A Blend of Essential Oils Plus Vitamin D3 Combined with Bacillus subtilis or with Virginiamycin on Performance, Dietary Energetics, and Carcass Traits of Lambs Finishing Under High Ambient Temperature

With the aim of evaluating the effects of combining a natural additive composed of a blend of essential oils plus vitamin D3 (EOD3) with Bacillus subtilis (BS) or with the antibiotic virginiamycin (VM) on growth-performance and carcass traits of finishing lambs subjected to high ambient heat load, 48 intact male lambs (initial weight = 33.71 ± 3.21 kg) were used in a feeding trial lasting 61 d. Treatments consisted of supplementing (as a feed basis) a total mixing finishing diet (90:10 concentrate-to-forage ratio) as follows: (1) without feed additives (Control); (2) supplemented with EOD3 (a blend of 100 mg essential oils plus 0.10 mg vitamin D3/kg diet); (3) diet supplemented with EOD3 plus 2 g Bacillus subtilis PB6/kg diet to provide 2.2 × 108 CFU (EOD3+BS); and (4) basal diet supplemented with EOD3 plus 25 mg virginiamycin/kg diet (EOD3+VM). The temperature–humidity index (THI) during the experiment averaged 82.52 ± 1.10. Compared to non-supplemented lambs, lambs that received EOD3 alone showed greater (p < 0.03) average daily gain (ADG; 15.3%), gain-to-feed ratio (GF; 12.3%), and observed-to-expected dietary NE ratio (5.9%). The combination of EOD3 with BS resulted in identical ADG but a numerically larger DMI (7.1%, p = 0.087) than the EOD3 treatment, leading to lower (p = 0.024) GF and a lower (p < 0.01) observed-to-expected dietary NE ratio. In contrast to our expectation, combining VM and EOD3 did not increase growth response (p = 0.69), GF (p = 0.49), or dietary energy utilization efficiency (p = 0.09) when compared to EOD3 alone. Because the lambs that received supplemental EOD3 alone and their combinations showed greater final weight, hot carcass weight was heavier than in controls; however, no effects were observed on the other carcass variables evaluated, including whole cuts and shoulder tissue composition. Lambs that received EOD3 and their combinations showed a lower relative intestinal mass than controls (p < 0.04). This result confirms the role of EOD3 as a tool to minimize the negative impact of high ambient temperature on the performance of fattening lambs. Combining EOD3 with Bacillus subtilis or with VM did not improve energy utilization efficiency or carcass traits compared with EOD3 supplementation alone in lambs fattening under high ambient heat load. Further research is needed to better understand the interactions among different additives and to propose more effective supplementation strategies in animals subjected to different environmental conditions.