Dose–response effects of dietary inclusion of agro-industrial by-products on in vitro ruminal fermentation and methane production

BACKGROUND
As the agro-industry produces considerable amounts of by-products globally, it is acknowledged that there is a need to address the environmental issues related to their disposal and the resource competition between food for humans and feed for animals. The aim of this study was to explore, in vitro, the effects of various by-products from the agro-industry on rumen fermentation and methane emission. Samples were collected from various food processing industries, including red and green apple pomace (RAP, GAP), hempseed cake (HC), coffee hulls (CH), coffee grounds (CG), spent mushroom compost (SMC) and distiller's dried grains with solubles (DDGS). In doses of 100, 200 and 300 g kg⁻¹, the tested by-products were incubated in rumen fluid, where the by-products replaced equal amounts of substrates.

RESULTS
Gas production (GP) and dry matter digestibility (DMD) decreased linearly for most of the tested by-products with the growth of doses (P < 0.001), while NH₃-N concentration increased linearly. Linear decreases were observed in CH₄ production with increasing doses of all by-products (P < 0.05). The reduction of CH₄ production ranged from 21.4% to 33.6% at doses of 100–300 g kg⁻¹, but reductions were only observed at a dose of 100 g kg⁻¹ when CH₄ productions were corrected by digested dry matter (P < 0.05). RAP, GAP and HC were higher than CH, CG and SMC for the comparison of key parameters including DMD, GP and volatile fatty acids. Better methane-mitigating effects were observed for RAP, GAP and HC than for the control group and CH, CG and SMC.

CONCLUSION
Most of the by-products tested were found to be a potential option for replacing conventional feed ingredients but should not exceed a dose at 200 g kg⁻¹.