SUPPLEMENTATION OF GRASS SILAGE-BASED DIETS WITH SMALL QUANTITIES OF CONCENTRATES - STRATEGIES FOR ALLOCATING CONCENTRATE CRUDE PROTEIN

Fifty-five multiparous Holstein-Friesian cows were used to evaluate th e short- and long-term effects of varying the crude protein (CP) conte nt of concentrates offered at a low level (5 kg/day) along with ad lib itum access to a high qualify grass silage. Three dietary treatment gr oups in lactation weeks 4 to 12 received concentrates containing eithe r 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 concent rate 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 concen trate 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 incre ased concentrate CP were of a similar magnitude in early- and midlacta tion; extra concentrate CP can recover depressed yields and concentrat ions of milk protein in established lactation. Production responses to concentrate CP involved a concomitant increase in silage voluntary in take.