Value and limitations of using tritiated water for predicting body composition and water intake in the African Zebu

The tritiated water space gave a 10% overestimate of the total body water pool obtained by dissection and drying, using 12 Zebu steers. The decrease in per cent body fat between a weight of 220 and 190 kg was significantly related to an increase in per cent body water pool. The per cent body fat could be predicted from the per cent tritiated water space, but the approach was considered questionable, particularly outside the bounds of these trials. A comparison between water input from food and drink plus calculated metabolic water and tritiated water turnover was made in 6-12 Zebu steers on four occasions, to give a total of 42 individual comparisons. It was found that there was a shortfall of 11% between input and turnover even after correction of the overestimate of the body pool by tritium. The shortfall could be accounted for by respiratory water exchange, which was estimated to represent 10% of the total water input. Although the mean ratio of input:tritiated water turnover was 0.96+-0.022, there were significant differences between individual ratios and experiments. The main reason for the difference was probably due to non-steady-state conditions during the periods of estimating water turnover. It was concluded that errors in the prediction of total water input from tritiated water turnover will be the rule rather than the exception for tropical herbivores in semi-arid areas where intermittent watering is practised unless the experiment is very carefully designed. (author)