Competition and Allelopathy Between Soybeans and Weeds

This research was initiated to determine how soybean [Glycine max (L.) Merr.] cultivars daer in competitive ability and also whether allelopathy functions to inhibit surrounding weed growth. Initially 280 soybean cultivars were screened in the field for competitive ability based on emergence, seedling vigor, quickness of canopy closure, and ability to inhibit growth of velvetleaf (Abutilon theophrasti Medic.) and foxtail millet [Setaria italica (L.) Beauv.]. Experiments were conducted at Lincoln, Nebr. on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudolls) during 1981 and 1982. Soybean cultivars of later maturity tended to compete more effectively with weeds. Competitive cultivars emerged quickly, rapidly formed a canopy, and were able to slow the growth of competing weed species. Twenty soybean cultivars of varying competitive ability were selected and grown in the greenhouse during 1981 and 1982 to determine the importance of allelopathy in competing with weeds. Exudates from roots of soybean cultivars grown in sand reduced the dry weight of 4-week-old velvetleaf plants an average of 15%, but foxtail millet was not inhibited. Incorporation of 1% ground soybean dry matter into Sharpsburg silty clay loam inhibited germination and dry weight of greenhouse grown velvetleaf an average of 46% each. Foxtail millet germination and dry weights were reduced an average of 82 and 65%, respectively. Undiluted soybean plant extracts of all cultivars tested slowed the germination and dry weight accumulation of 6-day-old velvetleaf and foxtail millet, but dilution of the extracts caused quite variable responses. Osmotic potential of the extract solution appeared to be the main cause of inhibition, however, extracts of some cultivars, diluted to decrease the osmotic potential, still inhibited growth. Allelopathy appears to be one mechanism for competition among soybeans and weeds. Soybean cultivars possessing the ability to chemically inhibit competing weed growth would be a great benefit to the soybean producer.