Effects of different rumen undegradable to rumen degradable protein ratios on performance, ruminal fermentation, urinary purine derivatives, and carcass characteristics of growing lambs fed a high wheat straw-based diet

•Different RUP:RDP ratios by inclusion of extruded soybean meal (RUP = 64 %) instead of conventional SBM (RUP = 26 %) were evaluated in lambs fed wheat straw based-diet.•Weight gain, N efficiency, microbial protein yield, blood glucose and insulin were enhanced with increasing RUP:RDP ratio.•Increasing RUP:RDP ratio increased dressing percentage and decreased mesenteric fat.•High dietary RUP:RDP ratio is recommendable in growing lambs received low-quality forage. The present study aimed to evaluate the effects of various rumen undegradable to rumen degradable protein ratios (RUP:RDP) on the performance, rumen fermentation pattern, urinary purine derivatives, and carcass characteristics in growing lambs when received a high dietary wheat straw level (420 g/kg, DM basis). Lambs (average initial live weight = 27.2 ± 2.4 kg) were individual housed (n = 18, 6 animals/treatment) to evaluate the effects of treatments. The RUP:RDP ratios tested were obtained through replacing extruded soybean meal (ESBM; RUP = 64 %, based on total CP) instead of conventional soybean meal (CSBM; RUP = 26 %, based on total CP) in proportion of 0, 50, and 100 % in order to obtain ratios of (1) low RUP:RDP ratio as LR diet (25:75); (2) moderate RUP:RDP ratio as MR diet (30:70) (3) high RUP:RDP ratio as HR diet (35:65). The study lasted 10 weeks. Dry matter intake did not differ among experimental treatments (P = 0.80), while average daily gain (ADG) and final BW were linearly improved (P < 0.05) with increased RUP:RDP ratio. Accordingly, the lowest feed efficiency observed in LR lambs and it was reduced linearly when RUP:RDP ratio was increased (P = 0.01). Although the amount of nitrogen intake was constant across experimental treatments (P = 0.69); however, the nitrogen to gain conversion ratio was increased as RUP:RDP being increased (P = 0.02). Digestibility of organic matter (P = 0.02) and crude protein (P = 0.03) as well as ruminal concentration of acetate (P = 0.05), urinary concentration of allantoin (P = 0.01), and blood glucose and insulin concentrations (P = 0.01) were linearly increased when lambs received diets contained high RUP:RDP ratio. In contrast, ruminal propionate concentration (P = 0.02) and urinary nitrogen concentration (P = 0.02) were reduced as RUP:RDP being increased. The dressing percentage was increased (P = 0.02) but mesenteric fat content (P = 0.03) and back-fat thickness (P = 0.01) were reduced when diets with higher RUP:RDP ratio being fed to lambs. In