A comparison was made of the responsiveness to metabolic challenges of 10 rams from each of the Massey University High and Low Backfat Southdown selection lines. The lines were established by screening ewes from commercial sources on the basis of high or low weight-corrected backfat depths. Since the initial screening, 2.3 generations of selection have been undertaken, again on the basis of weight-corrected backfat depth (ewes and rams). Progeny of the High Line currently have 47% greater phenotypic backfat depths than those of the Low Line at 8 months of age. Challenges involved acute intravenous injections of adrenalin, glucagon, insulin, and glucose, a 63.5 h fast, and 7.5 h refeeding period. Relative to those in the High Line, Low Line rams exhibited: lower baseline concentrations of plasma urea during both the fasting and refeeding periods; higher growth hormone concentrations during the fasting and refeeding periods; increased lipolytic responses to the glucagon challenge; and larger glucose distribution spaces. No significant between-line differences were detected in responsiveness to the adrenalin or insulin challenges. It is concluded that baseline plasma urea concentrations,andresponsivenessofplasmaNEFA 1

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