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 evaluat e the effects of level of fish oil inclusion in the diet on rumen fermentat ion parameters and digestion with 10 beef cattle offered grass silage ad li bitum 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, Ir eland) or 300 g (T300B) fish oil premix (J. Bibby Agriculture) per head per day. The concentrates were formulated to have similar concentrations of cr ude protein, effective rumen degradable protein, digestible undegradable pr otein and starch. The dry matter (DM), pH and ammonia nitrogen (N) concentr ations of the silage were 194 g kg(-1), 3.98 and 94 g kg(-1) N, respectivel y. Level or source of fish oil did not alter (P > 0.05) the disappearance o f DM, neutral detergent fibre or acid detergent fibre after 12 or 24 h rume n incubation intervals. Increasing the level of fish oil increased rumen am monia concentration (P < 0.001) but did not alter (P > 0.05) rumen pH or th e molar concentrations of the volatile fatty acids. The fish oil premix dec reased rumen ammonia concentration (P < 0.001), the molar concentrations of acetate (P < 0.05), the acetate:propionate (P < 0.05), acetate+butyrate/pr opionate (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 product ion 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. (C) 1999 Elsev ier Science B.V. All rights reserved.