Weed Response to Agronomic Practices and Herbicide Strategies in Grain Sorghum

CORE IDEAS: High sorghum density and narrow row spacing reduce biomass of troublesome weeds. High spatial crop uniformity extends large crabgrass and sicklepod control over time. Optimized crop density and row width may limit the need for postemergence herbicide in sorghum. Sicklepod [Senna obtusifolia (L.) H.S. Irwin & Barneby] and large crabgrass (Digitaria sanguinalis L.) are ranked among the top 10 most common or troublesome weeds in grain sorghum [Sorghum bicolor (L.) Moench] in the southeastern United States. Field studies conducted in North Carolina from 2012 to 2014 investigated the effects of three row widths, four sorghum populations, and three herbicide programs on weed control, density, biomass, and grain yield. Results indicated that 19‐ and 38‐cm rows provided greater sicklepod control 7 and 10 wk after planting (WAP) compared to 76 cm but had no effect on crabgrass control. Under weed‐controlled conditions, sorghum population ≥297,000 plants ha⁻¹ allowed to maintain higher late‐season sicklepod and crabgrass control as compared to lower crop densities. In the weedy control, row spacing had no effect on weed density and biomass, whereas crop population ≥297,000 plants ha⁻¹ reduced sicklepod density and biomass by 38 and 65%, respectively, compared to 99,000 plants ha⁻¹. Effect on crabgrass was less pronounced with density and biomass reduction by 18 and 45%, respectively, for 396,000 plants ha⁻¹ compared to 99,000 plants ha⁻¹. In the absence of water stress, the highest grain yields were obtained with high spatial uniformity, corresponding to 19‐cm rows and sorghum population ≥297,000 sorghum plants ha⁻¹. Our results indicate that increased sorghum density associated with narrow rows may reduce the need for a postemergence (POST) herbicide but underscore the importance of a timely activated preemergence (PRE) treatment to efficiently control sicklepod and crabgrass.