Effect of varying levels of dietary undegradable protein on nutrient intake, digestibility and growth performance of Awassi lambs fed on high wheat straw diets

The effect of feeding diets with varying levels of dietary undegradable protein (UP) on growth performance of Awassi lambs fed on high wheat straw diets was studied. Thirty lambs (17.2 kg) were assigned randomly to three isocaloric (2.32 Mcal/kg), isonitrogenous (16% CP) diets differing in their UP content in a completely randomized design. The experimental diets contained 16.1 (LUP), 22.9 (MUP) and 28.9% (HUP) of the dietary CP as UP. Lambs were housed in individual pens and fed the experimental diets ad libitum as total mixed diets. No significant difference was observed in DM and CP intakes for lambs fed the experimental diets, and averaged 985 and 157 g/d, respectively. However, lambs fed the HUP diet consumed more (P < 0.05) UP as compared to lambs fed the LUP diet. No significant (P > 0.05) difference was observed in estimated metabolizable energy intake (2.24 Mcal/d). Dry matter digestibility was higher for lambs fed the MUP and HUP diets (average = 60.4%) as compared with lambs fed the LUP diet (56.2%). Apparent CP digestibility increased (P > 0.05) with the increase in dietary UP level (57.3, 60.1, and 63.6% for the LUP, MUP, and HUP diets, respectively). Final weight for lambs fed the HUP and MUP diets were higher (P < 0.05) compared with lambs fed the LUP diet. Lambs fed the MUP and HUP diets had greater (P < 0.05) average daily gain (ADG; 264 g/d) as compared with lambs fed the LUP diet (219 g/d). When all data were pooled, a significant (P < 0.05) positive non-linear relationship was observed between UP and ADG. The higher the UP intake the higher the ADG observed for all the experimental animals until it plateaued at 50.3 g/d of UP. In summary, higher dietary UP intake improves both nutrient digestibility as well as growth performance of Awassi lambs. The results show that 50.3 g/d of UP is required for maximum growth of Awassi lambs regardless of the dietary concentration of UP.