Effect of fatty acids or calcium soaps on rumen and total nutrient digestibility of dairy rations

Two trials ascertained whether feeding fatty acids as preformed calcium soaps would permit normal digestibility of fiber without reducing availabilities of fatty acids or energy. Six diets contained no added fat (control) or the equivalent of 4.5% added fatty acids as tallow or soy fatty acids, tallow or soy calcium soaps, or tallow combined with the nonnutritive carrier verxite. Six cows were fed each diet in trial 1 to ascertain digestibilities of total nutrients, whereas control, tallow fatty acid, and tallow soap diets were fed to steers with duodenal cannulae in trial 2 to evaluate effects of fat source on rumen fermentation. Digestibility of total fiber in trial 1 was reduced 13% by tallow fatty acids. In trial 2, tallow fatty acids markedly reduced digestibility of dry matter and fiber in the rumen. Diets supplemented with tallow calcium soaps had normal digestibilities of fiber in the rumen and totally, but digestibility of fatty acids was reduced 6% and digestibility of energy reduced 3% compared to feeding tallow fatty acids. Efficiency of synthesis of microbial protein was 39% higher for tallow fatty acids than for control. Feeding tallow verxite also permitted normal digestibility of fiber in trial 1 but caused reduced availabilities of fatty acids and energy.(ABSTRACT TRUNCATED AT 250 WORDS)