Feed Intake, Rumen Fluid Volume and Turnover, Nitrogen and Mineral Balance and Acid-Base Status of Wethers Changed from Low to High Concentrate Diets

Intake and excretory patterns, N and mineral balance, rumen fluid characteristics and acid-base status were measured for 15 days in five nonfistulated and six rumen-fistulated wethers. The animals were placed in metabolism crates and fed alfalfa hay from day 1 through 5, a 65% concentrate diet from day 6 through 10 and an 85% concentration diet from days 11 through 15. All diets were pelleted and fed ad libitum. Blood samples were taken via cannula from the jugular vein of nonfistulated wethers on days 5, 6, 10, 11 and 15, and from fistulated wethers on days 5 through 15. Rumen fluid turnover was determined in fistulated wethers on days 5, 10 and 15; rumen fluid samples were taken on days 5 through 15. There were no significant differences between nonfistulated and fistulated wethers in any balance data or blood characteristics except plasma alkaline phosphatase activity. Increased concentrate intake caused (P<.05) the following: decreased daily dry matter intake; decreased urine volume; increased urine Ca, Mg and P excretion; decreased Ca and Mg retention, including negative Ca balance during the 65% concentrate diet feeding period; decreased blood hematocrit, HCO3 , plasma L-lactate and Ca concentration; increased plasma P and Mg concentrations; increased rumen fluid L-lactate concentration and decreased rumen fluid volume and turnover. Rumen fluid D-lactate concentration was not significantly altered, but rumen fluid turnover decreased (P<.05) with increased concentrate intake. Transient increases in rumen fluid Ca concentration and turnover occurred with increased concentrate intake. N retention was not affected by diet change. Plasma alkaline phosphatase activity was not affected either, except for an increase in activity in nonfistulated lambs on day 15. These data show that, although acute acidosis or wide fluctuation in dry matter intake did not occur, mineral metabolism and acid-base status were substantially affected by increased concentrate intake.