De Vries A
Department of Animal Sciences, University of Florida, Gainesville 32611.
J Dairy Sci. 2017 May;100(5):4184-4192. doi: 10.3168/jds.2016-11847. Epub 2017 Feb 16.
Genetic improvement in sires used for artificial insemination (AI) is increasing faster compared with a decade ago. The genetic merit of replacement heifers is also increasing faster and the genetic lag with older cows in the herd increases. This may trigger greater cow culling to capture this genetic improvement. On the other hand, lower culling rates are often viewed favorably because the costs and environmental effects of maintaining herd size are generally lower. Thus, there is an economic trade-off between genetic improvement and longevity in dairy cattle. The objective of this study was to investigate the principles, literature, and magnitude of these trade-offs. Data from the Council on Dairy Cattle Breeding show that the estimated breeding value of the trait productive life has increased for 50 yr but the actual time cows spend in the herd has not increased. The average annual herd cull rate remains at approximately 36% and cow longevity is approximately 59 mo. The annual increase in average estimated breeding value of the economic index lifetime net merit of Holstein sires is accelerating from $40/yr when the sire entered AI around 2002 to $171/yr for sires that entered AI around 2012. The expectation is therefore that heifers born in 2015 are approximately $50 more profitable per lactation than heifers born in 2014. Asset replacement theory shows that assets should be replaced sooner when the challenging asset is technically improved. Few studies have investigated the direct effects of genetic improvement on optimal cull rates. A 35-yr-old study found that the economically optimal cull rates were in the range of 25 to 27%, compared with the lowest possible involuntary cull rate of 20%. Only a small effect was observed of using the best surviving dams to generate the replacement heifer calves. Genetic improvement from sires had little effect on the optimal cull rate. Another study that optimized culling decisions for individual cows also showed that the effect of changes in genetic improvement of milk revenue minus feed cost on herd longevity was relatively small. Reduced involuntary cull rates improved profitability, but also increased optimal voluntary culling. Finally, an economically optimal culling model with prices from 2015 confirmed that optimal annual cull rates were insensitive to heifer prices and therefore insensitive to genetic improvement in heifers. In conclusion, genetic improvement is important but does not warrant short cow longevity. Economic cow longevity continues to depends more on cow depreciation than on accelerated genetic improvements in heifers. This is confirmed by old and new studies.
与十年前相比，用于人工授精（AI）的公牛的遗传改良速度正在加快。后备小母牛的遗传价值也在更快地提高，并且与牛群中年龄较大的母牛相比，遗传差距在增大。这可能会引发更多的母牛淘汰，以获取这种遗传改良。另一方面，较低的淘汰率通常被视为有利因素，因为维持牛群规模的成本和环境影响通常较低。因此，在奶牛的遗传改良和长寿之间存在经济权衡。本研究的目的是调查这些权衡的原则、文献和程度。美国奶牛育种委员会的数据显示，生产寿命性状的估计育种值在50年里有所提高，但母牛在牛群中实际度过的时间并未增加。牛群的年平均淘汰率仍保持在约36%，母牛的寿命约为59个月。荷斯坦公牛经济指数终身净效益的平均估计育种值的年增幅正在加快，从2002年左右进入人工授精的公牛的每年40美元，增加到2012年左右进入人工授精的公牛的每年171美元。因此，可以预期2015年出生的小母牛每泌乳期比2014年出生的小母牛大约多盈利50美元。资产替换理论表明，当有挑战性的资产在技术上得到改进时，资产应该更快地被替换。很少有研究调查遗传改良对最优淘汰率的直接影响。一项35年前的研究发现，经济上最优的淘汰率在25%至27%的范围内，而最低可能的非自愿淘汰率为20%。使用存活最好的母牛来繁育后备小母牛犊只观察到了很小的影响。公牛的遗传改良对最优淘汰率几乎没有影响。另一项针对个体母牛优化淘汰决策的研究也表明，牛奶收益减去饲料成本的遗传改良变化对牛群寿命的影响相对较小。降低非自愿淘汰率提高了盈利能力，但也增加了最优自愿淘汰率。最后，一个基于2015年价格的经济最优淘汰模型证实，最优年淘汰率对小母牛价格不敏感，因此对小母牛的遗传改良也不敏感。总之，遗传改良很重要，但并不保证母牛有较短的寿命。母牛的经济寿命仍然更多地取决于母牛的折旧，而不是小母牛遗传改良的加速。新旧研究都证实了这一点。
