Increases in yields of milk and milk protein have been observed from dairy cows offered a high protein supplement during the dry period (Van Saun, Idleman and Sniffen, 1993; Moorby, Dewhurst and Marsden, 1996). One possible mechanism for this is an accumulation of maternal body protein during late pregnancy and its later release during lactation. This experiment was designed to investigate the effect of diet on the potential of dairy cows to accumulate and release body nitrogen over the course of the dry period and the first 20 weeks of lactation.
Twelve multiparous Holstein-Friesian cows were offered one of three diets for 6 weeks prior to calving, with 4 animals per diet, in an continuous design experiment. Animals were offered ad libitum access to A) grass silage only (medium protein), B) a grass silage/barley straw mix (60:40 on a dry matter basis) (low protein), or C) grass silage plus 0.5 kg/d high protein maize gluten meal (high protein). After calving, all animals were offered a standard lactation diet based on ad libitum grass silage plus 10 kg/d concentrate to week 12 of lactation, with 7 kg/d thereafter. Animals were housed in individual stalls for 6 d N-balance procedures on three separate occasions in two groups of six animals: during the dry period (at approximately 3 to 4 weeks before calving), early-lactation (weeks 7 to 8 after calving) and mid-lactation (weeks 17 to 18).