The effect of different RUP: RDP ratios in sheep fed high wheat straw diet on ruminal fermentation, nutrient digestibility, blood metabolites, and microbial protein yield

Wheat straw (WS) has low nutritional value; however, because of simple availability as well as its low price rather than other forage sources has high consumption in lamb production. Different methods such as chemical processing for WS has been suggested that could be have some health concerns for both producer and animal. The other strategy is to improve fiber digestion of low quality forages with balanced nitrogen sources in diet. Greater rumen undegradable protein (RUP) level in diet of ruminansts those fed low quality forages caused greater efficiency. Therefore, in the present study we evaluated the effects of different ratios of RUP: RDP in sheep fed with high level of WS (400 g/d) on fermentation parameters, nutrients digestibility, blood metabolites, and purine derivatives excreted through urine.

The study was carried out on three ruminal fistulated sheep in 3 × 3 Latin square design with 21-d periods (the first 14-d for adaptation period and the last 7-d for sample collection). The treatments were; (1) LR; low RUP: RDP ratio (30:70); (2) MR; medium RUP: RDP ratio (35:65) (3) HR: high RUP: RDP ratio (40: 60). For RUP modification in diet and having different RUP: RDP ratios, processed soybean meal was replaced with soybean meal. The basal diet contained high amount of WS (400 g/kg, DM basis) as low quality forage. Ruminal fermentation pattern, microbial protein synthesis, nutrients digestibility, blood metabolites and urinary purine derivatives were evaluated in the current study.

The results show that the intake did not differed among treatments (P > 0.05). The concentration of short chain fatty acids (SCFA) was constant among treatments; however, acetate concentration (P = 0.05) as well as branched chain volatile fatty acid (BCVFA) concentrations (P = 0.04) were greater in LR diet. The greatest ruminal ammonia nitrogen was obtained for LR diet as well. Neutral detergent fiber digestibility was the greatest value for LR diet among treatments (P = 0.04). There was no significantly different for another nutrients digestibility. Considering blood metabolites, HR diet reduced blood urea nitrogen. But there was no other effect of treatments. The liver enzymes (AST and ALT) did not differed among treatments. The urinary excretion of allantoin (P = 0.03) was increased in HR diet and subsequently purine derivatives excretion (P = 0.04) was improved in this diet. Urinary urea nitrogen (UUN) (P = 0.05) concentration was reduced in HR diet that was a greater nitrogen efficiency.

The present study results show that the high level of RUP: RDP (40:60) caused increased MPY and reduced UUN. In conclusion lower level of RUP: RDP improved digestibility, but from the nitrogen efficiency perspective, the greater RUP: RDP could be recommendable in diets high in wheat straw as low quality forage.