Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 6/17/2006
Publication Date: 6/17/2006
Citation: Moran, P.J., Greenberg, S.M. 2006. Winter cover crops and vinegar as weed control techniques in sustainable cotton production. In: Proceedings of the Beltwide Cotton Conferences, January 3-6, 2006, San Antonio, Texas. 2006 CDROM. p. 2188-2195. Interpretive Summary: Weeds are an important obstacle to crop production, especially in organic agriculture, in which synthetic chemical herbicides cannot be used, and farmers must rely on turning the soil over (tillage), hand-weeding, the use of cover crops to suppress weed growth during the off-season, and a limited number of non-synthetic weed control chemicals that have been approved under the USDA’s National Organic Program. Organic cotton comprises a tiny fraction of U.S. cotton production and global yield, but the demand for clothing and other products derived from organically-produced cotton is growing. In the Lower Rio Grande Valley of South Texas, we partially simulated organic cotton production at our research farm by planting black oats and hairy vetch cover crops in the winter of 2004 and withholding synthetic fertilizers and herbicides. Pigweed, purslane, and several mustard and sunflower relatives were the dominant weeds observed in the winter. Total weed cover was about 20% higher under cover crops than in bare fields in which the soil had been turned over. We, therefore, concluded that tillage was slightly more effective as a winter weed control strategy than was cover crop planting. In the spring, we used tillage to kill the cover crops, and planted cotton. In some fields, only organic-approved fertilizers and insecticides were used, while other fields received conventional synthetic chemicals. The past presence of a cover crop in some organic fields did not increase the abundance of weeds in cotton rows compared to rows in organic fields that had not contained winter cover. Weeds were generally low in abundance in all fields due to continued tillage, but the ground area covered with weeds was 18% higher in conventional cotton fields that were left untilled than in any tilled organic or conventional cotton field. Tillage was clearly essential for spring weed control in cotton. To simulate organic chemical weed control, we sprayed pigweed, purslane, sunflower, and cotton seedlings grown together in a greenhouse with vinegar solutions containing 0.9%, 4.5%, and 9% acetic acid plus a soap to aid in penetration of the vinegar into leaf tissues. Young weeds (1.5 weeks-old) were 100% killed by 9% and 4.5 acid vinegar, but 9% vinegar also killed 78% of 1.5 week-old cotton and 4.5% vinegar killed 28% of cotton seedlings of this age. At the lowest dose (0.9% acid), vinegar was still effective against weeds, killing 48% or more of pigweed, purslane and sunflower, and it was much safer for cotton-0% of the seedlings were killed, although 92% of the plants suffered leaf damage. But, two weeks after being sprayed, damaged cotton plants had grown as tall or taller and had put on at least as many leaves as had other plants sprayed only with soap. The cotton plants thus recovered from vinegar damage. Older weeds and cotton (3.5 and 5.5 weeks-old) were much more resistant to 0.9% acid vinegar. A maximum of 10% pigweed and purslane, and only 17% of sunflower plants were killed by this dose. Full-strength (9% acid) vinegar killed up to 90% of sunflower and up to 68% of purslane plants, but only a maximum of 33% of pigweed was killed. No 3.5 and 5.5 week-old cotton plants were killed by vinegar, and live, damaged plants were still able to grow as tall as plants sprayed with soap only. In field tests, 9% vinegar was not as effective as in the greenhouse, killing only 27% of young pigweed and only 8% of mature, flowering pigweed. The usefulness of vinegar in organic cotton production is likely to be limited to pre-planting control of young weeds in tilled and untilled fields. Vinegar at high doses (10-20% acid) may also be useful for spot weed treatments along field edges. Additional studies are needed to find a combination of cover cropping, tillage, and organic chemical weed control strategies that maximizes weed control in organ
Technical Abstract: Organic cotton production was partially simulated in the Rio Grande Valley of South Texas. Black oat (Avena strigosa) and hairy vetch (Vicia villosa) winter cover crops were planted in plots. Six weeks after planting, pigweed (Palmer amaranth, Amaranthus palmeri), common purslane (Portulaca oleracea), and several members of the Asteraceae (sunflower family) were the most frequently-encountered winter weeds. Total weed cover was 17 to 24% higher in plots containing a cover crop than in tilled plots lacking cover, and purslane cover was 10% to 13% higher in cover crop plots. Cover crops were tilled under and cotton was planted in early spring. Seven weeks after planting, total weed, pigweed and purslane cover did not differ between plots that had contained winter cover and plots without cover. In untilled cotton plots in separate conventional fields, weed cover was 18% greater than in tilled organic or conventional plots. Cotton plants grown conventionally, with or without tillage, were 2.5-fold taller than cotton plants in the organic field. To simulate organic chemical weed control, weeds and cotton were grown together in pots in a greenhouse and exposed to foliar applications of vinegar. Young (1.5 wk-old) pigweed, purslane, and sunflower (Helianthus annuus) were 100% controlled by vinegar containing 9.0% and 4.5% acetic acid, but 78% and 28% of 1.5 wk-old cotton seedlings were also killed by these doses, respectively. Vinegar solution containing 0.9% acid caused 48% or higher mortality in 1.5 wk-old weeds, while causing no mortality in cotton. Most (92%) of these cotton plants had leaf damage, but the damaged plants attained heights and leaf counts similar to undamaged controls within 2 weeks of vinegar application. Older weeds and cotton (3.5 and 5.5 wks old) did not show significant mortality to 0.9% acid vinegar. Weeds of these ages were partially controlled (up to 33% pigweed, 68% purslane, 90% sunflower) by 9% acid vinegar. Only 10% of cotton plants of these ages were killed by 9% vinegar. In 1-m2 field plots, 9% acid vinegar killed 27% of young, vegetative and 8% of mature, flowering pigweed plants, and 27% of two-to four-leaf cotton plants. Winter cover crops do not control winter weeds as effectively as tillage, but could suppress winter and spring weed buildup under no-till conditions. Vinegar has limited utility as an organic weed control against young pigweed, purslane, and sunflower growing in cotton fields.