The effect of body weight loss and regain in mature wethers

An investigation was conducted to study the effect of body weight loss and regain on the relationship between the excretion and retention of nitrogen and sulphur in mature, cross-bred wethers. A reduction in dry matter intake from 800g/day to 200g/day resulted in about 25% loss of live weight after 7 weeks. The greatest loss occurred in the fat stores (5kg or 50% of initial weight). When re- fed the maintenance diet (i.e. rehabilitation) the wethers failed to fully recover their lost live weight over 10 weeks and total body water accounted for, on average, 88% of the live weight gain, protein 10% and ash 2%, fat made very little or no contribution to the weight regain. Reasons for the failure to completely recover the lost live weight, such as inadequate dietary energy, protein and/or minerals have been discussed. The balance trial indicated that during undernutrition therOtas'greater: retention of sulphur relative to nitrogen within the body, pre sumably for wool growth. The N:S excretion ratios, 15-37:1, were gen- erally wider than the 15:1, which would be expected if the excretion ratio reflected tissue protein catabolism. By including the losses of nitrogen and sulphur in wool grown during undernutrition, the N:S excretion ratios decreased to 9-15:1. During positive balance (i.e. rehabilitation) the N:S retention ratios of 10-15:1 were similar to those observed by other workers, but the deduction of nitrogen and sulphur deposited in wool resulted in wider ratios of 19-45:1. The - plasma levels of urea nitrogen and inorganic sulphate sulphur were not greatly affected by either undernutrition or rehabilitation. A subsequent balance trial which considered the effect of body weight loss, in respect to nitrogen and sulphur depletion in tissue,and nitrogen and sulphur balance, revealed that there was a disproportionate loss of nitrogen and sulphur from muscle tissue in that there was a greater retention of sulphur in muscle tissue particularly in the sheep which were fed higher dietary levels of nitrogen. However, there was no evidence to suggest that the higher dietary nitrogen levels in any way influenced overall body protein loss. There were large discrepancies between the results for protein depletion as assessed by the protein prediction equation of Donnelly and Freer (1974) and those obtained by nitrogen balance determinations, for both experiments. Of the possible errors discussed, it appears that the loss of ammonia from the urine before analysis, may be the major source of error even though precautions were taken to minimize such losses. The use of the measured amounts of sulphur in muscle tissue and sulphur balance data gave the better estimates of wool growth than-nitrogen balance data and accepted N:S tissue ratio of 15:1, when compared with the results obtained from dye-bands or shearing. The third experiment studied the effect of body weight loss and regain on the storage of copper, zinc and vitamin B 12 in liver. The losses of fat, water, protein and ash were not completely re- covered by refeeding the maintenance diet. Total body water regained, on average, 95% of its lost weight, protein 60%, ash 63% and fat only 32%. The loss of liver protein was accompanied by losses of copper, zinc and vitamin B12 but serum levels were not elevated during under- nutrition. The recovery of the undernourished animals were not improved by higher dietary levels of copper and cobalt. The excess trace elements were apparently excreted. The inadequate supp1y of energy was probably the major determinant of the failure of the sheep to completely recover their lost live weight in the present experiments. Wool growth was significantly reduced by undernutrition but there was very little evidence of reduced sulphur concentration in wool (experiment 2). Refeeding improved wool growth but not necessarily to the levels prior to undernutrition (experiment 3). The sulphur concentration in wool was reduced during the recovery period (experiment 1). However, the levels of copper and zinc in wool were not influenced by either undernutrition or rehabilitation.