Primary growths of perennial ryegrass (Lolium perenne) were cut on 22 May and 12 June and wilted for 24 h prior to ensiling. A 40: 60 mixture of the two silages was offered ad libitum with two pelleted supplements. Supplement Ba was a mixture of (dry matter (DM) basis) 932 g rolled barley and 68 g fish meal per kg. Supplement SBP/FF consisted of 555 g unmolassed beet pulp, 314 g extracted rice bran, 56 g fat prills and 75 g soya-bean meal per kg. 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 four 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 per day.Supplement Ba contained less ash (31 v. 94 g/kg), acid-detergent fibre (68 v. 218 g/kg) and fat (29 v. 77 g/kg) but more starch (558 v. 89 g/kg) and digestible organic matter in the dry matter in vitro (DOMD) (781 v. 627 g/kg) than SBP/FF. The concentration of total nitrogen (N) was similar at 25·6 g/kg. Silage had a DOMD of 643 g/kg, a pH of 3·8 and proportionately 0·82 of fermentation acids as lactic acid.Apparent digestibility of gross energy was higher for Ba diets (0·748) than for SBP/FF (0·704). Cows given SBP/FF ate 0·9 kg more silage DM than those given Ba (P < 0·01) but there was no difference in digestible energy intake or in substitution rate (−0·37 kg silage DM per kg additional supplement DM). Increasing the amount of supplement increased milk yield by 3·9 kg/day (P < 0·001). Cows given SBP/FF yielded on average 1·6 kg more milk than those given Ba (P < 0·05). However, this increased output consisted almost entirely of lactose and water as a result of a high concentration of fat in the milk of cows given Ba at the low level (46·3 g/kg). Further, the concentration of protein was less with SBP/FF (28·0 g/kg) than with Ba (29·2 g/kg).The effects of SBP/FF in early lactation were translated into a positive residual effect in mid lactation equivalent to the immediate effect. In contrast, raising yield by increasing the amount of supplement did not result in increased output subsequently. The results show that a supplement of fibre and fat despite having a lower digestibility than barley can produce more milk but a similar yield of fat and protein provided silage is offered ad libitum.

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