Two experiments were carried out to compare the feed conversion efficiency at high pasture allowances in early (EXP1) and mid-lactation (EXP2) of genetically heavy (H) or light (L) Holstein-Friesian cows (H = 487, L = 410 kg, P<0.001). Average milksolids (MS) yield was not significantly different in EXP1 (H = 1.73, L = 1.64 kg MS), but the H line had higher MS yield than the L line in EXP2 (H = 1.70, L = 1.54 kg/cow/day, P<0.001). The mean individual dry matter intakes (DMI, kg/cow/day), measured using the n-alkanes technique, were higher for H cows in both EXP1 (H = 15.5 and L = 13.9; P<0.05) and EXP2 (H = 12.2 and L = 10.8; P<0.05). However, herbage intake was not significantly different between the H and L cows when adjusted to a common MS yield. Feed conversion efficiency (g MS/kg DMI) was slightly higher for the L cows during EXP1 (H = 114, L = 120, P = <0.1), but not different for H and L cows during EXP2 (H = 144, L = 143). The calculated bite size of the H cows was on average 0.095 g DM/bite (pooled for the two experiments) heavier (P<0.05) than that of the L cows, but the L cows showed a consistently faster rate of biting than the H cows (H = 51, L = 56 bites/min, P<0.05). Although the H cows had higher MS yield than the L cows, especially in EXP2, the higher MS yield was not associated with increased efficiency, presumably because the H cows required more energy for maintenance than the L cows.
Proceedings of the New Zealand Society of Animal Production, Volume 58, , 128-131, 1998
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