Utilization by growing goats of diets that contain monensin and low or excess crude protein: comparative slaughter experiment.

Monensin was added at 0 or 23 mg/kg dry matter to low (8.3%) or high (17.5%) crude protein diets (2 X 2 factorial arrangement) that were fed to 28 growing wether goats for 194 to 256 d. Average daily gain increased 24% with monensin and 20% with higher crude protein (P less than .05). Feed conversion was improved by both monensin and higher crude protein (P less than .001). During the experiment, digestion and metabolism trials were conducted twice (Periods I and II) with each goat. Apparent dry matter and gross energy digestibility coefficients were not affected by monensin in either period. Digestibility of dietary N was enhanced 7% by monensin in Period II (P less than .01) and tended to be improved in Period I. Higher dietary protein increased all digestibility coefficients (P less than .001). Nitrogen retention was not affected by monensin but was greater (P less than .001) for high-protein-fed goats. Potential changes in empty body composition were assessed by comparative slaughter balance. No effects of dietary treatments on percentage composition of empty body were noted, except a trend for higher protein in monensin-fed goats compared with controls (P less than .1). Empty body protein gain as a percentage of protein intake improved 41% with monensin and decreased 37% with higher dietary protein content (P less than .05). Monensin increased conversion of dietary energy to body ether extract energy (12%, P less than .05), to protein energy (35%, P less than .001) and to total body energy (23%, P less than .001). Magnitude of improvement tended to be greater with low than high dietary protein treatments, suggesting a protein-sparing effect of monensin as determined by comparative slaughter method. However, N retention, measured by intake-excreta balances, was not affected by monensin, suggesting that these point estimates are less than adequate to evaluate true body protein gain.