The influence of forage quality and concentrate level on dry matter intake and milk production of British Friesian heifers

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Abstract
Thirty‐six British Friesian heifers were divided into two groups during weeks 3‐26 of lactation and received ad libitum either good (G) or average (A) quality grass silage. The in vitro digestible organic matter in the DM was 680 and 600 g kg‐1 for silages G and A, respectively. In addition, the heifers in each group were offered concentrate at either a low (4.3 kg DM d‐1), medium (64 kg DM d‐1) or high (8.4 kg DM d‐1) level. The concentrate contained 180 g (kg DM)‐1 of crude protein and had a calculated metabolizable energy concentration of 12.9 MJ (kg DM)‐1.Increasing concentrate level significantly increased milk yield (P < 0.01), milk protein concentration (P < 0.05), and yield of both milk protein depressed milk fat concentration (P < 0.001) and silage intake (P < 0.001).Although the overall effect of silage quality on milk yield was not significant, the milk yields for silage G at each concentrate level were higher, significantly so for low level, than the corresponding values for silage A. An improvement in silage quality did not affect milk fat concentration but significantly increased the concentration of milk protein (P < 005) and yields of both milk fat (P < 005) and protein (P < 0001). The response of silage DM intake to improved silage quality was 0 06 kg DM per 10 g rise in vitro DOMD value. Each additional kg concentrate DM depressed intake of silages G and A by 0‐63 and 0‐27 kg DM, respectively. Substitution rate was also significantly related to stage of lactation.With both silages, the digestibility coefficients determined in vivo for acid detergent fibre (ADF) decreased significantly (P < 0.001) when concentrate level was increased from low to high. Differences for DM, OM and total N digestibility coefficients between treatments were not established as significant at the 5% level.Estimates of mean efficiency of utilization of ME for lactation, made on a weekly basis, were 049 and 052 for the heifers given silages G and A. respectively, and 048, 052 and 052 for those given the low, medium and high levels of concentrate. Energy balances were calculated on a mean weekly basis.