| Abstract: | Four cows were used in a balanced 4×4 Latin square with 2 week experimental periods to investigate the effects of intraruminal infusions of volatile fatty acids and protein source on milk production and blood metabolites. The four treatments in a 2×2 factorial arrangement were isoenergetic intraruminal infusions of propionate (500 g day?1) or butyrate (417 g day?1) each given with isonitrogenous protein supplementation of fish meal (FM) or barley protein (BP). The cows were fed restrictively with 9 kg dry matter day?1 of formic acid treated grass silage and 8 kg day?1 of concentrate. Propionate infusion increased milk yield (24·9 vs 23·4 kg day?1; P<0·05), milk protein yield (832 vs 778 g day?1; P=0·05) and milk lactose content (44·7 vs 43·5 g kg?1; P<0·05) and yield (1113 vs 1023 g day?1; P<0·01), whereas butyrate infusion was associated with a higher milk fat content (44·7 vs 39·4 g kg?1; P<0·01) and yield (1033 vs 974 g day?1; P<0·01). FM tended (P<0·10) to increase milk yield, but had no significant effects on milk composition or milk component yields compared with BP. Butyrate infusion increased blood ketones, plasma non-esterified fatty acids and glycine relative to propionate infusion. The concentrations of ammonia N in rumen fluid and urea in plasma and milk were similar for both protein supplements. The profile of amino acids in plasma was similar for both protein supplements except for the higher concentrations of phenylalanine, proline and tyrosine with BP. The results show that protein utilisation can be improved by increasing the supply of propionate from rumen fermentation in cows given a grass silage-based diet. © 1998 SCI. |
