Diskin M G, Murphy J J, Sreenan J M
Teagasc, Animal Production Research Centre, Mellows Campus, Athenry, Co. Galway, Ireland.
Anim Reprod Sci. 2006 Dec;96(3-4):297-311. doi: 10.1016/j.anireprosci.2006.08.008. Epub 2006 Aug 3.
Efficient pasture-based milk production systems require a compact calving pattern aligned to the onset of the grazing season, a 365-day calving interval and low culling rates for infertility. Achievement of these targets requires high herd reproductive performance. While high genetic merit Holstein cows produce more milk in grass-based systems their fertility is compromised. Management of the modern high genetic merit Holstein dairy cow presents a major challenge in pasture-based systems of production. It appears that the extent of early embryo loss is greater (up to 20% points greater) in the modern high-producing dairy cow and that a much higher proportion of the embryos die before day 7 following insemination in contrast to heifers and lower yielding cows. About 7-8% of pregnancies are lost between days 30 and 90 of gestation with no evidence that loss rate is related to cow genetic merit, parity or level of production. Systemic concentrations of progesterone during both the cycle preceding and following insemination affect embryo survival rate with evidence that too low or indeed too high a concentration of progesterone been negatively associated with embryo survival rate. Peripheral concentrations of both progesterone and oestradiol are lowered by increased plane of feed intake due to increased metabolic clearance rate of the steroids, which is related to liver blood flow. It appears that high producing dairy cows have an increased risk of embryo death as a result of lowered peripheral concentrations of progesterone as a consequence of increased hepatic metabolism of progesterone. Uterine expression of mRNA for progesterone receptor, oestradiol receptor and retinol binding protein mRNA appears to be sensitive to changes in peripheral concentrations of progesterone during the first week after AI. It would appear that energy balance and dry matter intake during the 4 weeks, immediately after calving are critically important in determining conception rate when cows are inseminated at 70-100 days post-calving. Concentrate supplementation of cows at pasture during the breeding period has minimal affects on conception rates though sudden reduction in dietary intake should be avoided. For pasture-based systems of milk production more balanced breeding strategies, with greater emphasis on fertility and feed intake must be developed.
高效的基于牧场的牛奶生产系统需要紧凑的产犊模式，与放牧季节开始时间一致，产犊间隔为365天，不育淘汰率低。要实现这些目标需要牛群具有较高的繁殖性能。虽然遗传优势高的荷斯坦奶牛在以草为基础的系统中产奶量更高，但它们的繁殖力会受到影响。在基于牧场的生产系统中，管理现代遗传优势高的荷斯坦奶牛是一项重大挑战。现代高产奶牛早期胚胎损失的程度似乎更大（高达20个百分点），与小母牛和低产奶牛相比，在授精后第7天之前死亡的胚胎比例要高得多。大约7 - 8%的妊娠在妊娠30至90天之间丢失，没有证据表明丢失率与奶牛的遗传优势、胎次或生产水平有关。授精前后周期中孕酮的全身浓度会影响胚胎存活率，有证据表明孕酮浓度过低或过高都与胚胎存活率呈负相关。由于类固醇代谢清除率增加（这与肝血流量有关），饲料摄入量增加会降低孕酮和雌二醇的外周浓度。由于孕酮肝脏代谢增加导致外周孕酮浓度降低，高产奶牛似乎有更高的胚胎死亡风险。授精后第一周，子宫中孕酮受体、雌二醇受体和视黄醇结合蛋白mRNA的表达似乎对孕酮外周浓度的变化敏感。当奶牛在产后70 - 100天进行授精时，产犊后立即的4周内能量平衡和干物质摄入量对于确定受孕率至关重要。在繁殖期给牧场中的奶牛补充精料对受孕率影响最小，但应避免日粮摄入量突然减少。对于基于牧场的牛奶生产系统，必须制定更平衡的繁殖策略，更加注重繁殖力和采食量。
