Winter Cover Crop Blends for Cotton Fields

Cotton Incorporated and the Missouri State Support Committee funded two experiments designed to provide information to farmers for  improving soil health in cotton fields.  The first study compares pure stands of winter cover crops to “cocktail” mixtures of cover crops on sandy at the MU Rhodes Farm and silt loam soils at the MU Lee Farm.  Test fields were bedded in the fall after cotton harvest and cover crops drill in the row middles. We are studying wheat, cereal rye, Austrian winter peas, crimson clover, tillage radishes and rapeseed.

IMG_9331
Crimson clover mixed with winter wheat. 
Austrian winter peas mixed with cereal rye.
Austrian winter peas mixed with cereal rye.
Rapeseed mixed with cereal rye.
Rapeseed mixed with cereal rye.
Winter wheat, crimson cover and tillage radish blend.
Winter wheat, crimson cover and radishes.

 

Cotton yield increases from planting wheat in row middles to reduce blowing sand was reported in early studies at the Delta Center. The yield response varies from year to year depending on the severity of spring wind storms. In the last two years, we have not found a significant yield increase from using winter crops but numerically the highest yields compared to no cover controls have been with multiple cover crop blends.  The carbon:nitrogen ratio of the residue is reduced by mixing a legume such as crimson clover or Austrian winter peas with wheat or rye. This blend promotes mineralization rather than immobilization of N when the microbes decompose the residue.  It is a better diet for the microbes and ultimately the cotton plants. 

Blends versus Pure. One of the challenges of blends is knowing how much grass to mix with the legumes.  If too much wheat or rye is added to the blend, they may choke the legumes out. However, this is not a great concern. In cold winters, we observed that the grasses actually reduce winter kill in the legumes.  Cost is the biggest factor.  Since legumes seeds are expensive relative to grasses often the blend ratios are a economic decision.  

Solid versus Middles. In the fall,  farmers must decide whether to plant winter cover crops solid over the cotton rows or just in the row middles.  Pros and cons exist for selecting both systems.  The added mulch from planting cover crops solid helps suppress herbicide resistant weeds such as Mare’s tail and Palmer pigweed.  However, depending on spring rainfall, the solid cover crops deplete the moisture in the cotton seed bed and possibly delay cotton seed germination.  If you have center pivot irrigation be prepared to irrigate early if you broadcast cover crops across the cotton rows.

Termination Timing. Another decision that will be made in the spring is when to terminate the winter cover crop.  Before the Palmer pigweed problem developed less than 10 years ago, we always waited until after wheat stems jointed to allow the straw to develop enough stiffness to avoid a “melt down” .  But now some new pre-emergence herbicides have to be applied weeks in advance of cotton planting to avoid injury.   Crimson clover and Austrian winter peas produce most of the growth in a short time in early April.  Killing them early defeats the proposed of growth them in the first place. This is a challenge that I have not found a good solution.

Winter Kill.  In one out of four years, winter kill is a major problem with cover crops especially on the northern edge of cotton production. In 2013, we had a late cotton harvest due to below average temperatures.  This, in turn, resulted in late crop crop planting.  Coupled with a brutal winter, we had a weak stand of clover, peas, radishes, and rapeseed in the spring.  We might have been able to avoid this by flying on the seed before the cotton mature to give the cover crops time to establish before the winter freeze.   The downside of flying on seeds is that applying by airplane adds more cost.  Also broadcasting results in less soil to seed contact compared to a drill.