Influence of diet on amino acid absorption in beef cattle and sheep.

Six sheep and three steers were fed pelleted alfalfa hay prior to and during the first blood sampling period (BSP) and then switched to an 85% concentrate (HC) diet at least 28 days before a second BSP. Portal plasma flow rates were higher in steers than sheep (7.9 versus 5.0 liters/hr per kg0.75). With HC feeding, sheep and cattle had higher plasma femoral arterial concentrations of glycine and 3-methylhistidine and lower concentrations of valine and isoleucine than when fed hay. When fed HC diets, sheep, but not cattle, had lower glutamate and histidine and higher serine concentrations than when fed hay. Sheep had higher concentrations of aspartate, asparagine, threonine, glycine, tyrosine, phenylalanine, valine, leucine, citrulline, arginine, lysine, 3-methylhistidine and histidine than cattle. Diet did not affect the portal venous-arterial concentration difference or the net portal amino acid appearance (NPAAA) rate of any of the amino acids in sheep or cattle. With sheep, there was a tendency for the NPAAA rate to be higher for all amino acids, except aspartate, with HC compared to hay feeding. Expressed on the basis of metabolic body size, NPAAA rate was qualitatively and quantitatively similar between sheep and cattle.