Introduction - in dryland (rainfed) production in Kansas, winter wheat serves as the base crop, as wheat stubble provides excellent soil cover that minimizes wind and water erosion and stores water for row crops. Therefore, residue management and weed control after wheat harvest are critical to conserving plant-available water for corn, grain sorghum, and soybeans planted the following year. Increased adoption of conservation tillage practices is mostly attributed to reduced costs and the historically high efficacy of herbicides for weed control. However, repeated applications of these herbicides have resulted in the development of herbicide resistance in several weed species. Limited new herbicide options and the high cost of herbicide programs that include multiple sites of action present a major challenge in no-till crop production.
How Do Cover Crops Suppress Weeds? Research in Kansas indicates that growing cover crops after wheat harvest can suppress weeds and serve as an essential component of integrated weed management in no-till systems. Cover crops suppress weeds through competition for light and other resources, as well as through the release of allelopathic compounds. However, the ability of cover crops to suppress weeds is directly related to the amount of biomass produced, which depends on the cover crop species, planting time, seeding rate, termination time and method, and environmental conditions. In dryland regions where biomass production can be limited, grass cover crop species tend to produce more biomass and provide more effective weed suppression than legumes. For example, studies in southwest Kansas showed that winter triticale alone or triticale-hairy vetch mixed cover crops reduced kochia biomass by 98% compared to chemical fallow. However, monoculture winter pea or hairy vetch cover crops produced less biomass and had less effect on kochia biomass suppression. Studies in Trego County, KS, showed that oat-triticale-pea or oat-triticale cover crop mixtures provided 97% weed biomass suppression, and the decrease in weed biomass increased the efficacy of herbicide application to control weeds at the termination of cover crops.
Selecting and Managing Cover Crops - because water availability may limit cover crop establishment in dryland regions, selecting productive, well-adapted species that can produce more biomass will be key to achieving the best weed suppression. Biomass production from single, three-species, and multi-species cover crop mixtures tends to be similar. If weed suppression is the goal, planting grass cover crops like triticale, millets, or sorghum-sudangrass, alone or in simple mixtures that produce a high amount of biomass, may be the best option. Furthermore, the cost of simple mixtures or single-species cover crops is lower than that of more complex mixtures.
The challenge is growing cover crops that use less plant-available water so they don鈥檛 impact subsequent crop yields, but produce enough biomass for effective weed control. Using cover crops for forage provides economic benefits to offset revenue loss associated with potential decreased crop yields when cover crops are grown after wheat harvest ahead of a row crop. When grazing is desired, studies in western Kansas have shown that cover crops can be grazed without losing weed-suppression benefits. In that study, grazing or haying cover crops significantly reduced weed biomass compared with chemical fallow (which had received two herbicide applications) or standing cover crops.
Termination Timing and Weed Management - growing cover crops for a longer period will increase biomass and may provide greater weed suppression benefits than an earlier termination date. However, late termination will increase water use by cover crops and may affect subsequent crop yields due to the increased biomass production. A later termination date for cover crops may result in cover crop species such as triticale, rye, and sudangrass going to seed and having volunteer plants in the next crop. Terminating cover crops at heading or soft dough will provide adequate weed suppression while also avoiding issues with volunteers from mature seed. The planting window for cover crops will depend on the targeted weeds and the individual producer's crop rotation. For example, spring- and summer-planted cover crops will provide better weed suppression of summer annual weeds such as Palmer amaranth. Still, fall-seeded cover crops are needed to suppress kochia.
Take-Home Message
路 Cover crops after wheat harvest can provide significant weed suppression and reduce herbicide application costs.
路 Weed suppression by cover crops is directly related to the amount of biomass produced.
路 Grass cover crop species like triticale, millets, or sorghum-sudangrass, alone or in simple mixtures, produce higher amounts of biomass than legumes or complex mixtures.
路 Grazing cover crops does not impact their weed suppression benefits.
Information provided by K-State Research & Extension Agronomy specialist.