The effects of concentrate energy source on feed intake and rumen fermentation parameters of dairy cows offered a range of grass silages

The effects of concentrate energy source on feed intake and rumen fermentat ion parameters of lactating dairy cattle, offered one of three grass silage s differing in fermentation and intake characteristics, were evaluated in a partially balanced changeover design experiment involving four rumen fistu lated dairy cows. Three silages were harvested using different management p ractices prior to and at ensiling. Silages A and C and silage B were harves ted from primary or secondary regrowths either untreated or treated with a bacterial inoculant. For silages A:, B and C, dry matter (DM) concentration s were 334, 197 and 183 g/kg (S.E. 3.1), pH values 4.00, 4.79 and 4.80 (S.E . 0.042) and ammonia nitrogen (N) concentrations were 123, 319 and 295 g/kg total N (S.E. 20.0), respectively. Two concentrates were formulated to con tain similar crude protein, effective rumen degradable protein, digestible undegradable protein and metabolisable energy concentrations but using diff erent carbohydrate sources to achieve a wide range of starch concentrations . For the low and high starch concentrates starch concentrations were 17 an d 304 g/kg DM and acid detergent fibre concentrations were 170 and 80 g/kg DM, respectively. The silages were offered ad libitum, supplemented with 10 kg fresh concentrate daily. For silages A, B and C, DM intakes were 10.9, 7.2 and 8.6 kg/day and concentrate energy sources did not alter (P > 0.05) intake. Increasing the level of starch in the concentrate decreased the mol ar concentration of acetate (P < 0.05) and tended to increase the molar con centration of propionate (P < 0.1). Silage type altered the molar concentra tion of acetate (P < 0.01) and the acetate:propionate ratio (P < 0.05). The re were no silage type x concentrate interactions (P > 0.05) on silage inta ke or rumen fermentation parameters. It is concluded that when concentrate and silage form equal proportions of the diet, the composition of the silag e has an over-riding influence on rumen fermentation parameters. Furthermor e, the changes in milk fat concentration, observed in a concurrent producti on study, due to changes in silage and concentrate types can be accounted f or by changes in the ratio of lipogenic to glucogenic precursors in the rum en fluid. (C) 2001 Elsevier Science B.V. All rights reserved.