Reduced Fertility in High‐yielding Dairy Cows: Are the Oocyte and Embryo in Danger? Part I 
The Importance of Negative Energy Balance and Altered Corpus Luteum Function to the Reduction of Oocyte and Embryo Quality in High‐yielding Dairy Cows*

Abstract

Fertility in high yielding dairy cows is declining, and there is increasing evidence to presume that oocyte and embryo quality are major factors in the complex pathogenesis of reproductive failure. In this report we present an overview of possible mechanisms linking negative energy balance (NEB) and deficiencies in oocyte and embryo developmental competence; specifically, in the high producing dairy cow. Changes in follicular growth patterns during a period of NEB can indirectly affect oocyte quality. The endocrine and biochemical changes, which are associated with NEB, are reflected in the microenvironment of the growing and maturing female gamete, and likely result in the ovulation of a developmentally incompetent oocyte. Even after an oocyte is successfully ovulated and fertilized, a full-term pregnancy is still not guaranteed. Inadequate corpus luteum function, associated with reduced progesterone, and probably also low insulin-like growth factor concentrations, can cause a suboptimal microenvironment in the uterus that is incapable of sustaining early embryonic life. This may partly account for the low conception rates and the high incidence of early embryonic mortality in high yielding dairy cows.

Effect of prepartum somatotropin injection in late-pregnant Holstein heifers on metabolism, milk production and postpartum resumption of ovulation

“…Thus, primary follicles exposed to adverse conditions associated with the metabolically challenging period of NEB during the early postpartum period may be less capable of producing adequate amounts of estrogens. Furthermore, such follicles would be doomed to contain an inferior oocyte, which will then be ovulated approximately 60 to 80 days post partum (Leroy et al, 2008).…”

Genetic relationship between milk urea nitrogen and reproductive performance in Holstein dairy cows

“…An excessive intake of degradable protein and a relative shortage of energy to synthesize bacterial proteins will result in the accumulation of excessive ammonia in the rumen, which is absorbed through the ruminal wall and converted into urea in the liver. This detoxification process consumes energy and thus may exacerbate negative energy balance (NEB) early post partum (Leroy et al, 2008). The effect of NEB on the reproductive performance is reviewed in Leroy et al (2008).…”

“…This detoxification process consumes energy and thus may exacerbate negative energy balance (NEB) early post partum (Leroy et al, 2008). The effect of NEB on the reproductive performance is reviewed in Leroy et al (2008). NEB is associated with a high incidence of irregular cycles that can both increase the interval to first service and reduce conception rates.…”

Physiological adaptations and ovarian cyclicity of Holstein and Montbéliarde cows under two low-input production systems

“…Selection prioritizing milk yield changed physiological regulation of body fat mobilization and accretion, aggravated negative energy balance during early lactation and impacted multiple aspects of reproductive function. These effects are mediated in part by hormones and metabolites related to nutritional status, and changes in tissue responsiveness to key hormones (Butler, 2003;Chagas et al, 2007;Leroy et al, 2008). As a result, cows selected for conventional continuous calving systems experience longer periods of body fat mobilization, chronic low body condition score (BCS) and are prone to reproduction problems, which may render them incompatible with seasonal-calving grazing systems (Dillon et al, 2003b;Horan et al, 2005b;Macdonald et al, 2008;Piccand et al, 2013).…”