For heifers, beef and moderate-yielding dairy cows, fertilisation generally exceeds 90%. In high-producing dairy cows, it may be lower and possibly more variable. The major component of embryo loss occurs before Day 16 following breeding, with emerging evidence of greater losses before Day 8 in high-producing dairy cows. Late embryo loss causes serious economic losses because it is often recognised too late to rebreed females. Systemic concentrations of progesterone during the cycles both preceding and following insemination affect embryo survival; too-high or too-low a concentration has been shown to be negatively associated with survival rate. Energy balance and dry matter intake during the 4 weeks after calving are critically important in determining conception rate when cows are inseminated 70 to 100 days after calving. More balanced breeding strategies with greater emphasis on fertility, feed intake and energy must be developed. Genetic variability for fertility traits can be exploited; genomic technology will not only provide scientists with an improved understanding of the underlying biological processes involved in fertilisation and the establishment of pregnancy, but could identify genes responsible for improved embryo survival. Their incorporation into breeding objectives would increase the rate of genetic progress for embryo survival. There is a range of easily adoptable management factors, under producer control, that can either directly increase embryo survival or ameliorate the consequences of low embryo survival rates. The correction of minor deficits in several areas can have a substantial overall effect on herd reproductive performance.

Uterine secretions, or histotroph, are a critical component for early embryo survival, functioning as the sole supply of vitamins, minerals, enzymes, and other myriad of nutrients required by the developing conceptus before implantation. Histotroph is therefore a promising source for biomarkers of uterine function and for enhancing our understanding of the environment supporting early embryo development and survival. Utilizing label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) shotgun proteomics, we characterized the uterine proteome at two key preimplantation stages of the estrous cycle in high fertility cattle. We identified 300 proteins on Day 7 and 510 proteins on Day 13 including 281 proteins shared between days. Five proteins were more abundant (P < 0.05) on Day 7 compared with Day 13 and included novel histotroph proteins cytokeratin 10 and stathmin. Twenty-nine proteins were more abundant (P < 0.05) including 13 unique on Day 13 compared with Day 7 and included previously identified legumain, metalloprotease inhibitor-2, and novel histotroph proteins chromogranin A and pyridoxal kinase. Functional analysis of the 34 differentially expressed proteins (including 14 novel to histotroph) revealed distinct biological roles putatively involved in early pregnancy, including remodelling of the uterine environment in preparation for implantation; nutrient metabolism; embryo growth, development and protection; maintenance of uterine health; and maternal immune modulation. This study is the first reported LC-MS/MS based global proteomic characterization of the uterine environment in any domesticated species before implantation and provides novel information on the temporal alterations in histotroph composition during critical stages for early embryo development and uterine function during the early establishment of pregnancy.

For heifers, beef and moderate-yielding dairy cows, it appears that the fertilisation rate generally lies between 90% and 100%. For high-producing dairy cows, there is a less substantive body of literature, but it would appear that the fertilisation rate is somewhat lower and possibly more variable. In cattle, the major component of embryo loss occurs in the first 16 days following breeding (Day 0), with emerging evidence of greater losses before Day 8 in high-producing dairy cows. In cattle, late embryo mortality causes serious economic losses because it is often recognised too late to rebreed females. Systemic concentrations of progesterone during both the cycle preceding and following insemination affect embryo survival, with evidence of either excessive or insufficient concentrations being negatively associated with survival rate. The application of direct progesterone supplementation or treatments to increase endogenous output of progesterone to increase embryo survival cannot be recommended at this time. Energy balance and dry matter intake during the first 4 weeks after calving are critically important in determining pregnancies per AI when cows are inseminated at 70-100 days after calving. Level of concentrate supplementation of cows at pasture during the breeding period has minimal effects on conception rates, although 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 must be developed. There is genetic variability within the Holstein breed for fertility traits, which can be exploited. Genomic technology will not only provide scientists with an improved understanding of the underlying biological processes involved in fertilisation and the establishment of pregnancy, but also, in the future, could identify genes responsible for improved embryo survival. Such information could be incorporated into breeding objectives in order to increase the rate of genetic progress for embryo survival. In addition, there is a range of easily adoptable management factors, under producer control, that can either directly increase embryo survival or ameliorate the consequences of low embryo survival rates. The correction of minor deficits in several areas can have a substantial cumulative positive effect on herd reproductive performance.

Pregnancy per insemination is a major determinant of reproductive efficiency in cattle and is affected by concentrations of progesterone (P4) during early pregnancy. The relationship between pregnancy per artificial insemination (P/AI) and early luteal concentrations of P4, and repeatability of concentrations of P4 was examined on d 4, 5, 6, and 7 (day of standing estrus=d 0) in 118 Holstein Friesian heifers following 2 rounds of AI to 1 high-fertility sire. Repeatability estimates (R(e)) for P/AI were established following 4 rounds of AI. We found a linear and quadratic relationship between P/AI and concentrations of P4 on d 4 to 7 after estrus, as well as a linear and quadratic relationship between P/AI and the change in concentration of P4 from d 4 to 7 and from d 5 to 7. Optimum concentrations of P4 to maximize probability of P/AI were 2.5, 4.0, 5.0, 5.2, and 3.5 ng/mL for d 4, 5, 6, and 7, and the change from d 4 to 7, respectively. Repeatability of P/AI following 4 rounds of AI was low (R(e)=0.07). Repeatability estimates for concentrations of P4 from cycle to cycle indicated low repeatability between d 4 (R(e)=0.05) and 7 (R(e)=0.20). These data indicated the importance of P4 in the early luteal phase for pregnancy survival, but also demonstrated that high concentrations of P4 on these days have a deleterious effect on embryo viability. Early luteal (d 4 to 5) concentrations of P4 were a reasonable predictor of concentrations on d 7 and could be used as a diagnostic tool to identify animals at risk of subsequent embryo loss.