SummaryOne hundred and ninety-four lactating British Friesian cows were used over a 2-year period to determine the effects of mixing diet ingredients on dry-matter (D.M.) intake, animal performance, digestibility coefficients, rumen fermentation and efficiency of food utilization. Four treatments were imposed in both years and were complete diet and separate ingredients at ad libitum and restricted levels of feeding.In Expt 1 a complete diet at ad libitum and restricted levels of feeding was compared with separate ingredients. The diet consisted of 60% concentrates and 40% forage and contained 20, 20, 10 and 50% on a D.M. basis of maize silage, lucerne silage, dried sugar-beet pulp and dairy compound, respectively. Method and level of feeding were the same in Expt 2; however, the diet in the 2nd year consisted of 50% concentrates (40% dairy compound, 10% dried sugar-beet pulp) and 50% grass silage.In Expt 1 cows fed the complete diet ad libitum consumed 16·5 kg D.M./day which was significantly more than the 14·3 kg/day consumed by those offered the separate ingredients ad libitum. Intakes in Expt 2 were not significantly affected by method of feeding. Much of the difference in Expt 1 was attributed to the fact that many cows rejected lucerne silage when fed in an unmixed form and therefore to keep the concentrate to forage ratio constant other diet ingredients were reduced proportionately.Mixing diet ingredients did not significantly affect milk yields, which were for the complete diet ad libitum and restricted and the separate ingredients ad libitum and restricted 22·2, 22·2; 22·1 and 20·7 kg/day in Expt 1 and 23·6, 20·8; 24·2 and 21·5 kg/ day in Expt 2, respectively. In Expt 1 when the concentrate to forage ratio was 60:40, cows fed the complete diet ad libitum produced milk with a significantly higher milk fat concentration (39·2 g/kg) than those fed the separate ingredients (31·6 g/kg). Similar effects were not recorded in Expt 2 when the concentrate to forage ratio had been reduced to 50:50. Changes in milk protein concentration were more closely related to changes in intake rather than method of feeding. Mixing diet ingredients tended to decrease live-weight loss in early lactation and accelerate live-weight gain in midlactation.In vivo digestibility coefficients determined in lactating dairy cows showed that in Expt 1 the D.M. and organic-matter digestibility coefficients for the complete diets fed ad libitum (0·689 and 0·713) were lower than those recorded (0·712 and 0·732) for the unmixed diet ad libitum. Although the digestibility coefficient of the acid-detergent fibre of the complete diet was markedly higher at 0·519 compared with 0·478 for the separate ingredients, the difference was not significant. In Expt 2 the only difference in digestibility coefficients was that for nitrogen, which for the complete diet was significantly higher than that of the separate ingredients at both levels of feeding.At an ad libitum level of feoding in Expt 1, the molar proportions of acetate and butyrate were 620 and 116 mM/l for the complete diet which were slightly higher than 602 and 95 mM/1 for the separate ingredients ad libitum. This diet also had a slightly higher molar proportion of propionate, 249 compared with the 221 mM/l for the complete diet ad libitum. In Expt 2 there wore no significant differences between treatments for either molar proportions of rumen volatile fatty acids or the ratio of lipogenic to non-lipogenic acids.
