Global fertility in cows has steadily decreased over the last few years. In the UK, for example, first service calving rates fell from 55, 6 to 39, 7% between 1975-1982 and 1995-1998 - a 1% annual decline in first service pregnancy rate. In Spain the fertility indexes in cows have fallen over the last 10 years ago, accompanied by a frequency in the rate of postpartum disturbances and culling from infertility, which, in turn, is generally related to a higher milk production. Nonetheless, there remain a few herds of cows with both high yield and good fertility. Many studies have indicated that a high level of genetic merit rather than high production levels has played a part in reducing fertility in cows. Cows of a high level genetic merit have longer intervals in-between calving and first visual estrus as well as first service and first luteal activity, but a shorter estrus cycle and are less likely to ovulate post-partum. Oocytes from cows having a high level of genetic merit form fewer blastocysts and have a lower cleavage and blastocyst formation rates than those from cows having a medium level of genetic merit. Simultaneously, no differences have been observed in the number of oocytes recovered and developed in cows having either high or low milk production. Milk selected cows mobilize more body tissue in early lactation, have a better physical condition as well as delayed ovarian activity and greater risk of prolonged interval from calving to first ovulation than control cows, independent of the feeding system. Selecting for milk yield has been associated with high levels of insulin, and lower T4, GH, progesterone, estradiol, LH and IGF-1 concentrations. There seems evidence that concentrations of glucose, NEFA and ketones affect the selection process. Higher levels of genetic merit change cholesterol metabolism. Concentrations of cholesterol and lipoproteins lipids are correlated with a lower level of lipid infiltration of the liver. Hepatic lipidosis during early lactation has been associated with reduced reproductive efficiency.
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