Submitted to: Journal of Sustainable Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 29, 2007
Publication Date: August 1, 2008
Citation: Moran, P.J., Greenberg, S.M. 2008. Winter cover crops and vinegar for early-season weed control in sustainable cotton. Journal of Sustainable Agriculture. 32(3):483-506. Interpretive Summary: Weeds interfere with cotton production by taking water, light, and nutrients away from cotton plants, especially in the spring. In ‘conventional’ weed control, chemical herbicides are used to kill weeds before and after cotton planting, but they can also harm cotton plants, and they increase chemical, fuel, and labor costs. The term ‘sustainable weed control’ refers to a set of techniques that maximizes the use of on-farm resources to control weeds, and minimizes the need for synthetic off-farm resources, thus benefiting the environment. In this study, three such techniques were evaluated for two years in a cotton field in the Lower Rio Grande Valley in the southern tip of Texas. Cover cropping (the planting of black oats, and hairy vetch cover crops in the off-season) was tested, as was tillage (turning the soil over with a tractor-mounted plow). Finally, vinegar was tested as an organic herbicide, both to kill cover crops so cotton could be planted, and to control weeds that escaped the other two methods. Cover crops planted in November were checked for weeds six weeks later. Both black oats, and hairy vetch cover crops suppressed winter weeds like pigweed, purslane, and sunflower as well as did plowing in plots with no winter cover, although black oats kept weed coverage 8-17% lower than did hairy vetch. Household vinegar containing 9% acid reduced live hairy vetch cover to 5% or less in one of two tests, but was not effective in killing black oats. In the spring, the cover crops were, therefore, killed by plowing, and cotton was planted. To sustainably control spring weeds, the areas between cotton rows were plowed, and alfalfa strips were planted along field edges. Seven weeks later, these cotton fields were weedier than adjacent fields maintained with synthetic herbicides, but cotton plant heights were similar under sustainable, and conventional weed control regimes. Breakdowns in the sustainable weed control system, the withholding of tillage, or a failure of the alfalfa strips to grow, led to 60% or greater increases in weed cover, to levels expected to have negative effects on cotton. Vinegar containing 9% acid killed 80-100% of 30-day old or younger sunflower, and 10-day old pigweed, and purslane, but killed less than 50% of older, tougher pigweed, and purslane plants, and caused damage or death to young cotton plants. In conclusion, the combination of cover cropping and tillage is a good sustainable weed control technique for cotton in this region. Spraying vinegar could enhance the benefits of tillage in controlling young weed seedlings between cotton rows or in edge areas.
Technical Abstract: Weeds in cotton may be suppressed by winter cover crops and the use of organic herbicides such as vinegar. Black oat (Avena strigosa), and hairy vetch (Vicia villosa), winter cover crops were planted in a sustainable production field in the Lower Rio Grande Valley of Texas and were tilled prior to cotton planting. Palmer amaranth (Amaranthus palmeri), common purslane (Portulaca oleracea), and sunflower (Helianthus annuus)were frequently-encountered winter, and spring weeds. Both cover crops controlled winter weed coverage as well as did tillage without cover, but plots with black oats had 8–17% lower total weedy, and purslane coverage than did hairy vetch plots. Seven weeks after cotton planting, sustainable cotton plots had higher weedy cover than separate plots maintained with tillage and conventional herbicides. Cotton coverage was similar in former black oat plots and conventional plots, but was reduced 10–15% in other sustainable plots. Breakdowns in the sustainable weed management system (withholding of tillage or failure of alfalfa strips) caused spring weedy cover to increase by 60% or more in both former winter cover and no-cover plots. Application of vinegar containing 9% acid (1,550 L/ha) reduced live hairy vetch cover to less than 5% in one of two trials, but was not effective as a burndown on black oats cover. Application of 9% acid vinegar (2,980 L/ha) killed >80% of 30-day old or younger cotton, and sunflower, and 10-day old Palmer amaranth, and purslane in greenhouse, and field trials, but caused <50% mortality to mature Palmer amaranth, and purslane. More dilute vinegar solutions (0.9–4.5% acid) caused damage but little mortality on cover crops and weeds in the field. The use of covers controls winter weeds and can function as part of a tillage-based sustainable weed management system in cotton. Vinegar has limited potential to control young weeds in non-crop areas.