Reproduction in Domestic Animals

Embryo ⁄ Foetal Losses in Ruminants 265Energy balance at around the time of insemination andsubsequent conception rateDry matter intake is lower for cows grazing pastures thanfor cows fed maize-based TMRs. High-genetic merithigh-producing cows experience greater NEB in earlylactation under grazing conditions relative to lowergenetic merit cows, notwithstanding the somewhat higherDMI capacity of the former cow type (Horan et al.2004). Meta-analysis of the combined Kennedy et al.(2003) and Horan et al. (2004) data showed that therewas no service · study · supplementation rate (p >0.10), service · study rate (p > 0.10), study · supplementationrate (p > 0.10) interaction effects on conceptionrate (see Diskin et al. 2006). Yet, there was aservice · supplementation rate effect on conception rate(p < 0.05). The rate of supplementation had no effect onfirst service conception rate (56% vs 53%) but cows onthe low level of supplementation had a lower (p < 0.05)second service conception rate (39% vs 58%) comparedwith cows on the high level of supplementation. Interestingly,the withdrawal of the supplementation commencedat about the onset of the breeding period in bothstudies. From these studies, there is no clear evidence thatconcentrate supplementation of dairy cows at pastureimproves first service conception rate, but it may bebeneficial in maintaining second service conception rates.This clearly highlights the difficulty that improving theEB of the modern dairy cows presents at this stage oflactation where grazed grass is the predominant componentof the diet. Based on the studies of Sangsritavonget al. (2002), the increased milk production as a result ofthe concentrate supplementation may well be associatedwith increased hepatic blood flow resulting in increasedmetabolism of progesterone and consequently loweringof peripheral concentrations of progesterone andpre-disposing to greater risk of embryo death.Sudden reductions in feed intake and conception rateStudies from this laboratory (Dunne et al. 1999) showthat sudden reductions in DMI at around the time ofinsemination adversely affect embryo survival in heifers.When energy intake was reduced from a high level oftwice their maintenance requirement to 0.8 times maintenancefor 2 weeks immediately after AI, embryosurvival rate in heifers was consistently less than 40%.When heifers were provided with a constant level of feedintake or when they were changed from a low to ahigher level feed intake, embryo survival was consistentlyhigh at 65–71%. In that study, where heifers wereused, there was no indication of any association betweenenergy intake and systemic progesterone concentration.Unlike the situation in sheep and pigs, there was nochange in systemic progesterone following either anincrease or reduction in energy intake. Changes inprogesterone metabolism may have been balanced bychanges in progesterone production.Protein nutrition and conception rateDairy cows at pasture frequently ingest high quantitiesof protein, often with a high proportion of theingested protein being rapidly degradable in therumen. The effects of high intakes of crude proteinon conception rate are equivocal (Diskin et al. 2006).They concluded, based on the published evidence, thatelevated concentrations of urea per se are not detrimentalto embryo survival. Yet, it needs to be clarifiedwhether the observed adverse effects of urea onembryo survival are dependent on the energy statusof the animal.Future PossibilitiesIt is well established that embryo survival is criticallyimportant in all ruminant species but particularly inseasonally bred herds. The extent of late embryomortality is numerically much smaller than earlyembryo mortality loss but nevertheless causes seriouseconomic losses to producers, because it is often too lateto rebreed females when they repeat particularly inseasonally bred herds. The major challenge will be toimprove embryonic survival rate and therefore herdreproductive efficiency particularly given the antagonisticrelationship between production and embryo survivalrate. It is clear that genetic variability exists within theHolstein breed for important fertility (Berry et al. 2003)and DMI and EB (Berry et al. 2007) traits. Because ofthe difficulty in directly measuring DMI and ⁄ or EB inlarge numbers of dairy cows, it is not currently directlyincluded in international breeding objectives. Applyinga positive weighting to traits correlated with DMIand ⁄ or EB or the identification and application ofsuitable molecular markers would be expected toimprove DMI and EB and subsequent reproductiveperformance. Alternatively, strains of cows derived frommore balanced breeding objectives, such as the NewZealand Friesian (Harris and Kolver 2001) or alternativedairy breeds such as the Jersey or Norwegian Red,could be utilized in such production systems. Advancesin genomic technology have the potential not only toprovide scientists with an improved understanding ofthe underlying biological processes involved in fertilizationand the establishment of pregnancy but also toexplore gene profiles and potentially identify genesresponsible for improved embryo survival. Markerassistedselection will increase the rate of geneticprogress for reproductive traits such as embryo survivalby increasing the accuracy of selection and permittingselection in both genders and at a young age (Rohrer2004). Reproduction-enhancing technologies such as AI,embryo transfer and perhaps cloning will have animportant role in fully exploiting their potential (Georgeand Massey 1991).ReferencesAdams GP, Sumar J, Ginther OJ, 1991: Form and function ofthe corpus luteum in llamas. Anim Reprod Sci 24, 127–138.Arthur GH, Noakes DE, Pearson H, 1982: Reproduction inthe camel. In: Arthur GH, Noakes DE, Pearson H (ed.),Veterinary Reproduction and Obstetrics. Bailli-ere Tindall,London, 482–487.Ashworth JC, 1994: Nutritional factors related to embryonicmortality in domestic species. In: Zavy MT, Geisert RDÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag