A study of the protein requirements of the mature breeding ewe

Abstract

1. An experiment was carried out to study more precisely by nitrogen balance techniques the intake of digestible crude protein required for maintenance in the mature non-pregnant ewe. 2. Four isocaloric diets supplying adequate energy, approximately 90 kcal/kg W^0.73 metabolizable energy daily, and differing in crude protein content were each given to eight individually penned ewes. The diets provided 2.4, 4.9, 7.7 and 9.5 g digestible N per ewe per day. 3. The average weight of the ewes was 57.4 kg. They were rationed according to metabolic body-weight (W^0.73) at a rate of approximately 800 g dry matter per 50 kg ewe per day for a 4-week period before N balance studies were carried out over an 8-day collection period. 4. The mean apparent digestibilities of dry matter were 67.3±0.8, 68.1±0.7, 70.9±1.0 and 68.8±0.8 respectively. The apparent digestibilities of N, increasing with increasing N intake, were 30.6±2.1, 46.3±2.1, 58.2±0.6 and 61.5±1.3 respectively. 5. The intake of apparently digested N required for maintenance was calculated in three ways, from the regressions of apparently digested N on N retention or on urinary N and from the underlying relationship between N retention and urinary N. The estimates so obtained were respectively 0.185±0.037, 0.148±0.020 and 0.150±0.020 g N per kg W^0.78 per day, corresponding to 1.16, 0.93 and 0.94 g apparently digestible crude protein per kg W^0.73 per day. 6. Metabolic faecal N, determined by the extrapolation method, was 0.629±0.047 g/100 g dry matter consumed. 7. The results are discussed in relation to practical feeding standards and other research findings.