Effect of plane of nutrition on energy and nitrogen retention and on plasma urea concentrations in Southdown ram hoggets from high and low backfat selection lines

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Abstract
Tweuty-four Southdown ram hoggets (12 each from the Massey University high and low backfat selection lines) were offered lucerne chaff at 0.7 or 1.2 times maintenance requirement in a balanced factorial design. Energy and nitrogen (N) balances were measured over a 10-day period. Rams were then placed on a 2-hourly feeding pattern and differences in plasma hormone and metabolite concentrations were examined. No differences were observed between selection lines in components of energy or N balance at 0.7 maintenance. Low (backfat) line rams offered 1.2 maintenance exhibited greater diet selection than high (backfat) line rams with the result that they had lower energy intake and energy retention (intake less faecal and urinary losses). N intakes were also lower in low line rams at 1.2 maintenance but, because of their (not significantly) lower urinary N excretion, there were no differences between lines in N Despite lower energy retentions, low line rams exhibited higher concentrations of glucose in plasma. Differences in circulating insulin were not apparent, retention. suggesting that the pancreas of low line rams may be less sensitive to circulating glucose than that of high line rams. Low line rams exhibited greater concentrations of creatinine in plasma than high line rams, particularly at 0.7 maintenance. Differences in plasma urea concentration were apparent only at 1.2 maintenance, with concentrations being greater in high line rams than in low line rams. It is concluded that low backfat line rams break down muscle protein more readily during underfeeding whereas at high allowance they use dietary amino acids more efficiently for production. In contrast to studies with tleeceweightselected lines, differences in kidney function appear to contribute little to differences between high and low backfat line rams in plasma urea concentration.