The effect of fishmeal on the digestion of grass silage by growing cattle

The effect of two levels of fishmeal substitution (50 (FM1) and 150 (FM2) g/kg) of a grass silage control diet (C) on the rumen digestion of organic matter and nitrogen, and the small intestinal disappearance of amino acids was examined in young growing cattle each equipped with simple PVC cannulas in the dorsal sac of the reticulo-rumen, the proximal duodenum and the terminal ileum. The silage was a primary growth of perennial ryegrass (Lolium perenne) (+formic acid) with a total N content of 22 g/kg dry matter (DM) (diet C). Fishmeal substitution increased this to 26 (diet FM1) and 34 (diet FM2) g/kg DM. On diets C and FM1, approximately 0.71 of digestible organic matter intake was apparently digested in the rumen, but this was significantly (P less than 0.05) reduced on diet FM2 (0.60). Whilst duodenal flows of non-ammonia N and total amino acids were significantly (P less than 0.01) increased at the highest level of fishmeal inclusion only, the synthesis of microbial N was significantly (P less than 0.001) reduced by fishmeal inclusion, and feed N degradability declined progressively in response to increased fishmeal. Both levels of fishmeal addition caused a significant (P less than 0.05) reduction in the fractional outflow rate of water from the rumen, and on the highest level of fishmeal significant (P less than 0.05) increases in rumen ammonia concentration and rumen propionate molar proportions were observed. The net effect of the highest level of fishmeal substitution was to increase amino acid absorption from the small intestine by 0.47 compared with the control diet (P less than 0.05), but due to an elevated ileal flow of amino acid no such effect was detected at the lowest level of fishmeal substitution. Composition of the absorbed amino acid fraction was relatively unaffected by the treatments imposed, despite large changes in the composition of the duodenal protein. The apparent non-linearity of response to fishmeal substitution is discussed and the amino acid supply findings are compared with the protein retention findings obtained in an earlier study by Gill et al. (1987). By two methods of calculation it was estimated that the amino acid N fraction disappearing from the small intestine was utilized with an efficiency of between 0.51 and 0.53 and no apparent effects due to diet or level of amino acid supply were detected.