The effect of supplements of fishmeal and lactic acid on voluntary intake of silage by calves

In order to assess the relative effects of protein content and acid on intake, grass silage was given ad libitum to twelve entire male calves, initially 4 months of age and 117 kg liveweight (LW), either alone or supplemented with fishmeal at 50 g dry matter (DM) per kg silage DM. These two diets were offered alone or with additions of lactic acid at 50 g per kg silage DM in a partial changeover design in which lactic acid levels only were reversed between two 28-d measurement periods. In the absence of lactic acid, fishmeal significantly increased absolute intake but not intake relative to LW. There was a significant interaction between treatments in that the addition of lactic acid depressed DM intake in the absence of fishmeal (23·9 vs 21·1 g kg^-1 LW) but not in its presence (23·2 vs 23·1 g kg^-1 LW). Addition of fishmeal increased the digestibility of DM from a mean of 0·708 to 0·744 (P < 0·05), of gross energy from 0·646 to 0·694 (P < 0·01) and of N from 0·422 to 0·592 (P < 0·001) but had no significant effect on the digestibility of cellulose (0·766 vs 0·788 for treatments without and with fishmeal respectively). Lactic acid addition had no effect on the digestibility of plant components and, further, interaction effects between treatments were not significant. Supplementation with fishmeal reduced the total time spent eating and ruminating (Ro value) from a mean of 284 to 240 min kg^-1 DM (P < 0·01) and increased urine pH from 8·34 to 8·50. Lactic acid influenced neither Ro value nor urine pH and the interaction effect did not reach significance for these two parameters. The results indicate that the addition of fishmeal can overcome a depressant effect of lactic acid on intake and it is suggested that the responses to treatment were mediated via changes in the ratio of energy to protein supply rather than as a result of treatment effects on digestive efficiency or acid-base balance.