Factors affecting body composition and mineral retention in growing lambs
In the studies described in this thesis the amounts of Ca, P and Mg retained by growing lambs were determined following the slaughter of the lambs and their analysis. In experiment 1, the effects of growth rate on body composition and mineral retention were studied. The lambs were grown at different rates from 24 kg to 50 kg live-weight. The composition of empty-body gain (EBG) was estimated based on an initial slaughter group at the beginning of the trial. Fast growing lambs contained more fat and less protein, ash, Ca and P than slow growing ones. However, in the fat-free empty-body (FFEB), there were no significant differences found attributable to growth rate, except for Mg which was decreased with slow growth. Supplementary CaCO3 significantly increased ash, Ca and P retention in FFEB of the fast growing lambs. In experiment 2, the retention of minerals from lambs fed either forage or concentrate diets was studied. Three groups of 8 Suffolk x Blackface wether lambs were (a) allowed to graze the grass outdoors (b) fed-concentrate indoors and (c) fed grass indoors. The lambs were grown from 24 kg and slaughtered at about 40 kg live = weight over 100 days. Retentions of Ca and P in empty-body gain (EBG) and fat-free empty-body gain (FFEBG) were significantly higher in grass-fed lambs. Higher mineralisation in the latter group was attributed to differences in acid-base balance between the diets and was not a function of either exercise or sunlight. Experiment 3 was designed to re-assess the requirements of Ca and P for growing lambs. Four groups of 10 Suffolk x Blackface lambs were fed concentrate diets containing different concentrations of Ca and P over a 93 day experimental period. Lambs fed on normal Ca-normal P diet (based on TCORN's (1988) new estimate) appeared to meet their requirement. Lambs fed on high Ca-high P diet showed no increase in mineral retention while those on low Ca-low P diet were more poorly mineralised. A high Ca-low P group had low concentration of P in blood and rumen liquor. Radiographic studies of the metacarpus and metatarsus from these trials were of limited value in predicting the mineral content of the empty-bodies of lambs.