Dietary phase-feeding as feedlot strategy for Santa Ines lambs: performance, N retention and meat quality

We executed two studies to investigate the possibility of reducing crude protein (CP) content throughout feedlot of growing lambs, when they are changing their body composition, and passing from most protein retention to fat deposition for finishing. A performance trial (EXP 1) with 40 Santa Ines crossbred sheep (initial body weight (BW) of 15.6±1.6 kg) were carried out in a completely randomized design, with four treatments and 10 repetitions. The EXP 1 lasted a total of 85 days, with 15 for adaptation and 70 for data collection. The investigated diets were formulated with two CP contents, in which 150 g of CP/kg DM (High CP (HCP)) diets represented the adequate nutritional requirement for growing lambs, or 130 g of CP/kg DM (Low CP (LCP diets)), which represented the reduced content. We also evaluated the feedlot phases, in which the initial phase-feeding (IPF) comprised day 1 to 35, and the final phase-feeding (FPF) comprised day 36 to 70. The combination of these factors resulted in the following treatments (T): T1 - Animals fed with the lower CP content during all the feedlot period, 130 in IPF and 130 g of CP/kg DM in FPF (T1: LCP-LCP); T2-Animals were fed with increasing content of CP along feedlot (T2: LCP-HCP); T3- Animals were fed with decreasing content of CP during the phases (T3: HCP-LCP); T4 - Animals were fed with the greater CP content during all feedlot period (T4: HCP-HCP). We carried out a second study to investigate the ruminal fermentation parameters (EXP 2). Four adult Santa Ines sheep (average BW of 60.6±10.6 kg), fistulated in rumen, were distributed in four 2 × 2 Latin squares. The same CP contents from EXP 1 were evaluated: LCP diets (130 g of CP/kg DM) and HCP diets (150 g of CP/kg DM). There was effect (P> 0.05) of CP contents in FPF on CP intake and urinary N excretion, being higher for the animals fed with HCP diets. The phase-feeding did not affect the microbial CP synthesis and microbial efficiency (P> 0.05), and also did not affect (P > 0.05) meat quality. The animals fed with LCP or HCP diets in both IPF and FPF presented a similar ADG (P = 0.65 and P > 0.15, respectively) and final BW (FBW, P = 0.66 and P= 0.24, respectively). Sheep fed with HCP diets had higher concentrations of ruminal ammoniacal N (N-NH₃, P < 0.01), but demonstrated similar (P > 0.05) volatile fatty acids (VFA) production. Our results demonstrate that no benefits on rumen fermentation, performance and meat quality were found when more than LCP diets was provided during the feedlot, nor when phase-feeding was applied to crossbred Santa Ines lambs.