Fifty-five multiparous Holstein-Friesian cows were used to evaluate the short- and long-term effects of varying the crude protein (CP) content of concentrates offered at a low level (5 kg/day) along with ad libitum access to a high qualify grass silage. Three dietary treatment groups in lactation weeks 4 to 12 received concentrates containing either 156 (L), 247 (M) or 338 (H) g CP per kg dry matter; from weeks 13 to 21, half of the L animals changed over to the H concentrate and vice versa so that there were five treatment groups (LL, LH, MM, HL and HH). Feeding M or H compared with L increased silage voluntary intakes (P < 0.05) and the yields of milk (P < 0.05) fat (P < 0.05) and protein (P < 0.02). Milk protein concentration increased with level of concentrate CP (P < 0.05). Pattern of concentrate CP supply (comparison of LH, MM and HL) had no significant effect on intake or yields of milk and milk solids across the experiment (weeks 4 to 21), though cows gained less weight on treatment HL than on LH (P < 0.05) or MM. Intake, milk and component yields were all markedly affected by a change in concentrate CP at week 13; there were positive effects of additional CP (LL v. LH) and negative effects of reduced CP (HH v. HL) on silage intake (P < 0.05), as well as milk yield (P < 0.001), milk protein yield (P < 0.001) and milk protein concentration (P < 0.001). Responses to increased concentrate CP were of a similar magnitude in early- and midlactation; extra concentrate CP can recover depressed yields and concentrations of milk protein in established lactation. Production responses to concentrate CP involved a concomitant increase in silage voluntary intake.
