In vitro rumen degradation, fermentation, and methane production of four agro-industrial protein-rich co-products, compared with soyabean meal

Soyabean is considered an unsustainable protein source for livestock feeds because of the large quantity of input and energy required to cultivate and process it. Other protein-based agro-industrial co-products that are less input-intensive, can mitigate methane (CH ₄) production and may therefore be more sustainable options instead soyabean. The objective of this study was to compare the effect of replacing the same amount of protein (40 g/kg DM crude protein) as soyabean meal (SBM) with low-carbon local agro-industrial co-products, (brewers’ spent grains, BSG; dried wheat distillers’ grains, WDG; dried corn distillers’ grains, CDG and corn steep liquor CSL), on in vitro rumen degradation, fermentation and gas and methane production. The study used a 72-hour in vitro gas production method with a basal substrate of dried, ground grass silage and wheat. Gas volumes were measured at ten different specific intervals, and CH ₄ concentrations were analysed via gas chromatography. After 72 hours, in vitro DM degradability (IVDMD) and volatile fatty acid (VFA) concentrations were assessed. Gas and CH ₄ production curve profiles were fitted to models to determine asymptote production, the extent of degradation in rumen proper (RoP), and the fractional degradation rate (μ) (h ⁻¹) in the halfway 50 % of the asymptote production. The IVDMD and estimated RoP at 0.04 h and 0.025 h were lower (P<0.05) for BSG compared to the other treatments, by 4.9-6.6 %; 5.8–9.9 %; 5.2–9.0 %. Gas and CH ₄ yield (mL/g substrate and mL/g substrate degraded), and pH (SB = 6.77, BSG = 6.80, WDG = 6.74, CDG = 6.84, and CSL = 6.73; P>0.05), were not significantly affected by treatment. Butyrate and valerate were lower (P<0.05) for BSG compared to CSL, and caproate was lower (P<0.001) for BSG compared to the other treatments and in CSL compared to SBM. The results regarding degradability and VFAs concentrations of this study demonstrated that dried wheat distillers’ grains, dried corn distillers’ grains, and corn steep liquor have the potential to replace soyabean meal as protein sources for ruminants, but further reduction of CH ₄ emissions as a result of such practice may not be expected. Although slightly less degradable, based on their nutrient composition and the fact they did not affect rumen fermentation characteristics, brewers' spent grains can still play a complementary role in ruminant diets, especially in regions where they are locally readily available.