Soybean natural cross-pollination rates under field conditions

Recent concerns regarding within-crop transgene flow stimulated this research to update natural cross-pollination rates in conventional sowings of modern soybean [Glycine max (L.) Merr.] in the Mississippi Delta. Two experiments were conducted in 2001 and 2002 using two soybean cultivars, "Pace" (white-flowered) and "DP3588" (purple-flowered), selected for their equivalent flowering dates. The experiments utilized the dominance of purple flower color over white flower color to identify natural cross-pollinations. In the first experiment, 12 rows of Pace (white-flowered) flanked on each side by four rows of DP3588 (purple flowered) were sown on 10 May 2001. Seed were harvested by row from each of the Pace rows and examined for natural cross-pollinations in the next generation. In total, 73 512 potential hybrid plants were examined and natural cross-pollination rates ranged from 0.41% at 0.9 m from the pollen source to 0.03% at 5.4 m from the pollen source. These values were consistent with values previously reported in the literature. In the second experiment, seed of Pace and DP3588 were alternately sown 15.2 cm apart within a row in 2001. At maturity, 167 Pace plants (white-flowered) were harvested and a total of 19,151 progeny were evaluated for natural cross-pollinations in the next generation. The progeny of 56 (33.5%) of the 167 parent plants showed no evidence of natural cross-pollination. The progeny of the remaining 111 plants exhibited natural cross-pollination rates ranging from 0.65 to 6.32% and averaged 1.8%. The maximum rates reported here are considerably higher than most previously reported rates. The results of these two experiments highlight the potential for within-crop transgene flow in soybean.