4.1. Chemical Composition of Legumes
In this study, the cultivars of legume plants and soybean meal were characterized by a similar concentration of crude protein (CP), amino acids (AA), and starch as in the studies of Nalle [
29] and Abdulla et al. [
30]. Similar to our study, Hanczakowska et al. [
31] used the same lupin variety (Mister), though it contained a higher concentration of CP. Hanczakowska et al. [
31] recorded a lower concentration of NDF in the peas and yellow lupin compared to the varieties used in our study. Nevertheless, those values fitted the range for varieties presented by Nalle [
29]. A similar content of RFO was found by Gdala and Buraczewska [
32] in the seeds of yellow lupin, but, in the case of narrow-leaved lupins, the content was approximately 15 g/kg DM lower than that determined in the current study. In the case of peas, the level of RFO was approximately 8 g/kg DM lower compared to a study presented by Hejdysz et al. [
33]. This difference could be due to the genetics of the cultivars, agronomic management, growing location, and climatic conditions [
34,
35].
4.2. Fatteners Performance and Meat Quality
In the first fattening phase, the pigs consumed 1.75 kg of feed daily in the control group, and 1.62 kg in the experimental group. Lower consumption in the experimental group could result from a lower tastiness of the feed mix, in which the legume seeds (yellow lupin, pea) and RSM were included, or due to the different post-prandial glycaemic response of legume seed-based diets registered in some studies conducted [
36,
37,
38]. According to Hanczakowska and Ksiezak [
39], the lupin alkaloids may negatively affect the taste of feed, and therefore the pigs, especially the young ones, are less eager to eat it. Sobotka et al. [
40] showed that glucosinolates in rapeseed may also negatively affect the tastiness of feed. On the contrary, in the second fattening phase, the experimental pigs consumed more feed than the control ones. This could be a consequence of a lower feed digestibility. Considering the chemical composition of legume seeds, one should pay attention to a fact that the levels of ADF and NDF, as well as antinutritional factors (including phytic P and RFO), are quite high, which directly influences feed conversion ratio and its digestibility [
40]. In another study, a partial replacement of SBM with yellow lupin was proposed in a diet of (landrace × Yorkshire) × Duroc hybrids [
41]. The varied percentage content of yellow lupin was applied in animal diets. Namely, it was 7.5% in group II, 15% in group III, whereas the control group (I) was fed with complete feed mixes containing SBM. In this study, the pigs from groups I and II had a similar weight gain in the first fattening period, but in group III, where the content of yellow lupin in the diet was higher, the pigs had a lower weight gain. On the contrary, the pigs from group III had increased weight gain compared to the control group in the finishing fattening period. In terms of daily feed intake, the results recorded by those authors are similar to our results. Summarizing, the pigs in groups I and II had significantly higher body weight gain compared to the animals in groups III and IV, across the whole rearing period (83 days).
Sobotka [
42] replaced the SBM diet with a mix of peas and SRM. The content of peas varied between 8–15%, whereas the content of SRM was at a level of 10–15%. In the first fattening phase, the experimental group had a lower daily weight gain, but no differences were found in the second fattening phase. Body weight gain in the experimental groups was lower by about 4% through the whole fattening period, which the author explained by a decreased digestibility. Additionally, in their further study, Sobotka et al. [
40] partially (50%) replaced SBM in a feed mix. The protein source in their experimental diet was RSM “00”. The pigs were fattened after they had reached a weight of 65 kg and up to 115 kg. That period corresponded to the second fattening phase of this study. The daily weight gain was lower than observed here (different genotypes), however it was also better in the group fed a SBM diet. The FCR in that study was in the range of 3.14 kg/kg (SBM group) and 3.29 kg/kg (SBM + RSM group), which the authors explained by the effect of glucosinolates in the rapeseed. Okrouhlá et al. [
43] observed similar results of fattening as here. They studied the effect of replacing SBM with RSM on fattening outcome in DanBred × (CLW (Czech Large White) × CL (Czech Landrace)) pigs. They observed lower daily feed intake than here (control—2.78 kg: experimental group—2.56 kg). Daily weight gain exceeded 1000 g independent of the diet. The feed conversion ratio was slightly lower in the cited study. Plazak et al. [
44] studied body weight gain in PLW (Polish Large White) x PL (Polish Landrace) hybrids that were fed a complete feed mix with peas, potato protein and fish meal in the experimental group, and SBM in the control group. The fattening period lasted 98 days until the pigs gained ca. 100 kg of body weight. Average daily feed intake was about 2 kg (group I—2.14 kg; group II—2.23 kg). The feed conversion ratio was 2.87 kg in group I and 2.90 kg in group II. The experimental group had a 3.2% higher average body weight gain than the control group (group I—745 g, group II—769 g), however that difference was not statistically significant. The authors explained that a better tastiness of the feed and fiber content in the control diet determined a higher feed intake and higher body weight gain. Comparing those results to our study, a lower feed intake and lower body weight gain was observed by Plazak et al. [
44] in their PLW × PL hybrids than here. The feed conversion ratio was similar.
Here, the average back fat thickness from five measurements was significantly greater in pigs that were fed a diet based on SBM compared to the animals that were offered a diet containing RSM and legume plants. That result could be explained by the fact that pigs on the SBM diet had higher body weights at slaughter. On the contrary, Zaworska et al. [
7] recorded a reduced back fat thickness, however, the pig body weight at slaughter was lower in their study than here. Considering the physicochemical assessment of meat, a significant difference in the acidification of muscle tissue 45 min
post mortem was observed between the gilts and barrows. In female pigs, the pH
45 was higher, which could be explained by a faster glycogen turnover in muscles [
45]. Nevertheless, it should be emphasized that the acidification of muscle was similar for both sexes 48 h
post mortem. The pH is directly influenced by environmental factors and pre-slaughter treatment [
45]. A similar study was performed by Bocian et al. [
5] on F1 hybrids (PLW × PL). Two dietary groups were applied: the control pigs were fed a diet based on SBM, whereas the experimental pigs were fed complete feed mixes with 20% pea and 37.6% yellow lupin content in the first fattening phase, and 38.5% peas plus 20% yellow lupin in the second phase. In that study, the pH measured 48 h
post mortem was higher than here and amounted to 5.43.
In this study, the crude protein content in pork meat was found to be higher in the pigs that were fed RSM and legume plants compared to the other studies of Zralý et al. [
46] and Mordenti et al. [
47]. This parameter could be higher due to a simultaneous application of several legume species and RSM.
Another meat quality trait is the color. According to Szulc and Skrzypczak [
48], the meat color is directly correlated with its water holding capacity; the lighter the color of the meat, the higher its water content. Hanczakowska et al. [
31] assessed the meat color in fatteners that were fed diets containing yellow lupin (Mister variety). The authors recorded a lightness value of *L 51.50, whereas in our study, the meat in the experimental group had a lightness value of *L 50.60. Kuźniacką et al. [
49] studied highly productive white breeds and Pulawska pigs, and they did not show differences in meat quality between the pigs that were fed diets containing legume plants (yellow lupin, pea) with RSM, and pigs fed SBM. Similarly, Chrenková et al. [
50] did not find significant differences in pork meat between the pigs that were fed a diet containing peas (30%) and the fatteners that were fed a diet based on soybean meal.