The role of rumen protozoa in the utilization of paspalum (Paspalum dilatatum) hay by cattle.

Six Friesian heifers (250 kg live weight) with permanent cannulas in the rumen and abomasum were allocated at random into two groups of three. One group was treated with Teric GN9 (ICI (Aust.) Ltd) to defaunate the animals during the first two of the four periods of the experiment, after which they were refaunated. The second group was treated with Teric at the end of the first two periods. The dietary treatments were: paspalum (Paspalum dilatatum) hay (4.1 kg/d) given alone and the hay supplemented with urea (20 g/kg dry matter). Defaunation was not complete but the approximate volume of protozoa in the rumen of treated animals was less than 6% of that in the untreated animals. The amount of organic matter (OM) digested in the stomach was lower (P less than 0.01) in animals with reduced fauna than in those with normal fauna. There were reductions in both the apparent OM digestibility in the total tract (from 0.56 to 0.52, P less than 0.01) and the proportion of the digestible OM digested in the rumen (from 0.82 to 0.79, not significant) of animals with reduced fauna. Apparent digestibilities of acid-detergent fibre and neutral-detergent fibre were significantly lower (P less than 0.01) in animals with reduced fauna. The amount of nitrogen disappearing from the stomach was significantly higher (P less than 0.01) with the urea supplement; effects due to concentrations of protozoa were not significant. The flow of non-ammonia-N from the abomasum was higher (P less than 0.05) in animals with reduced fauna than in animals with normal fauna. The flows of bacterial N from the abomasum and the efficiencies of bacterial N synthesis were not significantly affected by the treatments. N retention was higher (P less than 0.01) in animals receiving the urea supplement but differences due to protozoa were not significant. Protozoal contribution to the microbial N flowing from the rumen of animals with normal fauna was estimated to be 24 and 27% with and without the urea supplement respectively. Concentrations of rumen-fluid ammonia-N were reduced (P less than 0.05) and those of volatile fatty acids were increased (P less than 0.01) with reduction in protozoal numbers. Molar proportions of propionic acid increased (P less than 0.05) and of butyric acid decreased (P less than 0.01) with reduced rumen fauna.(ABSTRACT TRUNCATED AT 250 WORDS)