Influence of Soybean (Glycine max) Row Spacing on Pitted Morningglory (Ipomoea lacunosa) Interference

Abstract

The interference and seed production potential of pitted morningglory (Ipomoea lacunosaL. # IPOLA) with conventional-row (1 m) and narrow-row (20 cm) ‘Hill’ soybeans [Glycine max(L.) Merr.] was studied for 2 yr. Pitted morningglory densities were 3.3, 10, 20, and 40 plants/m2while soybean densities were 23 and 50 plants/m2in conventional and narrow rows, respectively. Pitted morningglory interfered with soybean growth earlier in conventional-row soybeans than in narrow-row soybeans due to its rapid increase in leaf area index (LAI) and biomass from 4 to 8 weeks after emergence. Soybeans were competitive with pitted morningglory until the soybean reproductive stage began at 7 weeks after emergence. Pitted morningglory reduced soybean yields 17% more in a dry year than in a wet year. Yield of narrow-row soybeans was equal or greater than yield of conventional-row soybeans at all pitted morningglory densities. Conventional-row soybean yields were reduced an average of 42 and 81% at pitted morningglory densities of 3.3 and 40/m2, respectively, but yield of narrow-row soybeans was reduced only 6 and 62% at equivalent densities. Pitted morningglory grown without soybean interference produced an average of 52.3 million seeds/ha. Total seed production of pitted morningglory growing in soybeans increased as pitted morningglory density increased and was greater in conventional rows than in narrow rows. Narrow rows reduced total seed production an average of 90 and 68% at pitted morningglory densities of 3.3 and 40/m2, respectively.