The effect of selection for fleece weight upon urea metabolism and digestive function in Romney sheep

Abstract

The effect of 30 years genetic selection for greasy fleece weight upon rumen metabolism, apparent digestibility and nitrogen metabolism was investigated in Romney sheep.Fleece weight (Fwt) animals had a 1 mM lower plasma urea concentration than control (C) animals when fed on both diets given, but there was no difference in plasma urea IRL, urinary urea excretion or urea recycling to the digestive tract between the Fwt and C sheep when fed either diet. Relative to the C shccp, the Fwt animals had a higher creatinine clearance rate when fed the meadow hay diet, and a higher urea clearance rate when fed both diets.When fed the meadow hay diet there were no differences between the Fwt and C animals in voluntary feed intake (VFI) or apparent DM digestibility. However, when fed on the lucerne chaff diet the Fwt animals had a higher VFI than the C animals and a lower apparent DM digestibility. These changes were associated with an increase in the rumen fractional outflow rate (FOR) of lignin in the Fwt sheep, and a higher molar proportion of acetate and a lower proportion of propionate in rumen fluid.The differences in urea and creatinine clearance rates suggest that selection for fleece production may have altered kidney function, with expression of this and rumen FOR being related to nutritional level. A factor contributing to the superior wool production of the Fwt sheep is suggested.