Wathes D C, Pollott G E, Johnson K F, Richardson H, Cooke J S
1Department of Production and Population Health,Royal Veterinary College,Hawkshead Lane,North Mymms,Hatfield,Herts,AL9 7 TA,UK.
2Centre for Dairy Information,Scotsbridge House,Scots Hill,Rickmansworth,Herts,WD3 3BB,UK.
Animal. 2014 May;8 Suppl 1:91-104. doi: 10.1017/S1751731114000755. Epub 2014 Apr 3.
The rearing period has a key influence on the later performance of cattle, affecting future fertility and longevity. Producers usually aim to breed replacement heifers by 15 months to calve at 24 months. An age at first calving (AFC) close to 2 years (23 to 25 months) is optimum for economic performance as it minimises the non-productive period and maintains a seasonal calving pattern. This is rarely achieved in either dairy or beef herds, with average AFC for dairy herds usually between 26 and 30 months. Maintaining a low AFC requires good heifer management with adequate growth to ensure an appropriate BW and frame size at calving. Puberty should occur at least 6 weeks before the target breeding age to enable animals to undergo oestrous cycles before mating. Cattle reach puberty at a fairly consistent, but breed-dependent, proportion of mature BW. Heifer fertility is a critical component of AFC. In US Holsteins the conception rate peaked at 57% at 15 to 16 months, declining in older heifers. Wide variations in growth rates on the same farm often lead to some animals having delayed first breeding and/or conception. Oestrous synchronisation regimes and sexed semen can both be used but unless heifers have been previously well-managed the success rates may be unacceptably low. Altering the nutritional input above or below those needed for maintenance at any stage from birth to first calving clearly alters the average daily gain (ADG) in weight. In general an ADG of around 0.75 kg/day seems optimal for dairy heifers, with lower rates delaying puberty and AFC. There is some scope to vary ADG at different ages providing animals reach an adequate size by calving. Major periods of nutritional deficiency and/or severe calfhood disease will, however, compromise development with long-term adverse consequences. Infectious disease can also cause pregnancy loss/abortion. First lactation milk yield may be slightly lower in younger calving cows but lifetime production is higher as such animals usually have good fertility and survive longer. There is now extensive evidence that as long as the AFC is >23 months then future performance is not adversely influenced. On the other hand, delayed first calving >30 months is associated with poor survival. Underfeeding of young heifers reduces their milk production potential and is a greater problem than overfeeding. Farmers are more likely to meet the optimum AFC target of 23 to 25 months if they monitor growth rates and adjust feed accordingly.
育成期对牛的后期生产性能有关键影响，会影响其未来的繁殖力和寿命。生产者通常旨在让后备小母牛在15月龄时配种，以便在24月龄时产犊。初产年龄（AFC）接近2岁（23至25个月）对经济效益最为有利，因为这样可将非生产期减至最短，并维持季节性产犊模式。但无论是奶牛群还是肉牛群，这一目标很少能够实现，奶牛群的平均AFC通常在26至30个月之间。要保持较低的AFC，需要对小母牛进行良好管理，使其充分生长，以确保产犊时具有合适的体重和体格大小。初情期应在目标配种年龄前至少6周出现，以便动物在配种前经历发情周期。牛达到初情期时的体重占成熟体重的比例相当稳定，但因品种而异。小母牛的繁殖力是AFC的一个关键组成部分。在美国荷斯坦奶牛中，受孕率在15至16月龄时达到峰值57%，年龄较大的小母牛受孕率则会下降。同一养殖场内生长速度差异很大，常常导致一些动物首次配种和/或受孕延迟。发情同步方案和性别分选精液都可使用，但除非此前对小母牛管理得当，否则成功率可能低得令人无法接受。从出生到首次产犊的任何阶段，改变维持所需营养投入之上或之下的营养供给，显然会改变体重的平均日增重（ADG）。一般来说，奶牛小母牛的ADG约为每天0.75千克似乎最为适宜，增重速度较低会延迟初情期和AFC。在不同年龄阶段，只要动物在产犊时达到足够的大小，就有一定的空间来改变ADG。然而，营养严重缺乏和/或犊牛期严重疾病的主要阶段会损害发育，并产生长期不良后果。传染病也可能导致妊娠丢失/流产。产犊年龄较小的母牛头胎产奶量可能略低，但终生产量较高，因为这类动物通常繁殖力良好且存活时间更长。现在有大量证据表明，只要AFC大于23个月，那么对未来生产性能就不会产生不利影响。另一方面，首次产犊延迟超过30个月与存活率低有关。对年轻小母牛饲养不足会降低其产奶潜力，这比饲养过度更是个问题。如果养殖者监测生长速度并相应调整饲料，就更有可能实现23至25个月的最佳AFC目标。
