Reproduction in Domestic Animals

Embryo ⁄ Foetal Losses in Ruminants 263et al. (1992) concluded, based on results from embryotransfer and breed-crossing studies, that genetic differencesamong embryos were not of any consequence forvariation in embryo survival rate, aside from factorssuch as chromosomal abnormalities, specific lethal genesand genetic interaction. Breed differences in embryosurvival as a trait of the ewe have been examined inmany studies based on employing embryo transferprocedures and on estimation of embryo survival usinginformation on ovulation rate and the consequent littersize. The general conclusion from the embryo transferstudies is that breed of recipient ewe did not play animportant role in determining embryo survival despitethe fact that these studies often involved breeds withvery different levels of natural prolificacy (Hanrahan1994), though there is some evidence (Hanrahan andPiper 1992) that embryo survival may be better when theFinn breed was used as an embryo recipient. There isquite a substantial literature on estimation of embryosurvival based on the information on ovulation andassociated litter size and provides a good indication ofbreed variation in embryo survival. Hanrahan (1994),following an extensive review of the literature, suggestedthat the likely breed differences for the probability ofembryo survival are 0.15, 0.13 and 0.11 for twin, tripletand quadruplet ovulations, respectively. If the essentiallylinear relationship between ovulation rate andlitter size is the same in all breeds, then a global figure of0.12 has been suggested by Hanrahan (1994) as thelikely range of breed means for embryo survivaladjusted for ovulation rate. Variation among individualswithin a population, measured as repeatability ofdifferences in embryo survival, is also quite low (»0.05)(Hanrahan 1982; Ricordeau et al. 1986), and consequentlythe heritability of these differences is alsoextremely low (Ricordeau et al. 1986).Maternal ageWhile the overall embryo survival rates would appearto be largely similar in heifers, beef cows and low–moderate-producing dairy cows, embryo survival rate islower in high-producing dairy cows. Yet, it is improbablethat this is an age- or parity-related phenomenonand is more probable due to the direct and indirectconsequences of milk production (see later). Recentdata from the US (Kuhn et al. 2006) would suggestboth that heifer conception rate is at a maximum at 15–16 month of age. Breeding heifers at 26 months of ageor older resulted in a 13% lower conception ratepresumably due to a lower embryo survival rate. Thereis evidence, at least in some breeds of sheep (Galway:Quirke and Hanrahan 1977; Romney: McMillan andMcDonald 1985), that embryo survival is lower in ewelambs than in adult ewes. These later studies and the asubsequent study of Quirke and Hanrahan (1983) allemploying embryo transfer indicate that the impairedembryo survival associated with ewe lambs is attributableto the inherent quality of embryo rather than anydeficiency of uterine environment. Ricordeau et al.(1982, 1986) found no significant differences in embryosurvival between ewe lambs and adults of the Romanovbreed. This may be dependent on the degree of sexualmaturity attained at breeding as there is evidence thatconception rate increases between first and later oestrusperiods in ewe lambs. Among age groups other thanewe lambs, there is little evidence of differences inembryo survival rate (Hanrahan 1990; Kleeman et al.1990).Progesterone during the cycle immediately prior toinsemination and embryo survival rateThere is now good evidence linking circulating concentrationof progesterone during the cycle immediatelyprior to insemination as well as during the early lutealphase of the cycle following insemination with lowembryo survival ⁄ conception rate (see review by Diskinet al. 2006). Recent data from this laboratory (Diskinet al. 2006) clearly show that there is a positive linearassociation between the concentrations of progesteroneon the day of PGF-2a-induced luteolysis and subsequentembryo survival rate. Potential mechanisms by whichlow concentrations of progesterone during the precedingoestrous cycle might reduce fertilization and or embryosurvival rates include the production of oocytes that areat a more advanced stage of maturation at time ofovulation from persistent dominant follicles; increasedfrequency of pulses of LH which in turn inducesincreased secretion of oestradiol-17b or an alterationin endometrial morphology (see review by Diskin et al.2006). The more probable effect of low concentrationsof progesterone in the cycle preceding oestrus onsubsequent embryo survival rate is to result inpre-mature oocyte maturation, which subsequentlycompromises its ability to continue normal embryodevelopment after its fertilization.Post-insemination progesterone and embryo survival rateRecent studies (Stronge et al. 2005; Diskin et al. 2006)that examined the relationship between early and midlutealphase concentrations of progesterone and subsequentembryo survival ⁄ conception rate have employedlogistic regression techniques to model the relationshipbetween the binomially distributed dependent variable(conception ⁄ embryo survival rate) and the continuouslydistributed independent variable (progesterone). In thestudy of Stronge et al. (2005) (Fig. 2), there was apositive linear and quadratic relationship between milkconcentrations of progesterone on days 5, 6, 7 postinseminationand between the rate of change in concentrationsof progesterone between days 4 and 7 andembryo survival rate. Further analysis of this data setreveals that 75%, 72% and 56% of dairy cows haveconcentrations of progesterone that are optimal forconception on days 5, 6 and 7 post-insemination,respectively. In beef heifers, Diskin et al. (2006) haveshown that a similar linear and quadratic associationbetween peripheral concentrations of progesterone andembryo survival also exists. These recent data for cattleare inline with results from earlier sheep studies of Parret al. (1987) of an inverse relationship between circulatingconcentrations of progesterone and embryo survivalin these species. Furthermore, there is evidence for sheep(Parr et al. 1987) and dairy cows (Starbuck et al. 2001)Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag