Effects of supplementing a yeast culture in a pelleted total mixed ration on fiber degradation, fermentation parameters, and the bacterial community in the rumen of sheep

Feeding animals with pelleted total mixed ration (TMR) is a widespread practice as it reduces feed losses during storage, feeding and the animal’s sorting. Supplementation of yeast culture can promote rumen fiber degradation, which may occur through stabilizing rumen pH. There are few studies, however, evaluating the optimal supplementation level of yeast culture in pelleted TMR in which rumen pH has been continuously measured. This study aimed to determine the optimal level of a commercial yeast culture product (of which the number of viable CFU was not measured) mixed and pelleted with the TMR, on fiber degradation in situ, rumen pH dynamics, rumen fermentation variables, and the bacterial community composition of rumen fluid. Under a partial Latin square design, a total of twelve wethers equipped with permanent rumen fistulas were randomly assigned to four treatments of yeast culture at 0, 0.5, 1.0, or 2.0% dietary dry matter. After a 28 d period comprising 21 d for adaptation and 7 d for measurements, each group of three wethers was randomly reassigned to another treatment. Thus, each yeast supplementation level was fed to a total of six animals. In situ dry matter (DMD) and neutral detergent fiber (NDFD) disappearance of corn stover, mean rumen pH, total volatile fatty acids concentration, acetate, propionate, and n-butyrate molar percentages, and the acetate to propionate ratio were influenced (P ≤ 0.05) by dietary treatments. Quadratic (P ≤ 0.03) responses to increasing yeast culture supplementation level were observed for mean rumen pH, daily duration of pH below a 5.8 threshold considered inhibitory for fibrolysis, and the molar percentages of propionate, n-butyrate, and the acetate to propionate ratio. The highest in situ DMD and NDFD occurred when yeast was supplemented at 1%. Increasing yeast culture supplementation level tended to quadratically increase the abundance of fibrolytic bacterial genera NK4A214 and FD2005, which peaked at 1% yeast culture in the diet DM. The molar percentage of n-butyrate was positively correlated with the relative abundance of Rikenellaceae_RC9 and NK4A214. In our study, supplementation of pelleted TMR with 1% yeast culture provided as residual yeast medium components, was optimal for fiber degradation of forage fed to ruminants. Members of genera NK4A214 and FD2005 may contribute extra n-butyrate.