Authors: A. Shekoofa, S. Safikhan, T. Raper, and S. Butler

 For those that have cover crops, you’re likely contemplating when to terminate.  While delayed termination can increase the amount of biomass produced and prolong the length of time that biomass remains in the system, delayed termination can also bring a few challenges- one of which is allelopathic impacts on our cash crop.  This article highlights recent research at the University of Tennessee examining how termination timing might allow us to harness these chemicals for weed suppression while minimizing the impacts on our cash crop (Fig. 1).

What is allelopathy?

Allelopathy is defined as the effect of one plant on another through the production of chemical compounds. The allelopathic toxicity of cover crops can suppress weeds, but these chemicals can also impact the following cotton crop.

 What impact might termination timing have on these chemicals?

Termination timing can change the quality and quantity of residue and the allelopathic properties of that residue. Proper termination timing could potentially reduce the allelopathic toxicity which negatively impacts cotton germination and early seedling growth.

Summary of 2018 research activity and findings

In a study during 2018, a multi-species cover crop was terminated at several different timings. These timings included: termination at planting, termination three weeks prior to planting, termination six weeks prior to planting, and termination of an ~10” band in the area where cotton rows are to be established with the remaining cover crop falling in the row middles terminated at planting. Top growth biomass of cover crops were clipped and processed in the laboratory.  The sample flow is listed in Fig. 2.

To determine the effect of potential allelopathic chemicals on cotton seedlings, diluted plant extracts were added to filter paper within petri dishes containing 10 cotton seeds, and germination properties were measured (Fig. 3).

Figure 2. The cover crop extracts were collected following the above steps for both cover crop types and termination timings of blend species.

Selecting the right termination timings

Among the termination timing treatments, the allelopathic impacts on germination were most detrimental from the ‘at planting’ timing (Fig. 4).  As termination timing moved earlier in the year, the strength of the allelopathic chemicals declined. Similar, consistent trends were noted in germination rates, the most reduction under the extract compared to water (Table 1). Earlier termination timings greatly reduced the allelopathic impacts of the leachate (Table 1).

What management strategies can be recommended?

Selection of a termination timing prior to planting minimized the potential allelopathic impacts on the emerging cotton crop, but will likely also minimize the allelopathic impacts on emerging weeds.  Based on our data from this experiment, you should exercise caution if delaying cover crop termination until planting.  Furthermore, terminating cover crops within the row has shown tremendous potential to minimize the negative impacts of the cover crop on the cash crop while maximizing the benefits of the cover crop in the system. Monitoring cover crop growth and the proper timing of strip termination will be crucial to ensure remaining covers do not overtake established strips.