Reproduction in Domestic Animals

Embryo ⁄ Foetal Losses in Ruminants 261are neither environmentally nor nutritionally challengedand mated to fertile rams differences in fertilizationfailure rate are likely to be small or non-existent(Mitchell et al. 1999).Extent and Timing of Early Embryo Loss inCattleWhile fertilization rate is apparently similar in high- andmoderate-producing cows and unlikely to be affected bywhether cows are on pasture or high input total mixedration (TMR) diets, nevertheless the average calving rateto a single service is significantly lower in high- thaneither in low-producing cows or in heifers. Sreenan andDiskin (1986) calculated an embryonic and foetalmortality rate (excluding fertilization failure) of about40% for moderate-producing cows based on a fertilizationrate of 90% and an average calving rate of about55% with an estimated 70–80% of it sustained betweendays 8 and 16 after insemination. The comparativefigure for high-producing dairy cows based on afertilization rate of 90% and a calving rate of 40% is56%.There is some evidence that the pattern of earlyembryo death in the modern high-producing cow maybe different to that observed in heifers and loweryielding dairy cows. For example, in the study of Sartoriet al. (2002) conducted during the summer in Wisconsin,a significantly higher proportion of the embryos recoveredon days 6–7 were of higher quality grade fromheifers (72%) compared with lactating cows (33%).Similarly, during the winter component of the samestudy, Sartori et al. (2002) also recorded a higherproportion of higher quality grade embryos from dry(83%) compared to lactating (53%) cows. This trend ofa comparatively higher proportion of low-quality gradeor abnormal or retarded embryos from high-yieldingdairy cows is also evident in the study of Wiebold(1988), who found that 52% of the embryos from highyieldingdairy cows on days 6–7 were abnormal, basedon morphological criteria. Ryan et al. (1993) reportedthat the proportions of high-quality grade embryosrecovered under the conditions of high (58.5%) and low(51.6%) ambient temperature were similar. A feature ofthese recent studies is the consistent evidence that a high(41–67%) proportion of embryos recovered on day 7post-oestrus from moderate- or high-producing dairycows were classified as abnormal in contrast to a muchlower incidence (17–28%) in heifers and non-lactatingcows in the same studies or the 6% (average of fourstudies) in the report of Sreenan and Diskin (1986). Itwould appear that early embryo loss is greater in themodern high-producing dairy cow and that a muchhigher proportion of the embryos die before day 7following insemination. Based on the published evidence,the expected outcome of 100 inseminations ofBritish Friesian and Holstein–Friesian cows is summarizedin Fig. 1.Late Embryo and Foetal Loss in CattleOver the past 10 years, there has been significantinterest in the problem of late embryo and early foetalBritish Friesian 1980 Holstein Friesian 2006Late embryo mortality7% Early7%embryodeath 28%EarlyCalvingCalvingembryo40%55%10%death 43%Fertilisation failure10%Fig. 1. Reproductive outcomes in British–Friesian versus Holstein–Friesian cows (Source: Diskin et al. 2006)mortality, which has generally been defined as the deathof the embryo after about day 24 of gestation. With theadvent of ultrasound scanning, it has been comparativelyeasy to accurately establish the extent and timingof late embryo ⁄ foetal mortality. Silke et al. (2001)quantified the extent and pattern of embryo ⁄ foetal lossfrom days 28 to 84 of gestation in 1046 lactating dairycows and 162 dairy heifers managed on pasture-basedsystems of milk production. The overall loss ratesbetween days 28 and 84 of gestation and the pattern ofloss over this period are similar for cows (7.2%)producing on average 7247 kg of milk and heifers(6.1%). Almost half (47.5%) of the total recorded lossoccurred between days 28 and 42 of gestation. Thereare no significant association between level of milkproduction or milk energy output measured on day 120of lactation, milk fat concentration, milk proteinconcentration or milk lactose concentration andembryo ⁄ foetal loss rate. The extent and pattern ofembryo ⁄ foetal was not related to either cow or cow siregenetic merit. A more recent study by Horan et al.(2004) recorded a similar overall late embryo ⁄ foetal lossrate of 7.5% between days 30 and 67 of gestation indairy cows managed under pasture-based systems ofproduction. The extent of late embryo ⁄ foetal mortalityrecorded in these two pasture-based studies is muchlower than that reported for some US-based studies(Silke et al. 2001), though the causes for the apparentdifference are not clear. While the extent of late embryoloss is numerically much smaller than early embryoloss, it nevertheless causes serious economic losses toproducers because it is too late to rebreed cows whenthe loss occurs resulting in increased culling particularlyin seasonal calving herds.Extent and Timing of Embryo Loss in SmallRuminantsUnlike with cattle, there are a much smaller number ofreports of the serial slaughter of other ruminants atspecific time points following mating to determine thepattern of embryo loss. Most studies have measuredembryo survival at about the time of implantation andor at term. Furthermore, most studies have includedfertilization failure in their estimate of embryo loss.This is not unreasonable given that fertilization rate isusually close to 100%. In ewes and does, it is clearthat embryo survival rate is a function of ovulationÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag