Allelopathy--Cover Crops
Cover crops have long been recognized for their potential to provide soil cover that will curtail erosion between crop growing seasons, and to provide residue that is available to increase soil organic matter.
With the increasing occurrence of herbicide-resistant (HR) weeds, cover crops are now being evaluated for their allelopathic potential to control weeds.
Current thought is that cover crops and their residues may provide weed suppression through their physical presence on the soil surface and/or by the release of allelochemicals that may inhibit weed seed germination and/or early seedling development (Weston, Agronomy Journal, Vol. 88, 1996; Weston, 2005). Weston also suggests that allelopathic crops offer potential for the development of herbicides, as well as for providing germplasm from which to select for allelopathic products and chemistry.
Thus, allelopathic potential of cover crops for weed suppression is touted, and research has shown that cover crop extracts can inhibit early plant development (Kelton, Price, and Mosjidis, Intech Press, 2012).
In a study by Petersen et al (Agronomy Journal, Vol. 93, 2001), isothiocyanates released by a turnip-rape mulch were determined to suppress weed germination after incorporation of the mulch into the soil. They concluded that the potential of cover crops to release these compounds should be further investigated as a tool for weed control/suppression in integrated cropping systems.
In a recent report in Agronomy Journal (Vol. 104, 2012), Lawley, Teasdale, and Weil determined that early and competitive fall growth of a forage radish cover crop is the dominant mechanism for early-spring weed suppression resulting from use of this species as a cover crop. They did not measure any allelopathic activity that limited seed germination or seedling development. Thus, they concluded that cover crop management strategies should be directed towards practices that ensure maximum cover crop development in the fall to ensure maximum physical weed-suppression activity the following spring.
Cereal cover crops will produce more biomass than will legume cover crops (Reddy, Weed Technology, Vol. 15, 2001). This increased physical barrier, coupled with the slower degradation of residues from cereals compared to that of legumes, should result in more and longer-lasting weed control/suppression from using cereal cover crops.
There are four important points regarding the use of cover crops for weed control either by physical suppression or by allelopathy.
● Differentiating between allelopathy and the mulching effect of cover crops is difficult. As stated above, it is accepted that increased cover crop biomass on the soil surface can suppress weeds, but to what extent and with what resulting value in a soybean production system is not known.
● The variability in allelopathic effects from plant residues presently negates their consideration as a stand-alone weed control option in large-scale crop production systems.
● A likely system will be using cover crops that are proven to physically or allelopathically suppress weeds to offset some herbicide use.
● The additional cost associated with using cover crops in a crop production system must be considered (Reddy, Weed Technology, Vol. 15, 2001). In other words, the additional cost of using cover crops for potential weed control must be compensated for by increased soybean yield and/or reduced herbicide usage/cost. Otherwise, producers will be reluctant to insert cover crops into soybean production systems for any reason.
Composed by Larry G. Heatherly, Nov. 2012, larryheatherly@bellsouth.com