Previous work has shown that dry period protein feeding can have important residual effects on the subsequent lactational performance in dairy cows (Moorby et al., 1996), although the results have been variable. This variability may be due to an animals nutrient requirements during late lactation and its ability to compensate during the dry period for previously inadequate supplies. This experiment was designed to investigate the effects of heifer age and level of concentrate offered during the second half of the first lactation on the second lactation performance. The size of the Longissimus dorsi was monitored as a marker of skeletal muscle use for lactation.
Thirty-eight Holstein-Friesian heifers were treated as described by Dewhurst et al. (1997). Briefly, animals were divided into 4 treatment groups differing in age at first calving (2 or 3 years old) and level of concentrate feeding for the last 18-20 weeks of their first lactation (2 kg/d, ‘L’, or 7 kg/d, ‘H’; n=11, 11, 9 and 7 for treatments 2L, 2H, 3L and 3H respectively). Six weeks before predicted calving date, animals were dried off and offered a relatively low quality diet of ad libitum access to a grass silage:straw mix (40:60 on a DM basis), designed to offer them little chance to compensate for previous differences in diet. After calving, animals were offered a diet of ad libitum access to grass silage plus 8 kg/d concentrate to day 120 of lactation, and 5 kg/d thereafter. Live weight was recorded weekly. After calving, milk yields were recorded daily, and milk samples taken weekly, to week 20 of lactation.