The in vitro assay suggested roasting cowpeas increased colon fibrolysis while sprouting shifted non-enzymatic cowpea digestion (likely the spontaneous solid macromolecular disintegration) to the 3rd step, effects which implied increased, readily fermentable substrate for lower pig gut microbes. Unfortunately, despite insignificant effect on DM digestibility, the gravimetric analyses excluded molecular insight into the nature of the apparent biochemical shifts which could have important implications on pig nutrition.
In contrast to in vitro digestion, roasting and sprouting compromised total in vivo apparent DM digestibility, which could reflect either disparate digestion and, or, the result of increased endogenous faecal excretion. Compared to a commercial product such as Viscozyme, greater diversity of pig gut microbial fibrolytic activity (Fushai et al., 2019) should confer higher true digestibility of fibrous plant embryonic secondary tissue in cowpea sprouts, and of crosslinked compounds in over processed roasted cowpeas. Tendency (P = 0.053) toward pig genotypic x diet interaction on DM digestibility was consistent with numerically greater Windsnyer pig capacity to digest sprouted cowpea dietary fibre. The fibrolytic advantage of the indigenous pig gut was previously explained by genomic evaluation of faecal microbiota (Kanengoni et al., 2015), which might present a mechanism for broader tolerance of feed ANF’s present in their more complex native diet. Unfortunately, in this study, neither of in vitro/in vivo evaluation measured ileal protein or amino acid digestibility, to predict processing effects on protein quality (Mosenthin et al., 2000; Święch, 2017). Though growing pigs are considered highly N efficient, as high as 40% on standard diets (Rotz, 2004), in the current study, N balance was low (16.4 ± 1.0 %), despite high (98%) DM digestibility, which implied inferior dietary amino acid profiles, and consequently, low efficiency of tissue utilization (Smiricky et al., 2002). In previous studies, both sprouting (Urbano et al., 2005) and thermal processing (Doblado et al., (2007) of cowpeas improved protein digestibility, though excessive heating reduced digestibility, reflecting negative effects of non-enzymatic (Maillard) reactions between the reducing sugars and proteins, and thermally induced amino acid cross-linking (Tuśnio et al., 2017). El-Jasser (2011) reported as high as 75–79% in vitro protein digestibility of sprouted cowpea. However in current study DM digestibility was high from 86–88 % for raw 88%, sprouted 86% and roasted 87% cowpea.
Significant difference should be expected between dietary DM, protein and amino acid (AA) digestibility, particularly in diets in which the legume feed contains significant antinutritional factors (Kumar et al., 2006; Kayembe, 2013). Gut protein extraction for assimilation in pig tissues occurs with variable wastage via the urinary or faecal routes (Ball et al., 2013), with complex gut-systemic exchange of endogenous protein, AA and urea, which is dependent on diet quality (Ball et al., 2013) and intake (Ball et al., 2013). In the present study, the impact of processing on protein quality was complex and overall unexpected. Roasting and sprouting both depressed cowpea protein quality as reflected by the N balance. While roasting significantly increased the urinary N, sprouting induced numerically low N intake, greater faecal wastage, and significantly lower relative N balance when expressed on body weight, but not on metabolic body weight basis. Imbalance in amino acids supplied for protein synthesis for growth and other functions results in catabolism of excess amino acids, with excess N converted to urea, which increase its excretion in urine (Ball et al., 2013). On the other hand, protein indigestion in the upper tract diverts N to colon fermentation (Bindelle et al., 2009). With adequate fermentable energy supply, colon bacteria assimilate both endogenous and dietary N, to lock it and shift excretion from urea in urine to microbial protein in faeces (Bindelle et al., 2009). If energy is deficient, increased colon protein fermentation may also produce toxic metabolites (Tuśnio et al., 2017). Potentially toxic byproducts of protein fermentation include ammonia, amines and N-nitroso compounds (Bindelle et al., 2009). Yang et al., (2007) reported no detrimental effects on the growth of pigs from faba bean legume dietary inclusion levels of 20 to 37%. Genotype X diet X period interaction suggested superior relative FCR for Windsnyer, Land race x Large white and Land race x Large white genotype on the raw, sprouted and roasted cowpea diet, whereby pigs gained more weight while consuming lesser on period 2, which could be an indication of greater tolerance of the ANFs contained in the cowpea. Umapathy and Erlwanger (2008) reported depressed growth and feed intake in 30% raw dietary cowpea-fed pigs, in contrast to pigs fed thermally processed cowpea diets. Rats fed cooked cowpea gained more weight than those fed uncooked cowpea (El-Jasser, 2011).