that most issues with fertility of lactating cows is relatedto poor oocyte quality and/or inadequate uterineenvironment to support early embryo growth before day7. Although recent findings argue for significantimportance of some blood parameters (i.e. glucose) forembryo development after day 7 (Green et al., 2012).Despite of that, later findings indicate that oocytequality can be improved in lactating cows throughgreater pre-ovulatory circulating P4 concentrations(Wiltbank et al., 2012), but little is actually knownabout the capacity of the uterus of lactating cows tocope with adequate fertilization and early embryosurvival until day 7 after ovulation.Although scarce, later publications were able toshed light on the events within the uterine tract from day2 to day 7 after ovulation by utilizing laparoscopictransfer of IVF embryos into the oviduct (Rizos et al.,2010; Maillo et al., 2012). Both reports describeddrastic reductions in proportion of embryos thatremained viable from day 2 to day 7 of the estrous cyclewhen these embryos were transferred into lactatingcows as compared to nulliparous heifers (Rizos et al.,2010) or postpartum cows that were dried-off at calving(Maillo et al., 2012). The authors hypothesize that acombination of factors and complex associations mayresult in less than ideal endometrial environment in thelactating cow. Such factors are high milk productioninducing low circulating P4 due to greater steroidmetabolism, negative energy balance that lactating cowsundergo at beginning of lactation, with remarkableincrease in circulating non-esterified fatty acids (NEFA)as well as lower blood calcium, glucose and IGF-1.Lower circulating P4 before and/or afterovulation has been extensively studied and associatedwith embryonic growth and conception results(Wiltbank et al., 2012); however, other bloodparameters such as calcium and glucose concentrationshave been largely overlooked and only recently studiedmore in depth to unravel their importance on dairy cowfertility. For example, low circulating calciumconcentrations (<8.59 mg/dL) in the postpartum seem tobe a key element to explain poorer immunity, uterineenvironment, and overall health in the lactating cow asdescribed by Martinez et al. (2012). Additionally, lowercirculating glucose and the resulting insulin-resistantstate of postpartum cows to cope with high levels ofmilk production has only recently been associated withreduced embryonic growth (Green et al., 2012; Lucy etal., 2014). Thus, besides the classical findings on theimportance of P4 levels post-ovulation on embryodevelopment and maternal recognition, circulatingcalcium and glucose levels also appear to have centralroles in providing an ideal uterine environment for thegrowing embryo and pregnancy. Further comprehensivestudies looking into endometrial gene expression andhistiotrophic milieu are urgently needed to bridge some
of these missing concepts. Thus, later studies brought
some important insights to help elucidate that the
lactation status of the cow seem to be unarguably
detrimental not onl
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