The effect of trace element addition to mono-digestion of grass silage at high organic loading rates

•Dairy slurry when co-digested with grass silage provided sufficient trace elements.•Low addition of slurry (20% VS) exhibited stable VFA profiles and high SMYs.•Mono-digestion of grass silage at high loading rates required trace element addition.•Supplementation of cobalt, nickel and iron to mono-digestion increased SMY by 12%.

This study investigated the effect of trace element addition to mono-digestion of grass silage at high organic loading rates. Two continuous reactors were compared. The first mono-digested grass silage whilst the second operated in co-digestion, 80% grass silage with 20% dairy slurry (VS basis). The reactors were run for 65 weeks with a further 5 weeks taken for trace element supplementation for the mono-digestion of grass silage. The co-digestion reactor reported a higher biomethane efficiency (1.01) than mono-digestion (0.90) at an OLR of 4.0 kg VS m−3 d−1 prior to addition of trace elements. Addition of cobalt, iron and nickel, led to an increase in the SMY in mono-digestion of grass silage by 12% to 404 L CH4 kg−1 VS and attained a biomethane efficiency of 1.01.