Effect of brachytic dwarf forage sorghum or corn silage harvested in the summer or fall and supplemented with soybean meal or mechanically pressed cottonseed meal on performance of lactating dairy cows

A 6-wk randomized design trial with a 4 × 2 factorial arrangement of treatments was conducted to evaluate the production response of 48 Holstein lactating cows fed diets based on corn or forage sorghum silage harvested in the summer or fall and supplemented with soybean meal (SBM) or mechanically pressed cottonseed meal (CSM). Corn was planted in April and harvested in July (CSS), and a second crop was planted in August and harvested in November (CSF). Forage sorghum was planted in April, harvested in July (FSS), allowed to ratoon, and harvested again in November. Forages provided 41.67% of the dietary DM in the experimental diets, and CSM replaced a portion of the N provided by SBM. Cows were fed a corn silage-based diet for 2 wk before beginning the 4-wk experimental period. No differences were observed in DMI or milk yield among treatments. An interaction of forage source and protein supplement was observed for milk fat, which was lowest for CSF-CSM compared with the other treatments. No differences were observed in yield or concentration of milk protein, lactose, or solids-not-fat. An interaction was observed for efficiency of milk production, which was lowest for CSS-SBM and CSF-CSM compared with the other treatments. Concentrations of milk urea nitrogen were less for diets based on corn silage compared with forage sorghum and for CSM compared with SBM. Results of this trial indicate that diets based on corn silage or forage sorghum harvested in spring or fall and supplemented with CSM or SBM can support similar performance.