Effect of dietary physically effective fiber on ruminal fermentation and the fatty acid profile of milk in dairy goats.

The objective of this experiment was to characterize the relationship among rumen fermentation variables, milk fatty acid profile, and dietary physically effective neutral detergent fiber (peNDF) content in a study that controlled for the potential confounding effects of dissimilar dry matter intake among treatments. Ten multiparous Xinong Saanen dairy goats were divided into 2 groups with 2 ruminally cannulated goats per group. Goats in each group were assigned to 1 of 2 dietary treatments (high and low peNDF) according to a 2×2 crossover design with 2 periods. The peNDF content of alfalfa hay (proportion of neutral detergent fiber retained on an 8.0-mm screen) was 42.1% for the high-peNDF and 14.5% for the low-peNDF group. To ensure similar dry matter intake, each morning the amount of alfalfa hay consumed on the prior day by the high-peNDF group was determined (amount offered minus morning refusals), and this was the amount of hay offered to the low-peNDF group that day. Each adaptation period consisted of 21d, followed by a 9-d sampling period. Dry matter intake and milk production and composition were similar between treatments. Milk energy efficiency increased with low dietary peNDF. Duration of pH below 5.60 was longer for goats fed the low-peNDF ration compared with the high-peNDF ration (4.08 vs. 0.41h/d); however, mean rumen pH (6.05 vs. 6.13) was not different between treatments. Reducing dietary peNDF increased rumen total volatile fatty acids (114.6 vs. 95.1mM) and decreased chewing time (404 vs. 673min/d), but did not affect the ratios of acetate, propionate, and butyrate. The relative abundance of Fibrobacter succinogenes and Ruminococcus flavefaciens increased with reduced dietary peNDF, but Ruminococcus albus proportions were not influenced by treatment. Reducing dietary peNDF decreased the proportion of iso C14:0, iso C15:0, and trans-11 C18:1 in milk fat, whereas the iso C17:0 and trans-10 C18:1 increased. This study demonstrated that low dietary peNDF in dairy goats increases rumen volatile fatty acids, reduces chewing time, and is correlated to the amount of F. succinogenes and R. flavefaciens.