178. Feed Smarter: Probiotics to Improve Nutrient Digestibility in Dairy Cows.

Abstract Feed efficiency in dairy cattle is driven by genetic, metabolic, nutritional and physiological factors. From a nutritional perspective, ruminal fermentation and post-ruminal digestion of nutrients significantly influence nutrient utilization efficiency. Direct-fed microbials are commonly used feed additives for lactating dairy cows due to their ability to modulate ruminal fermentation through several mechanisms. Lactic acid bacteria facilitate the growth of ruminal microorganisms tolerant to the presence of lactic acid in the rumen and to stimulate lactic acid–utilizing microorganisms increasing ruminal pH and alleviating ruminal acidosis. Similarly, live-cell yeasts decrease lactic acid production or increase utilization of lactic acid by some bacteria, hence increasing diet consumption and stabilizing redox and ruminal pH. But these additives are more efficient when dairy cows are fed diets rich in rapidly degradable carbohydrates. Previously, we observed improved total tract NDF digestibility and corresponding milk fat concentration when diets containing 30% starch were supplemented with live-cell yeasts. More recently, Bacillus-based direct-fed microbials have garnered a lot of interest. Improved digestibility of OM, CP, NDF, and ADF was observed with the supplementation of Bacillus licheniformis supposedly by stimulating the activity of cellulose-digesting bacteria. Alternatively, studies with Bacillus subtilis suggest a different mode of action by modulating VFA production in the rumen through selective microbial growth. Recently, we evaluated diets varying in starch concentration (21 vs 27%) and supplemented without or with a direct-fed microbial containing Bacillus subtilis 810 and Bacillus licheniformis 809. Digestibility of DM, OM, CP, ether extract, and NDF were greater for diets supplemented with direct-fed microbials, but this response was of greater magnitude for the reduced-starch diet. Overall, feeding direct-fed microbials to dairy cattle has potential to improve nutrient digestibility through diverse mechanisms. However, the response varies based on type of directed-fed microbial, strains, dose, animal category and diet formulation strategies.