Influence of branched-chain fatty acid supplementation on voluntary intake, site and extent of digestion, ruminal fermentation, digesta kinetics and microbial protein synthesis in beef heifers consuming grass hay.

Four heifers (British x British; average BW 372 kg) cannulated at the rumen and duodenum and consuming a grass hay (fescue-orchardgrass) diet were used in a 4 x 4 Latin square and supplemented with four levels (0, 20, 40, and 60 g.head-1.d-1) of supplemental four- and five-carbon VFA (BCFA). Forage OM, ADF, NDF and N intakes and digestibilities were not affected (P greater than .10) by BCFA supplementation. Likewise, duodenal N (microbial, feed and ammonia) flows and microbial efficiency were not altered (P greater than .10) by BCFA supplementation. Neither particulate and fluid passage rate nor in situ rate of NDF digestion was affected (P greater than .10) by treatment. Ruminal pH, ammonia concentrations and total VFA concentrations were similar (P greater than .10) among treatments. Ruminal proportions of acetate and propionate were not affected (P greater than .10) by treatment; however, butyrate responded in a cubic (P less than .05) fashion to BCFA, with the lowest proportion of butyrate at the 40 g BCFA feeding level. A time x treatment interaction (P less than .05) was noted for isobutyrate, isovalerate and valerate proportions; they were increased as a function of BCFA dosage at 2 to 8 h postdosing. Supplemental four- and five-carbon VFA had no effect on digestion and fermentation of grass hay. Supplementation of low-quality roughages with BCFA is not justified.