Microbial Degradation of Vitamin a in Rumen Fluid From Steers Fed Concentrate, Hay or Straw Diets

The effect of diet on degradation of vitamin A by rumen microorganisms in vitro was investigated. Retinyl acetate was incubated in vitro with rumen fluid from cattle fed high-concentrate (C), hay (H) or straw (S) diets. Residual retinyl acetate and retinol were measured by high-performance liquid chromatography (HPLC), 24 h after retinyl acetate addition to rumen fluid. Retinyl acetate was degraded faster and more extensively in C rumen fluid than in H or S rumen fluid. Retinol levels increased in H and S rumen fluid throughout the incubation period but decreased in C rumen fluid (P < 0.05). Estimated effective ruminal degradation of biologically active vitamin A was 67, 16 and 19% for cattle fed C, H and S diets, respectively. Thirty-three strains of rumen bacteria were surveyed for their ability to degrade retinyl propionate. Of the nine strains that were positive for retinyl propionate degradation, seven were strains associated with starch digestion. The results imply that cattle fed high-concentrate diets will have considerably greater vitamin A requirements than those fed high-forage diets, if oral supplementation of vitamin A is used. Because of the extensive ruminal destruction of vitamin A, intramuscular injection would be a more efficient mode of supplementation for cattle fed high-concentrate diets.