Combinations of Allelopathic Crop Extracts Reduce Digitaria spp. and Setaria faberi Seed Germination

Allelopathic cover crops contain compounds that deter other types of plant seeds from germinating or inhibiting established plants’ growth. Sunflower (Helianthus annuus, SF), buckwheat (Fagopyrum esculentum Moench, BW), sorghum-sudangrass (Sorghum × drummondii [Nees ex. Steud.] Millsp. & Chase, SSG), and winter rye (Secale cereale) are all known allelopathic cover crops. However, there is little information about the use of these allelopathic cover crops used together and their combined impact on weed seed germination. Laboratory bioassays were conducted to determine the effect of the aforementioned cover crops alone and in combinations in reducing the germination rate of Digitaria spp. (crabgrass) and Setaria faberi (giant foxtail) through extract application. Two separate experiments were arranged as a 7 treatment × 3 extract rate factorial set out in a completely random design with three replicates. The first experiment used winter rye, sunflower, and sorghum-sudangrass with Digitaria spp., and the second experiment used sunflower, sorghum-sudangrass, and buckwheat with S. faberi. The 7 treatments were extracts of each cover crop species alone and in various binary and tertiary combinations. Each extract was applied at three concentrations: 3, 4, and 5% (g/v) extract. A water control was included. Winter rye alone or in combination with sunflower resulted in the lowest Digitaria spp. seed germination at extract concentrations 4% and 5%. The 5% sorghum-sudangrass extract caused the greatest reduction in the number of S. faberi seeds germinated and the greatest reduction in the rate at which they germinated. This is congruent with the fact that extracts used individually were more effective than the control at reducing total and the rate of germination. In addition, binary combinations were also more effective than the control in reducing germination rate. The data that binary combinations are more effective at reducing S. faberi germination than the control suggest a synergistic effect by various extracts used together at certain concentrations. This indicates that some of these cover crops may have potential value being used together in cover crop mixes to reduce Digitaria spp. and S. faberi weed pressure