Reproductive Abscission Levels and Patterns for Eleven Determinate Soybean Cultivars

Although soybeans (Glycine max L. Merrill) are considered to have relatively high levels of reproductive abscission, most studies have used only one or two cultivars and cultivar differences for abscission have rarely been reported. This study determined the abscission levels for 11 field-grown, determinate soybean cultivars (Maturity Groups V, VI, VII, VIII) and tested for cultivar differences in abscission. Numbers of flowers and young pods (2 an long) produced and pods at harvest were determined for 15 plants of each cultivar. New flowers were counted on alternate days; whereas, young pods were counted weekly. Production of each reproductive structure was recorded for each node (including any branches) of each plant. Seed number and weight at harvest were also determined. Cultivars differed in number of flowers, young pods, and pods produced per plant. Number of flowers roduced per plant rangaf from 170 to 332 and average 232. Number of pods at harvest ranged from 41 to 90 and averaged 57 per plant. Cultivars also differed in the percentage of fruit less than 2 cm in length which abscissed and percent total abscission. Percent total abscission rang ed from 67% for ‘McNair 800’ to 82% for ‘Hale 3’ and averaged 75% Small pods (less than 2 cm in length) and flowers accounted for an average of 87% of the total abscission. Flower production of most cultivars was concentrated in the top two-thirds of the canopy. The to one-thiid canopy region of most cultivars accounted for a majority of the harvestable pods. Percent total abscission increased with depth into the canopy. Yield (seed weight per plant) was related positively (r= 0.50) to numbers of flowers produced. It is concluded that determinate soybean cultivars differ in percent abscission. That a deficit of available carbohydrate is at least one of the factors involved in soybean reproductive abscission is discussed.