The effects of level of fish oil inclusion in the diet on rumen digestion and fermentation parameters in cattle offered grass silage based diets

A partially balanced changeover design experiment was undertaken to evaluate the effects of level of fish oil inclusion in the diet on rumen fermentation parameters and digestion with 10 beef cattle offered grass silage ad libitum as the basal forage supplemented with 5 kg concentrates head−1 day−1. Concentrates were prepared to provide either 0 (T0), 150 (T150), 300 (T300) or 450 g (T450) fish oil (Fish Industries, Killybegs, Co. Donegal, Ireland) or 300 g (T300B) fish oil premix (J. Bibby Agriculture) per head per day. The concentrates were formulated to have similar concentrations of crude protein, effective rumen degradable protein, digestible undegradable protein and starch. The dry matter (DM), pH and ammonia nitrogen (N) concentrations of the silage were 194 g kg−1, 3.98 and 94 g kg−1 N, respectively. Level or source of fish oil did not alter (P > 0.05) the disappearance of DM, neutral detergent fibre or acid detergent fibre after 12 or 24 h rumen incubation intervals. Increasing the level of fish oil increased rumen ammonia concentration (P < 0.001) but did not alter (P > 0.05) rumen pH or the molar concentrations of the volatile fatty acids. The fish oil premix decreased rumen ammonia concentration (P < 0.001), the molar concentrations of acetate (P < 0.05), the acetate : propionate (P < 0.05), acetate + butyrate/propionate (P < 0.01) and non-glucogenic (P < 0.05) ratios and increased the molar concentration of propionate (P < 0.01). It is concluded that changes in rumen fermentation parameters do not account for the depressions in milk butterfat content with fish oil inclusion observed in a concurrent production study in which lactating dairy cows were offered similar diets to those used in the present study. Furthermore, the changes in rumen fermentation parameters with inclusion of a fish oil premix are probably associated with the carrier or the source of fish oil used in that product.