Embryo survival is a major factor affecting production and economic efficiency in all systems of ruminant milk and meat production. For heifers, beef and moderate yielding dairy cows, does and camelids it appears that fertilization generally lies between 90% and 100%. In high-producing dairy cows there is a less substantive body of literature, but it would appear that it is somewhat lower and perhaps more variable. In cattle, the major component of embryo loss occurs before day 16 following breeding with some evidence of greater losses before day 8 in high-producing dairy cows. In cattle late embryo loss, while numerically much smaller than early embryo mortality loss, nevertheless, causes serious economic losses to producers because it is often too late to rebreed females when they repeat. In multiple ovulating small ruminants, the loss rate is positively related to ovulation rate. Systemic concentrations of progesterone, during both the cycle preceding and following insemination, affect embryo survival rate with evidence that too high or indeed too low a concentration being negatively associated with survival rate. Uterine expression of mRNA for progesterone receptor, oestradiol receptor and retinol-binding protein appears to be sensitive to changes in peripheral concentrations of progesterone during the first week after artificial insemination. Energy balance and dry matter intake during 4 weeks 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 effects on conception rates though sudden reductions in dietary intake should be avoided. For all systems of milk production, more balanced breeding strategies with greater emphasis on fertility and feed intake and/or energy balance must be developed. There is sufficient genetic variability within the Holstein breed for fertility traits. Alternative dairy breeds such as the Jersey or Norwegian Red could also be utilized. Genomic technology will not only provide scientists with an improved understanding of the underlying biological processes involved in fertilization and the establishment of pregnancy, but also, in the future, identify genes responsible for improved embryo survival. Its incorporation into breeding objectives would increase the rate of genetic progress for embryo survival.

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Background The corpus luteum develops from the follicular cells which remain following ovulation and secretes progesterone which is essential for normal pregnancy in domestic animals. In sheep, the corpus luteum must be present for at least 50 days if normal pregnancy is to be maintained (Casida & Warwick, 1945). Progesterone is produced in the corpus luteum of domestic animals by two morphologically and functionally distinct steroidogenic cell types termed large and small luteal cells. In sheep, large cells average 29.2 p.m in diameter (Rodgers et al., 1984) and can be morphologically distinguished by a spherical nucleus, stacks of rough endoplasmic reticulum, secretory granules and a highly involuted plasma membrane. Small cells average 15.8 p.m in diameter (Rodgers et al., 1984) and are distinguished by an irregularly shaped nucleus, large quantities of smooth endoplasmic reticulum, and numerous lipid droplets (for review see Niswender & Nett, 1988). After separation into relatively pure populations, biochemical differences have been clearly recognized between the two steroidogenic cell types (

Efficient milk production in Ireland in both spring and autumn calving herds is very much dependent on compact calving. This requires an early resumption of ovarian activity, high submission rate, and pregnancy rate to service. Since 1984 there has been an increase in the genetic merit of Irish dairy herds. High genetic merit animals have reduced reproductive performance due to the demands of high milk yield (Nebel and McGilliard, 1993) and the loss of body weight and body condition (Butler and Smith, 1989). However, little comparative data on the reproductive performance of dairy cows of different genetic merit are available. The objective of this study was therefore, to determine the relationship between genetic merit of dairy cows and follicular growth patterns, milk production, body weight loss, insulin and glucose levels in early lactation.

Based on their pedigree indices for milk production traits, 48 high (HM) and 48 medium (MM) genetic merit cows were studied in each of two years to determine the possible relationships between genetic merit, milk production, body condition, metabolite and hormone concentrations, feed intake and reproductive performance. The high genetic merit cows had higher milk production, greater body condition loss between calving and first service, lower plasma glucose and insulin-like growth factor-1 (IGF-1) and lower first and second service conception rates than the medium genetic merit cows. Furthermore, more high genetic merit cows were not pregnant at the end of the breeding period compared with medium genetic merit cows. Cows that did not conceive to first service were retrospectively compared to those that conceived to first service within each genetic merit category. Within the high and medium genetic merit category there were no significant differences between the cows that did not conceive and those that conceived to first service in terms of milk production, or plasma concentrations of glucose, non-esterified fatty acids (NEFA). Although medium genetic merit cows that did not conceive to first service lost more body condition score between calving and first service than those that conceived to first service, there was no difference in body condition score (BCS) change between calving and first service between high genetic merit cows that did not conceive and those that conceived to first service. In the present study, high genetic merit cows had higher milk production and reduced reproductive performance in comparison with medium genetic merit cows. However, reproductive performance was not associated with milk production, energy balance or plasma concentrations of glucose, NEFA or IGF-1 between calving and insemination