A comparison of the responses by lactating cows given grass silage to changes in the degradability or quantity of protein offered in the supplement

Abstract

ABSTRACTTwenty-four lactating British Friesian type cows were used in a four period, partially balanced, changeover design experiment to evaluate eight treatments consisting of four protein sources in the supplement (soya-bean meal, formaldehyde treated soya-bean meal, fish meal and a mixture supplying equal crude protein levels from fish and soya-bean meal (respective degradabilities of total supplement nitrogen (N) were 0·56, 0·37, 0·32 and 0·44), each offered in supplements formulated to contain 150 and 200 g crude protein per kg fresh weight in a 4 × 2 factorial design. All supplements were offered at 8·0 kg/day in addition to ad libitum access to a medium digestibility grass silage (digestible organic matter as proportion of dry matter (DM), 0·66); with a relatively poor quality fermentation (pH, 4·5; ammonia-N proportion of total N, 0·21).There were no significant interactions between source and level of protein on any of the animal performance variables. Source of protein did not influence milk output or milk composition but increasing protein intake significantly increased milk yield (26·0 and 26·7 (s.e. 0·19) for the low and high protein supplements respectively). Blood urea levels were significantly influenced by both protein source and level.Total diet digestibility and energy and N utilization studies were undertaken with six replicates per treatment and the full data are presented. Neither protein source nor level had any significant effect on DM or energy digestibility. N digestibility was significantly reduced following formaldehyde treatment of soya-bean meal (0·69 v. 0·72 (s.e. 0·01)) and most N utilization variables were influenced by the level of protein intake but not by protein source.