Growth Performance, Rumen and Cecum Fermentation Parameters, and Microbial Protein Synthesis in Kermani Lambs with Divergent Residual Feed Intake Fed Forage and Concentrate Diets

< p style="text-align: justify;">It was hypothesized that some of the variations in rumen and cecum fermentation parameters can form a share of individual differences resulting in feed efficiency, which may be altered based on the type of diet. This research aimed to determine the effects of dietary effect and feed efficiency of growing lambs (Kermani lambs) on their rumen and cecum indices and also microbial protein synthesis in gastrointestinal tract. Lambs (n=40; bodyweight (BW)=16±1.5 kg) were fed either a concentrate (CONC; 11.8% crude protein (CP), 18% neutral detergent fiber (NDF), 2.66 Mcal/kg metabolizable energy (ME); n=20) or a forage-based diet (FOR; 15.6% CP, 36.8% NDF, 2.15 Mcal/kg ME; n=20). Individual intake was recorded and residual feed intake (RFI) was determined over 42 days. The 8 highest (Low-RFI) and 8 lowest efficiencies (High RFI) records of lambs from each dietary group were selected (n=16; average BW=20±2.1 kg), and the samples of rumen and cecum fluid, and also urine were collected at the end of the trial. Data were analyzed as a 2 × 2 factorial design with RFI class (high vs. low efficiency), their type of diet (FOR vs. CONC), and their interaction in the defined model. Based on the results, high-efficiency lambs had a higher level (p < 0.01) of total volatile fatty acid (VFA), proportional concentrations of acetate, propionate, and ammonia nitrogen in the rumen in comparison to low-efficiency lambs. Higher (p < 0.01) amounts of allantoin, xanthine + hypoxanthine, total purine derivative (PD), microbial nitrogen and microbial protein were observed in the high efficiency than low-efficiency lambs. The low efficiency lambs had greater (p < 0.01) proportional acetate, cecal pH and cecal ammonia N compared to high-efficiency lambs. The RFI class × diet type interaction was significant (p < 0.01) for the majority of parameters of the rumen, cecum, and microbial protein synthesis. The results of this experiment exhibited that hindgut fermentation especially cecum played a key role in the efficiency of feed utilization in lambs which have consumed larger amounts of fermentable substrates.