نمونه متن انگلیسی مقاله
During the past 50 years, the fertility of high-producing lactating dairy cows has decreased, associated with intensive selection for increased milk production. The physiological and metabolic changes associated with high milk production, including decreased (glucose, insulin, IGF-I) or increased (nonesterified fatty acids, ketone bodies) concentrations of circulating metabolites during nutrient partitioning associated with negative energy balance as well as uterine and nonuterine diseases have been linked with poor reproductive efficiency. Fertilization is typically above 80% and does not seem to be the principal factor responsible for the low fertility in dairy cows. However, early embryonic development is compromised in high-producing dairy cows, as observed by most embryonic losses occurring during the first 2 weeks after fertilization and may be linked to compromised oocyte quality due to a poor follicular microenvironment, suboptimal reproductive tract environment for the embryo, and/or inadequate maternal–embryonic communication. These and other factors related to embryo development will be discussed.
In the modern Holstein, calving rates are close to 55% to 60% in heifers and 35% to 40% in lactating dairy cows. The physiological changes associated with high milk production have been linked with poor reproductive efficiency in commercial dairy herds . Decreased (glucose, insulin, IGF-I) or increased (nonesterified fatty acids [NEFA], ketone bodies) concentrations of circulating metabolites during nutrient partitioning associated with low body condition score and disease status undoubtedly play a role in determining reproductive outcome. However, understanding the causes of infertility in dairy cattle is complex and may be attributable to impacts at numerous points along the developmental axis including compromised follicle development impacting on oocyte quality, a suboptimal reproductive tract environment incapable of supporting normal embryo development, or a combination of both .
Subfertility is a multifactorial issue resulting as a consequence of insults at various points along the developmental axis. Evidence exists implicating the oocyte, the embryo, and the reproductive tract in subfertility. Improved understanding of the developmental biology involved in conceptus development and uterine receptivity in cattle may contribute for the development of strategies to minimize embryonic losses and improve reproductive efficiency in cattle.