The indirect determination of energy retention in farm animals: II. Studies on the validity of the method

Abstract

1. A series of simultaneous determinations of the Na, K, N, C, ash, H₂O and energy retentions of calves have been made in order to test the validity of a method of estimating energy retention without recourse to respiration chamber technique. The experiments involved 72 days' calorimetric observation with six animals. A further series of fourteen experiments with sheep involving 58 days' calorimetric observation were made but are not reported in detail2. It was found that, with calves, water retention could be estimated by an indirect method from Na, K and ash metabolism without significant systematic error. The standard deviations attached to the indirect estimates appeared to be invariant with length of period. Thus, by increasing the length of the period the accuracy of the mean daily estimate of water retention could be considerably increased.3. With calves, energy retention measured indirectly from body-weight gain and the gain of ash, protein and water did not differ significantly from that determined calorimetrically by the C and N balance technique. The standard deviation of the mean daily difference fell with increasing length of period, being ± 407 Cal./24 hr. in an experiment lasting 8 days. It was calculated that experiments with four calves lasting 24 days would give results of the same precision as that obtained in a single calorimetric trial.4. The use of the ratio calorie retention/protein retention could permit the calculation of energy retention from the N metabolism of the animal. It is suggested that this method, if developed to take into account the effect of nutritional plane on the ratio, could have considerable practical value.5. It was found that cutaneous losses of K by the sheep in its skin secretions (suint) make it impossible to interpret the difference between the intake and faecal and urinary losses of K as a body gain of K and express it in terms of water. The method is not therefore suitable for the sheep.6. The difficulties involved in the application of the method to adult cattle are pointed out, particularly those that stem from the high K content of most of the foods they eat. A very high analytical accuracy is necessary if the estimates of Na and K retention are to have the necessary precision.