Optimizing pure line breeding strategies utilizing reproductive technologies.

The development of new reproductive techniques has had a great effect on schemes for dairy cattle improvement; AI resulted in progeny-testing schemes, and, more recently, multiple ovulation and embryo transfer has resulted in nucleus breeding schemes. In traditional breeding schemes, the use of multiple ovulation and embryo transfer resulted in only small increases of genetic gains because the selection intensity of bull dams was already high, but the selection intensity of cows in nucleus breeding schemes increased considerably. This increase makes nucleus schemes very competitive, especially when in vitro maturation and fertilization is used to increase female reproductive rates further. The increased genetic gain results in the selection of young females (i.e., optimal generation intervals are shortened). The combined effect of multiple ovulation and embryo transfer and shorter generation intervals increases the rate of gain by approximately 15% but also increases the rate of inbreeding and variances of the selection response (i.e., the risk of the breeding scheme) by about 80%. Recently, selection methods have been developed to reduce the rates of inbreeding or the variance of the selection response in breeding schemes. A scheme that maximized the selection differential while constraining the rate of inbreeding yielded about 30 to 60% more selection response than selection for BLUP estimated breeding value at the same rate of inbreeding. For the future, the combination of juvenile predictors of genetic merit (e.g., DNA markers) and techniques that further increase female reproductive rates seem very promising methods to increase the rates of genetic gain. As an example, the genetic gain of an optimized hybrid nucleus scheme could be increased 24% by the combined use of juvenile predictors and in vitro maturation and fertilization. Fertilization of fetal embryos was predicted to increase rates of gain by up to 18%.