Velvetleaf Interference Effects on Yield and Growth of Grain Sorghum

Weed competition studies with grain sorghum [Sorghum bicolor (L.) Moench] are limited. Improved knowledge of growth in response to weed competition could contribute to development of sorghum hybrids that compete better with weeds. Velvetleaf (Abutilon theophrasti Medic.) competition with sorghum hybrids was evaluated in field experiments on a Sharpsburg silty clay loam soil (fine smectitic, mesic Typic Arguidoll) in 1996 and 1997 at Lincoln, NE. The objective was to determine the effects of velvetleaf competition on interception of photosynthetically active radiation (PAR), growth, and yield of sorghum hybrids with different heights. Sorghum hybrids used were 'FS2' (1.8 to 2.4 m tall) and 'DK54' and 'X260' (1.2 to 1.5 m tall). Weed treatments included sorghum in monoculture and grown in mixture with velvetleaf. Medium-height hybrids had 0.4 greater mid-season leaf area indices (LAIs) than the tall hybrid in 1996, and in 1997, DK54 had 0.4 to 0.6 greater LAI than the other hybrids after 500 growing degree days. Interception of PAR was similar among hybrids except for 2 to 10% less for X260 in mixture in 1996. In 1996, when weed pressure was greater, the tall hybrid had greater relative growth rate and net assimilation rate but lower leaf area ratio than for the medium-height hybrids. Sorghum intercepted more radiation and produced more biomass in monoculture than in mixture, but the reduction in growth and yield in presence of velvetleaf was less for the tall hybrid. Development of tall sorghum hybrids with high LAI would be useful for integrated weed management.