Supplementation with natural additives such as essential oils (EO) or probiotics has resulted in comparable growth performance to that of supplemental monensin in fattening lambs in hot environments. Supra-supplementation levels of vitamin D3 improved the carcass weight and dressing percentage of steers fattened under tropical conditions. We hypothesized that certain combinations of these natural additives could be complementary. For this reason, a feeding trial was carried out using 48 Pelibuey × Katahdin non-castrated male lambs (107 ± 14 d age; 17.9 ± 2.51 kg LW). Lambs were fed an 88:12 concentrate to forage ratio basal diet supplemented (dry matter basis, DMI) with: (1) no additive (CON); (2) 28 mg monensin/kg diet (MON); (3) 150 mg of essential oils containing a combination of thymol, eugenol, vanillin, guaiac, and limonene plus 0.12 mg vitamin D3 (EO + D3)/kg diet; and (4) 300 mg of essential oils containing a combination of carvacrol and cynamaldehyde plus 2 g probiotic (2.2 × 10⁸ CFU of bacillus subtilis/kg diet, EO + BS). Lambs were grouped by initial weight and assigned within six weight groupings to 24 pens (2 lambs/pen, 6 replicas per treatment) in a randomized complete block design. The experiment lasted 121 days. Daily maximal THI exceeded the 80 “danger or “emergency” range for 119 days of the 121 days of the trial. Lambs supplemented with MON had similar DMI, growth performance, and dietary energetics to those of CON lambs. Lambs supplemented with EO + BS had a greater (9.2%, p ≤ 0.05) average daily gain (ADG) than the CON and MON groups due to enhanced (10.2%, p ≤ 0.05) dry matter intake. Thus, gain efficiency (GF) and estimated dietary energy were similar for CON, MON, and EO + BS. Lambs receiving EO + D3 had similar (0.254 vs. 0.262 kg/d) ADG but a lower DMI (8%, p < 0.05) compared with EO + BS lambs. Consequently, GF and estimated dietary net energy were greater (4.9 and 3.7%, respectively; p ≤ 0.05) for EO + D3 lambs. Even when ambient heat load was elevated, the efficiency of utilization of dietary energy (observed-to-expected dietary net energy) was close to 1.00 (0.992) expected for EO + D3 lambs. In contrast, efficiency of energy utilization was depressed by −4.4% for lambs on the other treatments. Compared with the other treatments, lambs receiving EO + D3 had greater longissimus muscle area (5.6%, p < 0.05) and lower kidney pelvic fat (21.8%, p ≤ 0.05). There were no treatment effects on shoulder tissue composition or whole cuts (expressed as % of cold carcass weight). Compared to CON, lambs that were fed with natural additives showed 3.5% lower (p ≤ 0.05) intestine mass. All supplemental additives decreased visceral fat mass, which was minimal with EO + D3 treatment. Combinations of essential oils with vitamins or probiotics were superior to antibiotic monensin in finishing diets for feedlot lambs. Combining EO with probiotics promoted DM intake and gain but not gain efficiency, while combining EO with vitamin D3 supra-supplementation increased dietary energy efficiency and improved some carcass characteristics in lambs fattening under high ambient heat loads.

Several studies have shown that supplementation with 100 to 130 mg of 3-nitrooxypropanol (3-NOP)/kg diet acts as a mitigating factor of enteric CH₄ production in ruminants. From an energy perspective, this effect could indicate improved feed energy utilization. Feed additives that reduce the acetate-to-propionate molar ratio and/or CH₄ production generally increase the efficiency of feed energy utilization and can alleviate the negative impact of high ambient heat loads on ruminant productivity. In seeking to test this assumption, the impact of supplementing 3-NOP in growing–finishing diets was evaluated in 24 intact male lambs (31.92 ± 3.77 kg). The experiment lasted 61 days. Treatments consisted of supplementing a total mixed growing–finishing diet (30:70 forage-to-concentrate ratio) with zero or 115 mg 3-NOP/kg diet. Lambs were assigned to 12 pens (two lambs/pen, six replicates per treatment). The temperature–humidity index (THI) during the experiment averaged 83.37 ± 6.4. The inclusion of 3-NOP tended to increase final weight (2.6%, p = 0.06) but increased dry matter intake by 10.6% (p = 0.03), thus decreasing the efficiency of dietary net energy utilization by 2.3% and 3%, respectively (p = 0.04). Lambs fed with 3-NOP showed greater (6.2%, p = 0.04) carcass weight and dressing percentage (3.3%, p = 0.03) without effects on the tissue shoulder composition. Supplemented lambs showed lower gastrointestinal (GIT) fill (9.3%, p = 0.02) and greater (1.3%) empty body weight (EBW, p < 0.01). Visceral organ mass (expressed as g/kg EBW) was not affected by 3-NOP supplementation. It was concluded that supplemental 3-NOP did not improve feed efficiency nor the efficiency of dietary energy utilization, but did improve carcass weight and dressing percentage in lambs fattened under high ambient heat load. The greater carcass weight observed in the present experiment was due mainly to a tendency for a greater final weight (p = 0.06) for 3-NOP lambs, whereas the improvement in dressing percentage was due mainly to a lower (p = 0.02) GIT fill. It is crucial to highlight that this is a pioneering study on the effect of 3-NOP on the productive efficiency of lambs subjected to high ambient heat loads. It is also important to note that enteric methane production was not measured in this experiment. Although the doses used in this experiment have consistently reduced methane production in several studies conducted under favorable climatic conditions, we cannot precisely determine the role of CH₄ production in the dietary net energy efficiency observed in lambs that received 3-NOP. The results presented here provide a basis for future research evaluating the anti-methanogenic and productive responses to the use of 3-NOP under high ambient temperature conditions.