Short Communication: Effects of planting density on nutritive value, dry matter yield, and predicted milk yield of dairy cows from 2 brown midrib forage sorghum hybrids

Our objective was to evaluate the effects of 5 planting densities (148,000, 198,000, 247,000, 297,000, and 346,000 seeds/ha) on the nutritive value, DM yield, and predicted milk yield from 2 brachytic dwarf (shortening of plants by reducing length between internodes) brown midrib-6 forage sorghum hybrids (Advanta Seeds, AF7401; Advanta Seeds, ADV F7232; Irving, TX). Replicated plots were planted and harvested on the same day. At harvest, weights of whole-plant sorghum forage were obtained to calculate DM yields. A commercial laboratory (Dairyland Laboratories Inc., Arcadia, WI) analyzed nutrients and predicted ruminal NDF digestibility at 30 h using near-infrared spectroscopy. Estimated milk yield per unit of forage and per hectare were calculated using Milk2006. Summative equations were used to predict TDN and NEl. Data were analyzed as a randomized complete block design with a 5 (planting density) × 2 (hybrid) factorial arrangement of treatments. Differences were detected between hybrids for DM, starch and NDF concentrations, NDF digestibility at 30 h, DM yield (Mg/ha), kilograms of predicted milk per megagram of DM, and kilograms of predicted milk per hectare, highlighting the importance of hybrid selection. Because of increased tillering at lower planting densities, reductions in NDF concentration and uNDF240om were observed as well as a lack of effects on DM yield and predicted milk yields, emphasizing the tolerance of brachytic dwarf brown midrib-6 forage sorghum to decreases in planting density. Because of greater DM yield and predicted milk yield, Advanta Seeds ADV F7232 outperformed Advanta Seeds AF7401. Furthermore, increasing planting density above 247,000 seeds∙ha−1 may not be feasible during a summer planting in Florida as biomass yield was not increased. Due to tillering, increasing planting density above 247,000 seeds∙ha−1 may be unnecessary and production costs may be reduced with the use of these reduced densities when similar hybrids are planted during the summer in the southeastern United States and grown under irrigation.