Relative retentions of the nitrogen of urea and groundnut in isoenergenetic diets for growing heifers

Abstract

1. Two experiments were conducted to compare the effect on nitrogen retention of three dietary levels of N given either as groundnut or as urea to growing heifers. Additions of maize starch or dextrose were made to the diets to equalize the inputs of digestible energy within and between experiments.2. In the first experiment, in which the maximum level of N supplementation was 69·0 g/d, the response to additional N was linear, and was identical for both sources of N. Differences in faecal N between treatments were small; differences in urinary N were large and were entirely attributable to level of N intake.3. In the second experiment, the maximum level of N supplementation was raised to 103·3 g/d. The response to additional N was again linear and identical for both sources of N; however, for a given level of N input, the amount of N retained was 4·6 g less than in Expt 1. This reduction in N retention may have been due to the change in the proportion of digestible energy derived from the fibrous components of the ration.4. Live-weight changes calculated from the observed N retentions have been compared with published responses to the inputs of N and energy used in these experiments.