PROTEIN NUTRITION OF DAIRY-COWS RECEIVING GRASS-SILAGE DIETS - EFFECTS OF FEEDING A PROTEIN-SUPPLEMENT OF UNBALANCED AMINO-ACID-COMPOSITION

Eight cows were used in a replicated 4 x 4 latin square design experiment with period lengths of 3 weeks to investigate the effects of protein supplementation of a grass silage diet on silage intake and milk production. The four treatments were (i) the basal diet, consisting of grass silage ad libitum plus 2 kg day-1 of molassed sugar beet pulp; (ii) basal diet plus 1 kg day-1 of fish meal (F); (iii) basal diet plus 0.8 kg day-1 of feather meal 1 (FE1); and (iv) basal diet plus 0.8 kg day -1 of feather meal 2 (FE2). Silage intake was significantly (P < 0.01) increased by the F supplement but was not affected by the FE supplements; values were 9.9, 10.6, 9-6 and 9.6 kg DM day-1 for basal, F, FE1 and FE2, respectively. Relative to the basal treatment, F increased (P < 0.001) the yield of milk and milk protein but, again, these were unaffected by FE; values were, for milk yield, 13.6, 16.3, 13.7 and 13.1 kg day-1; and, for milk protein yield, 420, 545, 429 and 414 g day-1 for basal, F, FE1 and FE2, respectively. However, supplementation with FE produced increases in the concentration (P < 0.00 1) and yield (P < 0.05) of milk fat; values were 42.8,39.9,46.6 and 47.8 g kg-1 and 575, 646, 628 and 618 g day-1, respectively, for the basal, F, FE1 and FE2 treatments. The profile of amino acids in blood plasma was characterised by markedly lower (at least P < 0.01) concentrations of methionine, lysine, tryptophan and histidine for the diets containing FE compared with F. The results show pronounced effects of the amino acid composition of the undegraded protein component of the supplement on both silage intake and milk production but also draw attention to a possible relationship between amino balance and the secretion of milk fat.