Joint evaluation of survival and fertility in dairy cattle with a linear model
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There is an intricate relationship between milk production, fertility and survival of cows, which is further confounded by management decisions on the farm. Holsteinization and selection for high milk production can be associated with reduction in cow fertility (Pryce et al., 1998; Silvia, 1998; Harris and Winkelman, 2000) because of its impact on physiological factors affecting reproduction such as energy balance, ovarian function, heat detection and conception in dairy cows (Buckley et al., 2000; Snijders et al., 2001; Westwood et al., 2002).Cite
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INTRODUCTION Fertility is considered a complex trait influenced by many physiological and disease-related variables. These is considerable scientific evidence to support the view that fertility performance is influenced by environment (E), genetics (G) and the interaction between G by E (GxE). Many hypothesis have been proposed to explain this including genetics, physiology, nutrition nad management, and these factors have been investigated at the animal, organ and celluar level at critical time points of the productive life of dairy cows ( Walsh et al. , 2011). It is largely known that, fertility in dairy cows strongly decreased over the last decades as milk production per cows has highly increased. Hence, the reproductive efficiency is became an high priority in all systems and it is considered higher in seasonal calving systems as the opportunity for cow to calve and become pregnant is time limited to ensure a calf per cow per year in synchrony with grass growth ( Dillon et al., 2006). Over the last 30 years, genetic selection for increased milk production, particularly within the North American Holstein-Friesian genotype, has been very successful. Between 1985 and 2003, the rate of phenotypic gain in milk production per cow per year has been 193 kg for the United States, 131 kg for the Netherlands, 35 kg for the New Zealand and 46 kg for Ireland ( Dillon et al. , 2006). In Italy, the rate of the phenotypic gain in milk production per cow per year has been 112 kg in Holstein-Friesian with an average increase of the calving interval of 1.4 d per year ( Cassandro and Penasa , 2010). Despite these countries having diverse production systems, genetic selection criteria and climatic conditions, they all report a sensible decline in reproductive performance during the same period inducing, in recent years, the emphasis within selection indices for Holstein-Friesian has shifted from predominantly production to funcional nonproduction traits associated with improved health and fertility ( Miglior et al. , 2005). Poor reproductive performace often leads to premature culling and decreased productive career of dairy cows. The association between the declines in fertility and milk production in the last decades, is evident in the Holstein population, as reported in Figure 1 (USDA-ARS AIPL , 2007).
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The fertility of dairy cows has declined worldwide and this change is surprising given the importance of good fertility to the dairy industry. The decline in fertility can be explained by management changes within the dairy industry and also negative genetic correlations between milk production and reproduction. Four primary mechanisms that depress fertility in lactating cows are anovulatory and behavioral anestrus (failure to cycle and display estrus), suboptimal and irregular estrous cyclicity (this category includes ovarian disease and subnormal luteal function after breeding), abnormal preimplantation embryo development (may be secondary to poor oocyte quality), and uterine/placental incompetence. The solution for improving fertility in high-producing dairy cows will include both short-term and long-terms components. For the immediate short-term, using high fertility sires and implementing controlled breeding programs will help. Controlled breeding programs improve reproductive efficiency in confinement-style dairy herds and can be combined with post-insemination treatments to enhance fertility. An additional immediate short-term solution involves changing the diet so that dietary ingredients invoke hormonal responses that benefit the reproduction of the cow. The short-term solutions described above do not address the fundamental need for correcting the underlying genetics for reproduction in high-producing dairy cows. Crossbreeding will improve reproductive performance perhaps because it alleviates inbreeding and also lowers production in cows with an extreme high milk production phenotype. The current crisis in dairy reproduction will be permanently solved, however, when the genetics for dairy reproduction are improved through a balanced genetic selection strategy.
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