Evaluating dietary starch concentrations in reduced-forage rations fed to lactating dairy cattle.

Increasing frequency of severe weather events such as droughts and floods may reduce the inventory of high-quality forage available for feeding dairy cattle. Thus, optimizing rumen health and milk production when feeding reduced-forage rations is critical for dairy farmers and nutritionists. Our experiment investigated the effects of replacing NDF with starch in diets with 12.6% forage NDF on rumen metabolism, rumen inflammation, feed intake, and milk production. We hypothesized that increasing dietary starch would increase inflammatory cytokines and alter T lymphocytes in rumen tissue. We also expected increasing starch and removing NDF to reduce milk fat production. Six ruminally cannulated Holstein cows were assigned to diets containing 24%, 28%, or 32% starch in a replicated 3 × 3 Latin square design. Starch concentration was increased by replacing soybean hulls with corn grain. Data were analyzed with linear mixed models. Increasing starch concentration reduced NDF digestibility but did not affect DMI or milk yield. Milk fat percentage and yield decreased with increasing starch, accompanied by reductions in short-chain milk fatty acids and an increase in trans-10,cis-12 conjugated linoleic acid. Energy-corrected milk was reduced when greater starch concentrations were fed. Rumen pH, ruminal immune cell populations, and plasma markers of inflammation were unaffected. We observed that IFN-γ, IL-1α, IL-4, and IP-10 increased in rumen tissue as corn grain increased, replacing soybean hulls. Based on our results, increasing dietary starch in reduced-forage rations may risk rumen tissue inflammation while resulting in reduced milk fat production and reduced fiber digestibility.