This study investigated the effect of genotype and diet on the gross feed conversion efficiency (FCE) of cows milked for 600-day extended lactations (24-month season) compared with 300-day lactations (12-month season). North American (NA) and New Zealand (NZ) Holstein-Friesians (HF) grazed pasture and were supplemented with either 0, 3 or 6 kg DM concentrate/cow/day. In a normal 12-month season, gross FCE was greater for NZ HF compared with NA HF cows when fed 0 or 3 kg DM concentrate (96 and 104 vs. 90 and 99 kg milksolids (MS)/t dry matter intake (DMI), respectively), but similar when fed 6 kg DM (101 kg MS/t DMI). However, during an extended 24-month season, NZ HF cows had a lower gross FCE than NA HF when fed 6 kg DM concentrate (86 vs. 99 kg MS/t DMI, respectively) and similar FCE when fed 0 or 3 kg DM (85 and 94 vs. 86 and 99 kg MS/t DMI, respectively). Regardless of diet, gross FCE in an extended lactation was 98% of that obtained in a normal season for NA HF cows, but only 88% for NZ HF, indicating that NA HF are better suited to converting feed into high MS production in extended lactations.
Proceedings of the New Zealand Society of Animal Production, Volume 68, Brisbane, Australia, 100-104, 2008
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