Earlier work at this institute with calves indicated that a concentrate containing a mixture of fibre and fat allowed higher voluntary intakes than a concentrate containing barley. The objective of the current trial was to compare the immediate and residual effects on milk yield and composition of supplements containing starch or a mixture of fibre and fat.

Primary growths of perennial ryegrass were cut on 22 May and 12 June and wilted for 24 h prior to ensiling. The earlier cut material was preserved with an additive containing a mixture of formic acid, sulphuric acid and formalin at 4 l/t, whilst the later cut herbage was ensiled with formic acid at 3 l/t. A 60:40 mixture of the two silages was offered ad_ libitum with 2 pelleted supplements. Supplement Ba was a mixture of (DM basis) rolled barley (932 g/kg) and fishmeal (68 g/kg). Supplement SBP/FF consisted of unmolassed beet pulp (555 g/kg), extracted rice bran (315 g/kg), fat prills (56 g/kg) and soya bean meal (75 g/kg). Sources of protein differed in an attempt to balance RDP:UDP supply. The factorial combination of the two supplements (Ba and SBP/FF) given at two levels, 6 kg DM (L) or 12 kg DM (H), provided the 4 treatments imposed over weeks 3 to 10 of lactation on 40 British Friesian cows. During weeks 12 to 20 of lactation (Post experiment period) the cows were given an equal mixture of the two concentrates at 9 kg DM/day. Milk output was adjusted by covariance according to yield in week 2 of lactation.