The potential protein sources such as leaf meals, oil cakes and seed or nut meals from indigenous plants are valued for their nutritional characteristics which can benefit the livestock feed industry if they are considered. Exploring other potential protein sources for non-ruminants is imperative to sustain the ever-increasing poultry enterprises as well as human population in Sub-Saharan Africa. To date, Soybean production has been criticized due its low production outputs in South Africa, thus majority (72%) is imported from other countries (Grain SA, 2016). This consequently leads to high purchasing value, hence disadvantaging many resource limited poultry farmers, therefore it of paramount importance that other local feed ingredients of local origin such MPSM be considered when formulating poultry diets.
Average daily feed intake showed a linear decrease with increasing inclusion levels of MPSM. Our results are in agreement with (Carew et al., 2003; Jayaweera et al., 2007) who also reported a decline in average daily feed intake of broilers. Generally, heat processing results in detoxification of the anti-nutritional factors (ANFs), hence, enhancing protein absorption, bioavailability of the amino acids in the gut and higher palatability (Emenalom et al., 2005; Tuleun and Igba, 2008; Gurumoorthi and Uma, 2011). However, the observed decrease in feed intake suggests that the secondary compounds of the seed meal such as condensed tannins, total free phenolic, phytic acid and oxalate present in Mucuna pruriens seeds might not be heat-labile and also increase with the inclusion level of the seed meal. When consumed, they bind dietary protein to complexes that are not readily digestible and inactivate digestible enzymes (Alhassan et al., 2019). On other hand, phytic acid, phenolic, and oxalate interfere with essential minerals absorption such as calcium, phosphorus, magnesium, and zinc (Natesh et al., 2017). Condensed tannins are known to as one of the major groups of ANFs that inhibit feed consumption in non-ruminants. Broilers were selective when feeding hence avoiding some of the feed due to unpleasant aroma of the anti-nutritional factors (Manyelo, 2018).
It is with no surprise that average daily gain decreases with increasing levels of MPSM as expected. Similar observations were reported by (Iyayi et al. 2006; Tuleun and Igba, 2008; Gurumoorthi and Uma, 2011). Poor nutrient intake mostly protein and absorption lead to the unavailability of essential amino acids from the gastrointestinal tract resulting in broilers utilising body reserves to sustain their nutrient needs subsequently leading to decreased body weight (Mabusela et al., 2018). Furthermore, anti-nutrients inhibit the activity of hydrolytic enzymes such as trypsin, chymotrypsin, lipase, and amylase which leads to high by-pass nutrients (Lampariello et al., 2012). Due to evident decrease in body weight gain, the observed increase in FCR with increasing levels of MPSM in diets was expected. The higher FCR indicates that the feed was not efficiently converted to muscle (meat). Our results corroborate with Miya et al. (2019) who also reported a linear increase in FCR on broiler chicken fed increasing levels of Vachellia leaf meal. The decrease in final body weight of broilers to increasing levels of MPSM might be attributed to the linear decrease in ADFI and ADG which indicates that nutrients absorption was constrained at higher levels of the seed meal.
In this study, the carcass weight decreased linearly with increasing levels of MPSM in diets. This decreasing carcass weight was also observed by Emenalom and Udedibie (2005); Iyayi et al., (2006). The carcass weight results trend obtained across treatments in the present study was expected due to broiler final body weight trend. The meat pH24 and colour was not influenced by diets. The pH24 of meat fell within the ideal range in all treatments (5.75 to 5.83) (Hambrecht et al., 2004). Generally, over 24 hours after slaughter the pH of poultry meat decreases from approximately from 7.0-7.2 down to a range of 5.5-5.8 for acceptable meat where there is no case of pale soft and exudative (PSE) meat (Hambrecht et al., 2004). Increasing levels of MPSM in the diets increased the meat shear force. These results may have been driven by the presence of condensed tannins that are not heat-labile in Mucuna pruriens seeds which triggers oxidation in meat (Seeram et al., 2005; Reddy et al., 2007). Increasing meat shear force implies that, as the level of MPSM is increased the tougher the meat becomes. The toughening of meat is related to the oxidation of myofibrillar proteins due to tanniferous diets, which promotes accumulation of muscle fibres thus the meat becomes less tender (Harris et al., 2001; Morán et al., 2012). Currently, there is no traceable evidence on the meat quality analysis of broiler meat fed increasing levels of MPSM. However, the determination of tenderness through sensory evaluation was done by Adzitey et al. (2010). In this study it was observed that the inclusion of MPSM seemed not to influence meat tenderness. While in other related legume seeds fed to broilers, Milczarek et al. (2016) observed that the inclusion of faba beans to broiler diet resulted in more tender of meat. In this case, the vast difference in meat tenderness between broilers fed Mucuna pruriens and faba bean seeds may have been influenced by factors such as the age at slaughter, sarcomere shortening during rigor mortis, the amount and solubility of connective tissues, and post-mortem proteolysis of myofibrillar proteins (Warner et al., 2010). Furthermore, the M. pruriens seeds contain a toxic amino acid called L-Dopa which is a precursor of dopamine that increases the skeletal muscles resulting in tougher meat (Reichart et al., 2011).
Thawing loss increased linearly with increasing levels of MPSM in diets. During thawing process, small muscle fibres attained from smaller carcass weight have high muscle water loss due to myofibrillar proteins denaturation caused by freezing temperatures (Waritthitham et al., 2010). However, breast meat thawing loss on Guinea fowls fed diet containing high level of MPSM was found to be lower (Dahouda et al., 2009). Since Guinea Fowls might be comparable to indigenous chicken, therefore the ability of their muscle fibres to retain intrinsic water becomes poor because of age at slaughter and the amount of connective tissues (Listrat et al., 2016; Soglia et al., 2016). This suggests that higher MPSM in a diet lowers the amount of substance lost during cooking as observed in the current study where cooking loss decreased linearly with increasing levels of MPSM. These findings are in line with a study by Dahouda et al. (2009), where lower cooking loss of breast meat from Guinea fowls fed 20% cooked, and toasted MPSM were observed. Cooking loss is known as the amount of water lost during cooking (Tornberg, 2005). These losses may include volatile substances from the volatile aromatic substances and the decomposition of fat (Thu, 2006). Moreover, they include nitrogenous and non-nitrogenous extractives and salts, which are beneficial to meat consumers (Adzitey et al., 2010).
According to Wolmarans (2009), animal tissue is known to contain high levels of saturated fatty acid composition when comparing to plant materials. Myristic (C14:0) and palmitic (C16:0) were observed to decrease linearly in response to increasing levels of MPSM, which is a positive observation. These fatty acids are recognised as saturated fatty acids and has also been identified as a potential risk factor on human health. Specifically, palmitic acid is known to increase blood cholesterol (Peña et al., 2009), while myristic is known to accumulate fat in the body (Verruck et al., 2019). Unfortunately, one of the saturated fatty acid [arachidic acid (C20:0)] was observed to increase with increasing levels of MPSM in diets. These saturated fatty acids have the capacity to increase triglycerides in animal blood which increases risk of cardiovascular disease and other chronic diseases in meat consumers (Verbeke et al., 1999; Pottel et al., 2014; Navidshad et al., 2015). This indicates that MPSM exhibit a wide variety of pharmacological properties such as anti-diabetic and antioxidants (Lampariello et al., 2012), which in this case may have resulted to decrease some saturated fatty acids on broiler meat. These medicinal properties may have played a significant role in increasing the composition of α-Linolenic acid (C18:3n3) as observed in this study. Furthermore, the finding observed by Ndukwe et al. (2011) showed an increase in white blood cell count, as well as in bilirubin concentrations and alkaline phosphatase and a decrease in alanine aminotransferase and aspartate aminotransferase in rats fed diets containing M. pruriens seed extracts. Which also confirms its medicinal properties (Lampariello et al., 2012).
In conclusion, incremental levels of MPSM in diets reduce broiler performance, does not alter meat pH24 and colour. However, shear force increases with increasing levels of MPSM in diets. The diets with MPSM lowered the composition of some unsaturated fatty acid (Myristic, palmitic and arachidic acid) but improved the composition of α-Linolenic acid in meat. This suggests that incorporating Mucuna pruriens seed in broiler diets may therefore have a low-density lipoprotein (LDL)-Cholesterol lowering effect, of which is a positive implication on consumers’ health. However, further confirmation needs to be done to provide a more convincing evidence.