Is Cover Crop Allelopathy Affecting Subsequent Row Crops?
Cover crops (CC) are being investigated throughout the U.S. for their ability to improve soil health and suppress problematic herbicide-resistant (HR) weeds. The use and effects of CC in crop production systems have been documented in a White Paper on this website. Numerous other articles on this website (e.g. here, here, here, here, here, and here) have presented the latest information on some of the myriad facets of CC use in row crop production. In particular, an article titled “Allelopathy–Cover Crops” that was posted in Nov. 2012 contains information that will supplement the information below, and the information in the below-cited and summarized article does not change any of the tenets presented in any of the above-linked articles on this website.
A recent article by Koehler-Cole et al. titled “Is allelopathy from winter cover crops affecting row crops?” appeared in the journal Agricultural & Environmental Letters (2020;5:e20015: https://doi.org/10.1002/ael2:20015). In this article, the authors reviewed studies that document allelopathic effects of CC on subsequent row crops in both field and (mostly) laboratory settings. Details about and results from that review follow.
• The review was conducted “to determine whether or not there is a clear cause-and-effect relationship of CC allelopathy on row crops. The questions were (a) Do CC have allelopathic effects on row crop seed germination and/or yield, and (b) What environmental and management factors influence allelopathic potential of CC?”
• A literature review of peer-reviewed journal articles was conducted for this analysis. Allelopathic effects of winter annual cereals, brassicas, and legumes (all common CC) on corn, soybean, cotton, and wheat were the principal targets of the review.
• Eight references were selected to match the selection criteria for the review. The only field studies included were those with cotton. All cited references for corn, soybean, and wheat presentedresults from laboratory studies.
• All corn studies were conducted in the laboratory (there were no field studies that met the criteria of the review). In those studies, germination was sometimes affected by high concentrations of aqueous extracts of winter cereals, brassicas, and legumes.
• The lone cited cotton study was conducted in the field. In that study, cereal rye and wheat CC reduced cotton height and lint and seed yield. It is likely that these cereal CC also resulted in N immobilization, but this would not have been an allelopathic effect.
• The one cited soybean study that met the criteria for review was conducted in the laboratory. Its results were that red clover extracts had no effect on seed germination and root length.
• The one cited laboratory wheat study found that aqueous extracts of hairy vetch, cereal rye, and wheat reduced germination and/or root length of wheat.
• The authors state that “Allelopathic effects seem to be dose dependent, but allelochemical concentrations needed to inhibit row crop growth under field conditions have not been established”. Note that this statement refers to the results from laboratory studies in the cited references.
• The criteria used for selecting articles to include in this review did not identify a single study–laboratory or field–that confirmed a CC allelopathic effect on soybean.
The authors provide the following conclusions drawn from their review.
• It appears that crops following a CC are most vulnerable to potential allelopathic chemicals during their germination. Thus, the interval between termination of the CC and germination of row crops is likely the critical period to mitigate potential CC allelopathic effects.
• The overlap between allelochemical release from a terminated CC and germination of a following crop can likely be avoided by delaying crop planting for at least 2 weeks after CC termination, or by planting the crop into a living CC and terminating the CC after crop emergence.
• These conclusions need to be confirmed with rigorous field testing, and that is lacking.
The findings presented in this review article confirm the following main tenet about allelopathy of cover crops–i.e., there is a dearth of studies that have explored the effects of CC allelopathy or allelopathy in general on crops in field settings. Even though numerous laboratory studies have shown that plant extracts or extracts from their residues may have an allelopathic effect of crop plants, these findings have rarely been explored and/or confirmed in field settings.
Composed by Larry G. Heatherly, Sept. 2020, larryheatherly@bellsouth.net