EFFECTS OF INTRARUMINAL INFUSIONS OF PROPIONATE AND BUTYRATE WITH 2 DIFFERENT PROTEIN SUPPLEMENTS ON MILK-PRODUCTION AND BLOOD METABOLITES IN DAIRY-COWS RECEIVING GRASS SILAGE-BASED DIET

Four cows were used in a balanced 4 x 4 Latin square with 2 week exper imental periods to investigate the effects of intraruminal infusions o f volatile fatty acids and protein source on milk production and blood metabolites. The four treatments in a 2 x 2 factorial arrangement wer e isoenergetic intraruminal infusions of propionate (500 g day(-1)) or butyrate (417 g day(-1)) each given with isonitrogenous protein suppl ementation of fish meal (FM) or barley protein (BP). The cows were fed restrictively with 9 kg dry matter day(-1) of formic acid treated gra ss silage and 8 kg day(-1) of concentrate. Propionate infusion increas ed 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), wh ereas 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 signi ficant effects on milk composition or milk component yields compared w ith BP. Butyrate infusion increased blood ketones, plasma non-esterifi ed fatty acids and glycine relative to propionate infusion. The concen trations of ammonia N in rumen fluid and urea in plasma and milk were similar for both protein supplements. The profile of amino acids in pl asma was similar for both protein supplements except for the higher co ncentrations of phenylalanine, proline and tyrosine with BP. The resul ts show that protein utilisation can be improved by increasing the sup ply of propionate from rumen fermentation in cows given a grass silage -based diet. (C) 1998 SCI.