Department of Animal and Range Sciences, Montana State University, Bozeman 59717, USA.
J Anim Sci. 2013 Mar;91(3):1329-35. doi: 10.2527/jas.2012-5704. Epub 2012 Oct 24.
Research emphasis has been placed on heifer development strategies in recent years, comparing traditional, more intensive systems to more extensive systems using less feed and relying on compensatory gain to reach a target BW. Recent research has indicated that developing heifers to a lighter target BW at breeding (i.e., 50 to 57% of mature BW compared with 60 to 65% BW) reduced development costs and did not impair reproductive performance. Research published through the late 1980s demonstrated greater negative effects of limited postweaning growth on age at puberty and pregnancy rates whereas more recent studies demonstrate less of a negative impact of delayed puberty on pregnancy rate. A limitation of most research concerning influences of nutrition on heifer development and cow reproductive performance is little or limited consideration of long-term implications. Longevity has relatively low heritability; therefore, heifer development and other management strategies have a greater potential to impact cow retention. Establishing the impact of heifer development protocols on longevity is complex, requiring consideration of nutritional factors after the start of breeding and through subsequent calvings. Lower-input heifer development, where all heifers are managed together after the postweaning period, did not impair rebreeding, but continued subsequent restriction in the form of marginal winter supplementation resulted in decreased retention in the breeding herd. Therefore, the compensatory BW gain period for restricted-growth heifers may be important to longevity and lifetime productivity. Adequate growth and development to ensure minimal calving difficulty can be of critical importance for longevity; however, providing additional supplemental feed during postweaning development to accomplish this may be less efficient than later in development. Restricting gain during postweaning development by limiting DMI or developing heifers on dormant winter forage resulted in increased economic advantages compared with developing heifers at greater rates of ADG to achieve a greater target BW. Implications of heifer development system on cow longevity must be considered when evaluating economics of a heifer enterprise; however, studies evaluating the effects of heifer development systems on cow longevity are extremely limited.
近年来，研究重点一直放在后备牛的发展策略上，将传统的、更加密集的系统与使用较少饲料并依靠补偿性生长来达到目标体重的更加广泛的系统进行比较。最近的研究表明，在配种时将后备牛的目标体重设定得更轻（即 50%至 57%的成熟体重，而不是 60%至 65%的体重）可以降低发展成本，且不会损害繁殖性能。20 世纪 80 年代末发表的研究表明，限制断奶后生长对初情期和妊娠率的负面影响更大，而最近的研究则表明，青春期延迟对妊娠率的负面影响较小。大多数关于营养对后备牛发育和奶牛繁殖性能影响的研究的一个局限性是，对长期影响的考虑很少或有限。牛的寿命遗传力相对较低；因此，后备牛的发育和其他管理策略对牛的保留率有更大的影响。确定后备牛发展方案对牛的寿命的影响很复杂，需要考虑配种开始后的营养因素以及随后的产犊。在断奶后时期，所有后备牛都一起管理的低投入后备牛发展不会影响再繁殖，但随后以边际冬季补充的形式继续限制，则会降低在繁殖群中的保留率。因此，限制生长后备牛的补偿体重增长期可能对牛的寿命和终生生产力很重要。确保最小分娩困难的充分生长和发育对牛的寿命至关重要；然而，为了达到这个目标，在断奶后发育期间提供额外的补充饲料可能不如在后期发育更有效。通过限制干物质采食量或在休眠冬季牧草上育成后备牛，限制断奶后发育时的生长速度，与通过增加 ADG 来达到更大目标体重的育成方式相比，可获得更大的经济效益。在评估后备牛企业的经济效益时，必须考虑后备牛发展系统对牛寿命的影响；然而，关于后备牛发展系统对牛寿命影响的研究极为有限。
