The study was conducted at a commercial farm, in a building designated exclusively for the experiment. This allowed the animals to be kept free from external disturbances and under controlled and standardized environmental and management conditions.

Thirty-two castrated male pigs from a commercial line, in the growing-finishing stage, with an average initial live weight of 62 ± 2.5 kg, were used. The animals were randomly assigned to four treatments, with eight replicates per treatment. Each pig was housed in an individual 1.2 m² pen, equipped with a hopper feeder and nipple drinker, and was considered the experimental unit. Feed and water were offered ad libitum throughout the experimental period. To maximize voluntary intake, 10% more feed was provided than the amount consumed the previous day.

The basal diets (Table 1) were formulated to meet or exceed the nutritional requirements for pigs in the growing and finishing phases. All treatments were supplemented with the following enzymes: phytase (10 g/100 kg), xylanase (15 g/100 kg), protease (40 g/100 kg), and a mycotoxin adsorbent (Micofix, 150 g/100 kg). The treatments were:

Control: Basal diet without additives (no ractopamine and no AO Natural Fit Pro supplements).AO NATURAL FIT PRO®: Basal diet supplemented with the additive for the 8 weeks prior to slaughter. AO NATURAL FIT PRO® is a bioactive compound derived from plants such as tannins, purple basil, oroval, Indian gooseberry, and garlic. These compounds function as antioxidants that help protect animals from oxidative stress.Ractopamine: Basal diet supplemented with 10 ppm of ractopamine for the 5 weeks prior to slaughter.AO NATURAL FIT PRO® + Ractopamine: Basal diet supplemented with both additives, administered according to the timings of treatments 2 and 3, respectively.

The study was conducted in two phases: 1) Evaluation of carcass characteristics at the slaughterhouse. At the end of the experimental period, the 32 pigs were transported to the municipal slaughterhouse in Toluca, State of Mexico, where they were slaughtered following humane methods approved by current regulations. At the time of slaughter, the pigs must be healthy and have rested adequately (2 to 4 hours), especially if they have traveled for many hours. It is very important that the pigs destined for slaughter are properly immobilized before stunning and bleeding; once this is completed, the slaughtering process takes place [23]-[25]. The hot carcass weight (HCW) was obtained, and the relative weights of the carcass and its components were calculated in relation to the final live weight (g/kg LW).

For the chemical analysis of the diets, 250 g samples of feed were collected weekly from each phase (grower and finisher). At the end of the study, the samples from each phase were mixed to form a composite sample, which was ground (1 mm sieve) and analyzed in duplicate to determine the dry matter, ash, crude protein, and ether extract content, according to AOAC methods [26]. Table 1 shows the ingredient and nutrient composition for pigs weighing 50 to 100 kg LW.

Table 1.Composition of ingredients of the basal diet and nutrient composition for pigs from 50 to 100 kg of live weight.

¹Included in all groups = Phytase 10 g/100 kg, Xylanase 15 g/100 kg, Protease 40 g/100 kg, Micofix 150 g/100 kg.

The yield of the hot carcass (immediately post-slaughter) and the chilled carcass (24 hours post-slaughter) was determined. Subsequently, the yield of the primary cuts was evaluated.

The data were analyzed using an analysis of variance (ANOVA) for a completely randomized design, using the GLM procedure of the SAS software [27]. The statistical model used was:

where y i j is the observation value,

μ is the population mean,

τ i is the treatment effect, and

ε i j is the experimental error.

It was assumed that the experimental error followed a normal distribution. Differences between treatment means were considered significant at a P < 0.05 level and were compared using Tukey’s test.

Table 2 shows the different carcass characteristics. It can be seen that the hot carcass weight (kg) (77.817, 78.767, and 79.071) and the cold carcass weight (kg) (76.252, 76.578, and 77.267) with the AO NATURAL FIT PRO®, ractopamine, and AO NATURAL FIT PRO® + ractopamine treatments, respectively, were higher (P < 0.05) compared to the control group. It was also observed that the pigs in the ractopamine group and the AO NATURAL FIT PRO® + ractopamine group had the largest leg diameter (72.467 and 74.11, respectively) compared to the pigs in the other treatments; however, the four treatments were statistically similar in all other parameters.

Table 2.Carcass characteristics of pigs in the growing-finishing stage.

¹Standard error of the mean. ^aGreater difference, ^bLesser difference.

For the volume and weight variables of the forelimbs, no statistically significant differences were detected between the experimental groups. The numerical data showed no statistically attributable effect to any of the treatments applied compared to the control group. In contrast, highly significant effects were observed in the volume of the hindlimbs: The control group (9133.3 cc) presented a significantly smaller volume compared to the three treated groups: AO NATURAL FIT PRO® (10372.3 cc), ractopamine (10562.7 cc), and the combination of both (11621.0 cc). For the weight of the hindlimbs: The results were consistent with those for volume, showing a highly significant difference (P = 0.005). The control group (9264.2 g) was statistically lower in weight compared to the groups supplemented with AO NATURAL FIT PRO® (10531.4 g), ractopamine (11124.0 g), and the combination (11147.0 g). No significant differences were found between the groups that received the additives (Table 3).

Table 3.Weight and volume of legs and arms.

¹Standard error of the mean. ^aGreater difference, ^bLesser difference.

Regarding small intestine pH, statistically significant differences were found for duodenal pH. The control group had the highest value (6.4), followed by AO NATURAL FIT PRO® (6.24) and AO NATURAL FIT PRO® + ractopamine (6.06). In the jejunum, the treatment containing AO NATURAL FIT PRO® had the highest pH (6.48) compared to the other treatments; while in the ileum, the pH values were similar among the treatments (Table 4).

Table 4.pH of the small intestine of pigs treated in the growth-finishing stage.

¹Standard error of the mean. ^aGreater difference, ^bLesser difference.

Treatments with ractopamine and AO NATURAL FIT PRO® + ractopamine significantly improved carcass characteristics. Hot carcass weights were higher in all treated groups (77.817, 78.767, and 79.071 kg for AO NATURAL FIT PRO®, ractopamine, and the combination, respectively) compared to the control, showing a consistent trend also in cold carcass weights (76.252, 76.578, and 77.267 kg).

These findings are lower than those reported by Ferreira et al. [28], who obtained yields of 81.35% with the addition of ractopamine. However, a meta-analysis that evaluated carcass yield characteristics and some meat cuts determined different yield percentages according to the quality and development of the pigs when ractopamine was administered: high quality 82.23%, medium quality 75.15%, and low quality 73.85% [29]. This places our overall results as medium quality in the case of the treatment with AO NATURAL FIT PRO® + ractopamine, with a yield of 74.80%.

Leg diameter also showed significant improvements in the groups with ractopamine (72.467 cm) and AO NATURAL FIT PRO® + ractopamine (74.11 cm), indicating better muscle development, although not necessarily better carcass quality [29]. The statistical similarity observed in other parameters suggests that these additives enhance specific feed efficiency characteristics without adversely affecting overall growth metrics [29]-[31].

It is important to consider that, although ractopamine improves certain carcass traits, it may not significantly influence meat quality attributes [29][32], indicating the need to evaluate the balance between growth performance and carcass quality.

The volume and weight parameters of the forelimbs showed no statistically significant differences between treatments. In contrast, the volume of the hindlimbs showed highly significant effects (P < 0.01). The groups supplemented with AO NATURAL FIT PRO®, ractopamine, and their combination showed larger volumes than the control group, with no statistically significant differences between the supplemented treatments.

The weight of the hind limbs confirmed these findings with significant differences, where the control group was statistically lower compared to all supplemented treatments. This differential response between anatomical regions suggests that the anabolic effects of ractopamine are predominantly manifested in the muscles of the hind limbs, which is consistent with previous studies documenting preferential muscle deposition induced by β-adrenergic agonists and fat reduction [29][33].

Ractopamine, as a β2-adrenergic agonist, stimulates muscle protein synthesis and reduces lipogenesis, resulting in higher lean meat yield and improved carcass conformation [34][35]. The absence of evident synergistic effects, similar to that observed in production parameters, suggests that AO NATURAL FIT PRO® may exert its effects through complementary mechanisms, potentially improving nutrient absorption and providing antioxidant effects that reduce lipid peroxidation [36] and, therefore, improve the sensory quality of the meat [37].

It is important to consider that, although these additives improve the quantitative characteristics of the carcass, their impact on meat quality can be limited or even adverse. Many of the sources used may contain toxic and antinutritional factors and have negative effects if used in large quantities [37], as evidenced in the color parameters [29]. This divergence between performance improvements and potential compromises in meat quality requires a comprehensive evaluation to optimize nutritional strategies in commercial swine production.

Intestinal pH analysis revealed significant variations between segments of the small intestine. In the duodenum, the control group had the highest pH (6.4), followed by AO NATURAL FIT PRO® and the combination with ractopamine. In the jejunum, the AO NATURAL FIT PRO® treatment showed the highest pH (6.48) compared to the other groups, while in the ileum, no significant differences were observed between treatments. A better understanding of phytogenic compounds in the three components of the gastrointestinal ecosystem (intestinal microbiota, intestinal physiology and immunology) and of the underlying mechanisms will possibly allow us to optimize the use of phytogenic substances for economically efficient and sustainable animal production [38].

These findings have important physiological implications for nutrient digestibility and absorption. The elevated pH in the duodenum of the control group may favor pancreatic enzyme activity and nutrient absorption [39], which could partially explain the good production results observed in all groups.

The higher jejunal pH observed with AO NATURAL FIT PRO® suggests optimized digestive conditions that could promote nutrient utilization and improved carcass conformation [34][35]. The stability of ileal pH across treatments indicates that absorption capacity in this terminal segment is not significantly affected by dietary interventions, maintaining a stable environment for the absorption of specific nutrients such as bile salts and vitamin B12 [40][41].

The correlation between optimized intestinal pH and improved carcass characteristics has been documented in studies evaluating dietary restrictions and energy levels [42][43]. However, it should be considered that elevated gastrointestinal pH can promote undesirable fermentations that compromise meat quality [44], indicating the existence of specific optimum ranges to maximize both productive performance and the quality of the final product.