The effect of dietary non-protein nitrogen content on the performance of finishing lambs

The effect of increasing non-protein nitrogen content on nutrient digestibility, performance, and carcass characteristics of South African Mutton Merino wether lambs on low-fibre finishing diets containing similar degradable protein was investigated. The production and digestibility studies consisted of 60 and 32 lambs, respectively. Four similar dietary treatments were formulated with different non-protein nitrogen contents (16.6 g/kg, 28.3 g/kg, 40 g/kg and 51.7 g/kg) on a dry matter basis. The production study was conducted over 71 d, and the digestibility study over 7 d. Crude protein digestibility increased with non-protein nitrogen content. Organic matter (40 g/kg and 51.7 g/kg), metabolizable energy content (51.7 g/kg), as well as the digestibility of neutral detergent fibre (40 g/kg) and acid detergent fibre (28.3 g/kg and 51.7 g/kg) also increased with a higher non-protein nitrogen content. A lower non-protein nitrogen content within the 16.6 g/kg and 28.3 g/kg treatments resulted in a higher average daily gain compared to the 51.7 g/kg treatment. Additionally, more effective energy was used for growth compared to treatments with 40 g/kg and 51.7 g/kg non-protein nitrogen. Dietary treatment left most of the carcass characteristics unaffected. In conclusion, a low non-protein nitrogen content of low-fibre lamb finishing diets with similar degradable protein content affected animal performance favourably, despite having an opposite effect on nutrient digestibility.