Bennett G L, Echternkamp S E, Gregory K E
U.S. Meat Animal Research Center, ARS, USDA, Clay Center, NE 68933-0166, USA.
J Anim Sci. 1998 Jul;76(7):1789-93. doi: 10.2527/1998.7671789x.
Genetic increases in twinning of cattle could result in increased frequency of triplet or higher-order births. There are no estimates of the incidence of triplets in populations with genetic levels of twinning over 40% because these populations either have not existed or have not been documented. A model of the distribution of litter size in cattle is proposed. Empirical estimates of ovulation rate distribution in sheep were combined with biological hypotheses about the fate of embryos in cattle. Two phases of embryo loss were hypothesized. The first phase is considered to be preimplantation. Losses in this phase occur independently (i.e., the loss of one embryo does not affect the loss of the remaining embryos). The second phase occurs after implantation. The loss of one embryo in this stage results in the loss of all embryos. Fewer than 5% triplet births are predicted when 50% of births are twins and triplets. Above 60% multiple births, increased triplets accounted for most of the increase in litter size. Predictions were compared with data from 5,142 calvings by 14 groups of heifers and cows with average litter sizes ranging from 1.14 to 1.36 calves. The predicted number of triplets was not significantly different (chi2 = 16.85, df = 14) from the observed number. The model also predicted differences in conception rates. A cow ovulating two ova was predicted to have the highest conception rate in a single breeding cycle. As mean ovulation rate increased, predicted conception to one breeding cycle increased. Conception to two or three breeding cycles decreased as mean ovulation increased because late-pregnancy failures increased. An alternative model of the fate of ova in cattle based on embryo and uterine competency predicts very similar proportions of singles, twins, and triplets but different conception rates. The proposed model of litter size distribution in cattle accurately predicts the proportion of triplets found in cattle with genetically high twinning rates. This model can be used in projecting efficiency changes resulting from genetically increasing the twinning rate in cattle.
牛双胎率的遗传增加可能会导致三胎或更高胎次出生频率的上升。对于双胎率超过40%的群体中三胎的发生率尚无估计，因为这些群体要么不存在，要么没有相关记录。本文提出了一个牛产仔数分布模型。将绵羊排卵率分布的实证估计与关于牛胚胎命运的生物学假设相结合。假设胚胎损失分为两个阶段。第一阶段被认为是着床前阶段。此阶段的损失是独立发生的（即一个胚胎的损失不影响其余胚胎的损失）。第二阶段发生在着床后。此阶段一个胚胎的损失会导致所有胚胎的损失。当50%的出生为双胎和三胎时，预测三胎出生的比例不到5%。多胎率超过60%时，三胎数量的增加占产仔数增加的大部分。将预测结果与14组小母牛和母牛的5142次产犊数据进行比较，这些牛的平均产仔数在1.14至1.36头犊牛之间。预测的三胎数量与观察到的数量没有显著差异（卡方 = 16.85，自由度 = 14）。该模型还预测了受孕率的差异。预计在单个繁殖周期中，排卵两个卵子的母牛受孕率最高。随着平均排卵率的增加，预计单次繁殖周期的受孕率会增加。随着平均排卵率的增加，两到三个繁殖周期的受孕率会下降，因为妊娠后期失败率增加。基于胚胎和子宫能力的牛卵子命运的另一种模型预测的单胎、双胎和三胎比例非常相似，但受孕率不同。所提出的牛产仔数分布模型准确地预测了双胎率遗传率高的牛群中三胎的比例。该模型可用于预测牛双胎率遗传增加所导致的效率变化。
