Short-season soybean genetic improvement evaluated in weed-free and weedy conditions
Previously we found that the genetic improvement rate of soybean [Glycine max (L.) Merr.] increased as plant population increased, indicating that new cultivars were more tolerant to higher population stress. The objectives of the current work were to determine genetic improvement rates under weed interference and to examine agronomic and seed composition responses to weed interference. Twenty maturity group 0 to 00 cultivars released from 1934 to 2007 were grown at Ottawa, ON, from 2006 to 2008 under weedfree and naturally weedy conditions in a split strip trial. Over all cultivars, yield loss due to weeds ranged from 11 to 80% over years. Genetic improvement under weed-free conditions ranged from 12.1 to 16.6 kg ha -1 yr -1, similar to previous estimates. Under weedy conditions, estimates of genetic improvement decreased as weed pressure increased, ranging from 14.2 kg ha -1 yr -1 with 11% yield loss to 1.2 kg ha -1 yr -1 with 80% yield loss. Weed interference resulted in inconsistent changes in maturity, plant height, lodging, seed size, and seed protein and oil concentration across the 3 yr of this study. Canopy development was estimated using a measurement of green area from digital images. In weed-free soybean canopies, newer soybean cultivars had a slower rate of development compared to older cultivars. New cultivars were not more tolerant to weed interference but also were not inferior to old cultivars under weedy conditions. © Crop Science Society of America.
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