The chemical-bromatological composition found in the present study (39.14% CP and 21.50% CF for MS and 45.22% CP and 1.69% CF for SBM) (Table 1) is similar to that reported in the tables of Rostagno et al. (2011). The urease activity values for MS and SBM were within the reference range indicated by Sindirações (2013) (Table 1). Szmigielski et al. (2010) demonstrated that micronization is effective in reducing urease activity levels (from 2.0 to 0.10 ΔpH) without compromising protein content. The protein solubility values obtained for SBM were lower than the minimum of 75% recommended by Sindirações (2013). Excessive heating of the ingredient is known to denature proteins, which reduces protein solubility (Woyengo et al., 2017). In the present study, the lower protein solubility of SBM compared to MS may have been responsible for the lower digestibility of the diets containing a higher level of SBM.
The digestibility results obtained for the pre-starter I diet differ from those reported by Valencia et al. (2008) who studied the influence of particle size of SBM and full-fat soybean on the digestibility of diets for piglets at 23 days of age. The lower digestibility of diets with a higher SBM percentage can be attributed to the greater crude fiber content of this ingredient compared to MS (5.3 vs. 1.36% according to Rostagno et al., 2011). The presence of fiber can negatively affect protein digestion because of the effect of the association with lignocellulose, which reduces protein availability (De Coca-Sinova et al., 2008). On the other hand, the quadratic effect observed for ADCDM, ADCGE, ADCCF, DE and ME might be related to the increase in dietary CF content with increasing replacement level of SBM with MS. Azain (2001) suggested that dietary fat reduces the rate of food passage through the gastrointestinal tract and consequently increases nutrient digestibility because of the longer exposure to digestive enzymes. This higher amount of fat may have exceeded the enzymatic capacity of the gastrointestinal tract of piglets at this age.
The lack of significant differences between variables (except ADCDM and ADCCF for the pre-starter II diet and ADCDM for the starter diet) agrees with the results reported by Berrocoso et al. (2014), who found no benefit of micronization of common SBM or high protein content. According to these authors, the effects of micronization appear to be clearer in studies on cereals compared to soybeans. Agunbiade et al. (1992) observed a higher digestibility coefficient of diets containing SBM and soybean oil compared to diets with full-fat soybean. The authors suggested that how this ingredient is present in the diet seems to be an important factor for the utilization of soybean products. Thus, the integral oil present in the plant cell may not be fully available for enzymatic digestion even after micronization.
The effects of micronization on nutrient digestibility have not been fully elucidated. Although improving diet digestibility, the reduction in the particle size of the food may also affect gastrointestinal motility and function and health of the piglet (Berrocoso et al., 2014). Thus, the two effects may oppose each other and the magnitude of the responses observed can vary among experiments and phases studied. Furthermore, the differences found in the nutrient and energy digestibility coefficients of the diets compared to those reported in the literature suggest lower digestibility in weaned piglets compared to older animals whose gastrointestinal tract is more developed (Latorre et al., 2004).
Growth performance during the starter, growing and finishing phases
With respect to the starter phase (Table 3), the results corroborate those reported by Trindade Neto et al. (2002) who demonstrated superiority of SBM compared to other protein sources, assuming that this soy by-product triggers less damage to the piglet’s digestive process and gastrointestinal system in the early stages of development. However, our findings differ from Berrocoso et al. (2014) who observed better FC in animals fed diets with micronized SBM compared to common SBM in the first week of the assay. Although soybean micronization is able to improve nutrient digestibility by improving the mixing with digestive enzymes, this process may also negatively affect gastrointestinal motility and function, as well as the “healthy” status of the piglet, compromising growth performance of the animal.
In general, feed intake was considered low during the starter phase (especially for the treatments containing a higher percentage of MS), which may have caused poor growth performance. According to Jha & Berrocoso (2015), newly weaned piglets do not consume the adequate amount of energy to meet their energy requirements for body weight gain, particularly during the first days after weaning, and may recruit fat reserves only to sustain their body weight. Another factor that may have affected feed intake during the starter phase is the fact that no flavoring was used in the diet. Diet intake by adult pigs is generally regulated so that energy needs can be met (Costa et al., 2018), but the ability to adapt feed intake according to the energy content of the diet is variable in piglets. According to Valencia et al. (2008), palatability of the food is the factor that most influences the feed intake level during this stage of the piglet’s life. It is possible that the palatability of MS was not acceptable, but scientific studies on this topic are scarce.
The texture of the diets with higher inclusion levels of MS may have also influenced feed intake. The processing and physical form of the diet directly influence the diet preference of newly weaned piglets. Completing this reasoning, after weaning, feed intake is basically controlled by palatability, which mainly includes flavor, but can also be estimated by the texture and physical properties of the dietary ingredients (Lee et al., 2019). As reported by Solà-Oriol et al. (2009) and also observed in studies on infants and children, they reject foods that are difficult to move around in the mouth and that take longer to be swallowed. It is possible that foods that require fewer chewing movements are also preferred by newly-weaned piglets. These authors also emphasized that, for piglets, particle size seems to be less important than the texture of the foods. In view of these considerations, the higher fat content of the diets containing MS may have altered their texture, with a subsequently negative influence on feed intake.
Although the urease activity and protein solubility values obtained were within the acceptable range (Table 1), other ANF that were not analyzed could have affected food utilization by the animal because they possess heat resistance features and consequently the growth performance of piglets in this study, in addition the amount of ANF consumed daily is more important than their concentration in the ingredient or diet (Woyengo et al., 2017).
The high fat content of the diets, especially in the early stages, may have influenced the results of the present study. Trindade Neto et al. (2002) attributed the poor growth performance of piglets fed diets with ground full-fat soybeans to their high fat content, which would exceed the piglet’s digestive capacity at this stage of life. According to Adeola & Cowieson (2011), the total fat digestive capacity is not reached until 40 kg of body weight. In addition, a marked reduction in the activity of different digestive enzymes such as lipase is observed immediately after weaning.
Regarding the subsequent phases (growing/finishing), animals fed diets with higher inclusion of MS entered the grower phase with a lower body weight. Thus, the lower feed intake observed from 64 to 90 days of age in animals previously fed increasing levels of MS would be due to lower body weight (lower gastric capacity) and the better feed efficiency would be related to the higher rate of protein deposition, which is more intense in smaller animals and requires twice less energy than that needed for the deposition of adipose tissue (Pierozan et al., 2016).
Latorre et al. (2004) reported higher feed intake and weight gain of male pigs during the growing and finishing phases compared to females of the same age. This fact might be explained by the greater intake capacity and lower maintenance requirements of males.
Good growth performance of piglets in the early stages of growth depends on many factors, including weight upon entry to the nursery, existence of some nutrient restriction, feed intake, health status of the animals, diet composition and digestibility, and the presence of ANF (Jha & Berrocoso, 2015; Woyengo et al., 2017). Thus, considering the same sanitary standards, homogeneous weight at the beginning of the treatments and formulation of diets that were approximately isoproteic and isoenergetic and that met the nutritional requirements of this phase, the factors that could have compromised the animals’ growth performance during the previous phase would be the low feed intake, digestibility of MS, or ANF.
Carcass and meat quality traits
The pH measured 45 min and 24 h after slaughter is one of the parameters most used in slaughterhouses to predict meat quality because of its reliability and easy application. In addition, studies have demonstrated an association between pH values at 45 min and 24 h and the results of technological meat evaluations, such as water retention capacity, color, and drip loss. Quality standard pork is defined when the pH 45 min is between 6.0 and 6.5 and the pH 24 h between 5.5 and 5.8 (Costa et al., 2018). Hence, despite the significant difference in pH at 45 min, with an interaction between gender and diet, these values were within the reference range. These findings show that the inclusion of MS in the starter diet for piglets did not influence the conversion of muscle to meat. The measurement of meat color is essential to ensure attractiveness by consumers. In addition, color measurement assists in the diagnosis of PSE (pale, soft, exudative) and DFD meat (dark, firm, dry). AMSA (2012) established normal L* values for pork meat between 49 and 60. However, other authors defined a L* value between 43 and 49 as normal meat (Bridi et al., 2008; Garbossa et al., 2013). Thus, considering the latter reference, male pigs fed SBM had a L* value outside the expected range, while the L* values were within the range for quality pork meat according to the first reference.
With respect to lightness, males of the MS0 treatment had meat with greater lightness, i.e., paler meat. In addition, males fed the MS0 and MS25 diets in the early stages had meat with greater lightness when compared to females of the same group. Genetic factors, production system, feeding, age, and the final pH of meat can exert important effects on the L*, a* and b* values. These values also tend to change with increasing slaughter weight due to the greater muscularity of the animal (Costa et al., 2018). With development of the muscle, the amount of myoglobin increases, fat deposition becomes more evident, and the amount of water in the muscle decreases, consequently reducing lightness. This fact was not observed in the present study since feeding males higher levels of SBM during the starter phase benefitted their growth performance, resulting in larger pigs with more intense muscle activity, which would favor myoglobin accumulation and fat deposition and consequently result in meat with lower lightness (Garbossa et al., 2013).
The drip loss values found for animals fed diets with higher SBM levels during the starter phase were above those proposed by Bridi et al. (2008) for fresh pork meat, who recommend a drip loss of up to 6%. However, other authors define a drip loss of about 9.8% for normal pork meat and of about 12.9% for PSE meat (Latorre et al., 2004).
Taken together, the pH 24 h, L* value and drip loss of animals fed higher inclusions of SBM during the starter phase suggest the occurrence of PSE meat, which would explain the presence of meat with greater lightness. However, considering the literature, the lack of a sudden pH drop within the first 45 min post mortem and the fact that pH values at 24 h post mortem were close to those recommended by more rigorous references and that the cooking loss was within the quality standard, we conclude that no PSE meat occurred. Similarly, the DFD classification cannot be applied to the present study since the meat derived from the treatments evaluated had a L* value>42, drip loss ≥ 5%, and final pH<6.0 (Bridi et al., 2008).
Analysis of the carcass traits of animals fed the MS100 diet showed that the lower final body weight of these animals resulted in a reduction of 8.22% in rib weight, of 9.26% in shoulder weight, of 9.6% in boneless shoulder weight, and of 16.48% in boneless sirloin weight. In addition, females fed the diet with MS inclusion exhibited a linear reduction in the weight of the sirloin, with the observation of a 25.4% decrease in treatment MS0 compared to MS100.
With respect to gender, the quality of carcasses from barrows was lower than that of female carcasses because of the higher amount of fat and lower carcass yield (about 0.9% lower) in the former. According to Bridi et al. (2008), castrated male pigs fed similar diets as females will exhibit a poorer carcass quality (lower carcass yield and greater fat deposition) at the same slaughter age. This fact was also observed in the present study in which backfat thickness in males was 8.77% higher than in females. Higher fat thickness values in males have also been reported by other authors (Peinado et al., 2008). Studies suggest that fat deposition is favored in males compared to females because of higher feed intake (Costa et al., 2018) and the lack of production of testicular steroids.
Rib and shoulder weights also differed (p<0.05) between genders, with the observation of a higher weight of these cuts in barrows. Taken together, the results show that animals fed the MS0 diet had a higher weight of some cuts and females exhibited better carcass traits. However, it is important to note that carcass yield and ham and pork chops weight, which are considered noble cuts with a high added value, were not influenced by the diets tested.
In the present study, the lower weight upon entry to the growing phase demonstrates poor growth performance of animals receiving increasing levels of MS during the starter phase. Literature data (Trindade Neto et al., 2002) and those obtained in this study show that the growth performance of animals during the starter phase influences subsequent phases. In this respect, the poor growth performance of animals fed increasing levels of MS in the early stages of growth, despite their better FC in the grower I phase, did not influence the other phases and did therefore not compensate for the losses observed at the beginning of their productive life, resulting in lower slaughter weight, cold and hot carcass weight, and lower retail cut weights.