The response of ram lambs from the Massey university backfat selection lines to a urea load were measured in order to ascertain whether there were line effects on urea space and to evaluate relationships between urea space and empty body water content. Thirty- six 6-8 month-old ram lambs held in metabolism crates and fed lucerne chaff at 1.3 maintenance, were given 120mg urea/kg through jugular catheters. Plasma urea levels were measured in samples taken before and after the urea load. All animals were slaughtered within 7d of the urea challenge and total body water was determined. In contrast to previous studies, baseline plasma urea levels were not significantly higher in the high-backfat-line animals and were not related to carcass fat or water content. The increases in urea levels following the urea load were greater for the high backfat line at 15min, but not at subsequent sampling times. Clearance rates following the load did not differ between the lines. Correlations between empty body water and estimates of urea space were low, suggesting that this is not a useful means of obtaining body composition information for live sheep at this stage. Urea space estimated in several ways was significantly higher for the low backfat line group.
Proceedings of the New Zealand Society of Animal Production, Volume 50, , 59-72, 1990
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