Soybean (Glycine max) – Velvetleaf (Abutilon theophrasti) Interspecific Competition

Abstract

In a 2-yr field study conducted on a Weswood silt loam soil (Fluventic Ustochrepts), interspecific competition between soybeans [Glycine max(L.) Merr. ‘Hutton′] and velvetleaf (Abutilon theophrastiMedik. # ABUTH) resulted in greater than 40 and 50% reductions in soybean and velvetleaf seed yield, respectively. Leaf area index, number of mainstem nodes, total number of leaves, and plant dry weight of monocultured and intercropped velvetleaf differed significantly as early as 4 weeks after emergence. Interspecific competition had litttle or no effect on soybean morphology before 8 weeks after emergence. Soil water extraction occurred to 1-m depths in a monoculture of velvetleaf (five plants/m2) in 1984 and 1985. Monocultured soybeans (32.5 plants/m2) extracted water from a 1.5-m or greater depth of the soil profile during the same years. Soil water extraction in the intercropped plots resembled that of the monocultured velvetleaf treatment until soybeans attained R6, when soil water was extracted to a 1.5-m depth. The potential for interspecific competition for water existed early in the season before late-season soybean root development. Relative water content and leaf water potential (Ψw1) did not differ (0.05) between monocultured and intercropped soybeans in 1984 or 1985. In 1985, Ψw1differed between monocultured and intercropped velvetleaf during anthesis. Leaf water potential values in the youngest, fully expanded leaves were approximately 0.3 and 0.4 MPa lower during midmorning and midday hours, respectively, in intercropped and monocultured velvetleaf. Transpiration and stomatal conductance did not differ between monocultured and intercropped soybeans or velvetleaf at any time during 1984. Photosynthetic and transpiration rates, stomatal conductance, and Ψw1were lower in intercropped than in monocultured velvetleaf during anthesis in 1985, suggesting interspecific competition for soil water. Soybean water relations were not affected in either year. The data suggest that soybean yield reductions in soybean-velvetleaf interspecific competition are attributable to resource limitations other than water in south-central Texas.