Effect of manipulating dietary fat and carbohydrates on methane potential of dairy cow faeces

• Supplementation of dietary fat in dairy cows affect faecal biogas production. • Maximum methane yield and production rate increase by feeding fat as rapeseed. • Palm kernel fatty acids supplementation did not affect biogas production. • Dietary carbohydrate sources for dairy cows affect faecal biogas production. • The effect of carbohydrate from forages and concentrates on biogas is additive. The objective of this study was to evaluate the effect on the in vitro methane yield from dairy cow faeces, when the diet is manipulated for either source or concentration of dietary fat (rapeseed or palm kernel fatty acids; 20 to 50 g/kg DM), or a source of carbohydrate (grass-clover vs maize silage, and barley vs dried beet pulp). The faecal samples from two nutrition trials were used in an in vitro system for anaerobic digestion for measuring gas volume and methane concentration at given times, from 5 to 90 d. The ultimate methane yield measured at d 90, and the coefficients estimated from a modified Gompertz model (maximum methane yield, methane production rate, and lag time) were analysed in a linear mixed model. Increasing dietary concentration of fat increased the maximum methane yield, the methane production rate, and the lag time when rapeseed was used as fat source, but use of palm kernel fatty acids in the diet did not result in an increase of any parameter. The effect of the carbohydrate inclusion from forage and concentrate was additive, so no interaction was observed, and maximum methane yield increased with increasing dietary starch concentration. Ultimate and maximum methane yield were highly correlated, with the latter underestimating ultimate methane yield by 3-4%. Estimated coefficients from a modified Gompertz model are useful to understand the effect of diet on biogas yield and production rate from faeces, but slight under-estimation of maximum methane yield was observed at incubation time of three months.