Authors: Avat Shekoofa, Sara Safikhan, Tyson Raper, and Shawn Butler
Cover crops have been used to suppress weeds, reduce erosion, and increase water infiltration for many years. While cover crops can improve soil quality and physical properties, integration of cover crops into row crop production-specifically cotton- remains challenging. One potential negative impact on cotton growth may come from allelopathy. Allelopathy is defined as the direct or indirect harmful or beneficial effects of one plant on another through the production of chemical compounds that escape into the environment (Fig. 1). Although allelopathic toxicity of cover crops can suppress weeds and therefore assist in weed control, they may also suppress cotton germination and reduce stand. Little is currently known about the actual allelopathic effects of cover crops on germination and seedling growth of cotton. Proper selection of cover crop species and termination timing could potentially reduce the allelopathic toxicity which negatively impacts cotton germination and early seedling growth.
During 2018, we completed two allelopathy studies that will be of particular interest to those considering planting cover crops in front of their 2019 cotton crop. The first study examined allelopathic impacts of five common cover crops species (oats, winterpea, rye, vetch, and wheat). Unfortunately, the radish and turnips planted winter-killed and were not included in this evaluation. The second study examined termination timings of a multi-species cover crop; termination timings included termination at planting, termination 3 weeks prior to planting, termination 6 weeks prior to planting, and termination of the cover crop within the furrow 6 weeks prior to planting with the remaining cover crop in the middles terminated at planting. Each treatment was tested to determine the allelopathic impacts of cover crops on cotton seedling germination and growth.
Topgrowth biomass samples of cover crops were harvested and delivered to the laboratory at West Tennessee Research and Education Center (WTREC), University of Tennessee in Jackson to collect samples leachates. The sample flow is listed in Fig. 2. For both the cover crop species and termination timings, the stock extracts were diluted. Then, ten seeds of ‘PHY 490 W3FE’ (Dow AgroSciences, Indianapolis, IN) were uniformly placed on a piece of filter paper containing 4 milliliters of the diluted extract in each petri dish per treatment. Four milliliters of the diluted extract were applied to the filter papers every other day during the experiment, per treatment (Fig. 3).
Selecting the right cover crop types and termination timings
While all cover crop leachates negatively impacted cotton germination and seedling growth, the germination percentage of cotton seeds were most inhibited by winterpea; the highest concentrate of winterpea extract inhibited cotton germination by almost 55% and drastically reduced the root length of cotton seeding (Fig. 4).
Among the termination timing treatments, termination ‘at planting’ suppressed cotton germination percentage the most, with the germination of cotton seeds reduced by as much as 62% at the highest extract concentration. Earlier termination timings greatly reduced the allelopathic impacts of the leachate.
Although all five tested cover crop species negatively impacted germination and root growth due to their allelopathic impacts, winterpea inhibited germination of cotton seeds the most. Keep in mind radish and turnips were not evaluated due to winterkill.
Terminating ‘at planting’ drastically reduced the germination percentage. Earlier terminations reduced the negative allelopathic impacts.
Proper selection of your cover crop species and management prior to planting cotton will be critical in maximizing the benefits and minimizing the risks of a cover crop.