top as alternatives to fluometuron plus MSMA applied early POST directed. Fluometuron plus MSMA was more effective than bromoxynil or pyrithiobac sodium on tall morningglory, large crabgrass, goosegrass, and broadleaf signalgrass. Bromoxynil and fluometuron plus MSMA were similarly effective on common lambsquarters, common ragweed, and eclipta and more effective than pyrithiobac sodium. Pyrithiobac sodium and fluometuron plus MSMA were similarly effective on smooth pigweed and Palmer amaranth and more effective than bromoxynil. Prickly sida control by bromoxynil and pyrithiobac sodium was equal to or greater than control by fluometuron plus MSMA. All early POST herbicides controlled pitted morningglory similarly. Regardless of the early POST herbicides used, fluometuron applied PRE and cyanazine plus MSMA applied late POST directed increased control of most weeds and increased cotton yield and net returns. Bromoxynil and pyrithiobac sodium effectively substituted for fluometuron plus MSMA only in systems that included fluometuron applied PRE and cyanazine plus MSMA applied late POST directed. Effects of herbicide systems on cotton fiber quality were minor.

The influence of two weed management systems was determined on weed seed and seedling populations and on yields of cantaloupe (Cucumis melo L. var. reticulatus Naudin ‘Perlita′), bell pepper (Capsicum annuum L. var. grossum ‘Grande Rio 66′), cotton (Gossypium hirsutum L. ‘CP 3374′), onion (Allium cepa L. ‘1015Y′), and cabbage (Brassica oleracea, var. capitata L. 'Sanibel′) sequentially grown in two 3-yr cycles. Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) did not exist initially, but hurricane-introduced seed populations increased to 1.1 billion/ha as seed populations of common purslane (Portulaca oleracea L. # POROL) decreased from 786 million/ha to 124 million/ha in the 6-yr period, without weeding or herbicide. Use of herbicides and handweeding reduced Palmer amaranth seed populations 98%, but 18 million/ha still remained after 6 yr. The use of herbicides and Palmer amaranth interference decreased the seed populations of common purslane by 84%, but handweeding was inefficient. Yields of all but the first crop of cantalouple were almost totally eliminated by season-long interference of Palmer amaranth. Savings with the utilization of herbicides rather than handweeding ranged from $62/ha for cotton to $4703/ha for bell pepper.

Over two years, acetolactate synthase (ALS)-inhibiting herbicides and herbicides with different mechanisms of action were tested individually and in combination for control of ALS-resistant Palmer amaranth in soybean. As expected, ALS-inhibiting herbicides did not control the resistant Palmer amaranth. Lactofen at 210 g/ha and acifluorfen at 560 g/ha gave the best postemergence control. Tank mixes of lactofen with either imazethapyr or chlorimuron plus thifensulfuron did not significantly increase control over lactofen alone. Sequential treatment with a soil-applied herbicide, either SAN 582 or pendimethalin, followed by lactofen postemergence, controlled weeds best (greater than 85%). The extent of cross-resistance of Palmer amaranth to ALS-inhibiting herbicides was determined in the greenhouse. Sixteen POST ALS-inhibiting herbicides were sprayed at ½, 1, 2, 4, and 8X field use rates on resistant and susceptible biotypes. All ALS-inhibiting herbicides at all rates controlled the susceptible biotype. The resistant biotype was cross-resistant to all ALS-inhibiting herbicides. Metsulfuron and imazapyr at 8X rates were the only treatments that provided 80% or greater control of the ALS-resistant Palmer amaranth. Chlorsulfuron and tribenuron were the only other herbicides that gave partial control at the highest rate. Control with these four herbicides decreased as the rate was decreased. Experimental results suggest that cultural practices and herbicides with different mechanisms of action will be required to manage this ALS-resistant Palmer amaranth biotype.

The control of puncturevine in southern peas with imazethapyr and imazaquin and the potential for carryover of these herbicides to spinach was evaluated in Arkansas. Imazethapyr (0.07 kg ai ha−1) and imazaquin (0.14 kg ha−1) applied to southern peas either PPI tank-mixed with trifluralin (0.84 kg ha−1) or PRE or POST following trifluralin controlled puncturevine at least 70%. Imazaquin and imazethapyr remaining in soil after use caused injury and yield reduction to spinach planted 3 to 4 mo after herbicide application in the fall after southern pea harvest. In spring spinach planted 8 to 9 mo after herbicide application, the only injury observed was from soil residues of imazethapyr applied POST in one of two years, with no yield reduction. In rate by timing experiments, imazethapyr at 0.07 kg ha−1 applied PPI, PRE, and 3 and 6 d after puncturevine emergence controlled puncturevine at least 95% at the 2-wk rating. At the 4-wk rating, imazethapyr at 0.07 kg ha−1 applied PRE provided this level of control. Imazaquin at 0.07 and 0.14 kg ha−1 controlled puncturevine greater than 93% when applied up to 12 d after emergence. Puncturevine control was less with both herbicides at lower rates and as application timing increased.

Field experiments were conducted in 1990 and 1991 near Artesia and Las Cruces, NM to evaluate directed POST treatments of oxyfluorfen for the control of broadleaf weeds that emerge after thinning chile peppers. At Artesia, Wright groundcherry and prostrate pigweed reduced red pepper yield by 33% in untreated plots in 1990 and clusterflower reduced yield by 19% in 1991. At Las Cruces, Wright groundcherry, Palmer amaranth and common lambsquarters reduced yield by 61% in 1990 and by 76% in 1991. A sequential treatment of oxyfluorfen at 0.14 kg ai ha-1 2 wk after thinning, when the peppers were 15 cm tall, followed by 0.14 kg ha-1 4 wk after thinning controlled all species except common lambsquarters without injuring the peppers.

Acifluorfen, lactofen, chlorimuron, thifensulfuron, imazethapyr, and imazaquin were evaluated for control of Palmer amaranth, common waterhemp, redroot pigweed, and tumble pigweed at three application timings in field and greenhouse experiments. Results from field studies indicated that most herbicides provided greater than 90% control of common waterhemp, redroot pigweed, and tumble pigweed regardless of time of application. Palmer amaranth was the most difficult species to control, and only thifensulfuron and imazethapyr provided greater than 80% control at all application timings. In the greenhouse experiment, herbicides were applied when pigweeds averaged 10 cm, 20 cm, and 30 cm in height. Results were similar to the field experiment, except that common waterhemp was more difficult to control.

Introduction of glyphosate resistance into crops through genetic modification has revolutionized crop protection. Glyphosate is a broad-spectrum herbicide with favorable environmental characteristics and effective broad-spectrum weed control that has greatly improved crop protection efficiency. However, in less than a decade, the utility of this technology is threatened by the occurrence of glyphosate-tolerant and glyphosate-resistant weed species. Factors that have contributed to this shift in weed species composition in Georgia cotton production are reviewed, along with the implications of continued overreliance on this technology. Potential scenarios for managing glyphosate-resistant populations, as well as implications on the role of various sectors for dealing with this purported tragedy of the commons, are presented. Benghal dayflower, a glyphosate-tolerant species, continues to spread through Georgia and surrounding states, whereas glyphosate susceptibility in Palmer amaranth is endangered in Georgia and other cotton-producing states in the southern United States. Improved understanding of how glyphosate susceptibility in our weed species spectrum was compromised (either through occurrence of herbicide-tolerant or -resistant weed species) may allow us to avoid repeating these mistakes with the next herbicide-resistant technology.

Field studies were conducted in 1997 and 1999 to determine the effect of a broadcast-seeded glyphosate-tolerant/conventional soybean seed mix on soybean seed yield, gross profit margin from weed management, and early-season weed suppression prior to a single glyphosate application. Tests were established by broadcasting and incorporating 494,000 seeds/ha glyphosate-tolerant soybean in combination with 0 to 1,976,000 seeds/ha conventional soybean. Glyphosate at 1.12 kg ai/ha was applied at the V5 to V6 soybean growth stage to kill all conventional soybean plants and emerged weeds. The broadcast planting technique was effective in establishing a soybean stand on a silt loam but was ineffective on a silty clay soil. Hemp sesbania and pitted morningglory control, resulting from interference with soybean, were correlated with percent soybean groundcover (R 2 = 0.961 to 0.971) and population (R 2 = 0.821 to 0.860) at the V5 to V6 soybean growth stage. Although not significant, increased yield and gross profit margin of dryland soybean occurred as the total seeding rate of the seed mix increased from 494,000 to 741,000 seeds/ha. A dryland seeding rate of 741,000 compared to 494,000 seeds/ha resulted in 438 kg/ha and $93.78/ha increase in seed yield and gross profit margin, respectively. Interference between the two soybean cultivars at 1,976,000 seeds/ha reduced yield. Irrigated soybean at 988,000 seeds/ha produced the greatest seed yield and gross profit margin at 2,987 kg/ha and $555.86/ha, respectively. Maximum yields were attributed to early-season weed suppression.