Informe de Auditoría

1.5.3.1 Milk progesterone

As discussed earlier, the traits used to construct the FI are open to management bias which explains their relatively low heritability (h2 = 0.018-0.037; Wall et al., 2003). Therefore, in the long term it will not be a sustainable solution to improve fertility by traditional phenotypic selection approaches as they do not reflect the actual physiological characteristics of the cows. So in order for the selection index to be more effective in improving fertility and identifying the most fertile animals, management and environmental effects must be reduced (Royal et al., 2000a). One approach is through measuring physiological parameters, such as the time of the

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commencement of luteal activity postpartum (CLA), which are less affected by management bias (Veerkamp et al., 1997, Royal et al., 2000a). Milk progesterone level is measured thrice weekly from shortly after calving and continued until a minimum of 100 days of lactation. These measurements can be used then to determine CLA from the first elevation of milk progesterone level of ≥3ng/ml (Darwash et al., 1997; Royal et al., 2000a). This information will help to identify on average when the daughters of each bull have returned to cyclicity after calving. A significant and unfavourable genetic correlation exists between milk yield traits and endocrine fertility traits. Thus, CLA tends to be longer in cows with higher genetic merit for milk yield. The magnitude of this relationship is estimated to be an increase of about 1.4 days in CLA for every 1kg daily increase in milk yield over the first 100 days of lactation (Veerkamp et al., 1997). Furthermore, CLA was found to be positively correlated with calving interval (CI) indicating that cows with a genetically longer CI are more likely to have longer CLA. CLA increases by 1.1% (approximately 0.3d) for every 1 day increase in CI. A negative genetic correlation was established between CLA and body condition score, implying that cows that tend to be thinner are more likely to have longer intervals to CLA postpartum. Every unit increase in BCS (on a scale of 1 to 9) is associated with a decline in CLA by 22.4% (approximately 6 days; Royal et al., 2002b; Royal et al., 2002a). The heritability of CLA was found to be within the range of 0.16 and 0.23 (Royal et al., 2002b). Therefore, it has been concluded that endocrine measurements for fertility such as CLA might provide useful tools to improve the reliability of fertility breeding values when used with the traditional fertility traits of the selection index (Royal et al., 2002b). In the future, it is likely that milk progesterone recorded in automated parlours using biosensors will be incorporated into the selection index (Wall et al., 2003).

1.5.3.2 Juvenile predictor

Moreover, as bulls cannot have fertility proofs until they have daughters in milk, including them in the fertility index takes at least 4 years. So it would beneficial if the bulls with fertile daughters could be identified early in life before they are progeny tested. This approach is feasible as genes controlling fertility are expressed

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early in life. As mentioned before there are many hormones regulating ovarian function such as gonadotrophin releasing hormone (GnRH) which is secreted from the hypothalamus and then travels to the pituitary gland where it will trigger the secretion of gonadotrophin (LH or FSH). These hormones stimulate the development of eggs and sperm in the ovaries and testes. So an experiment has been conducted by Royal and her colleagues to measure circulating LH level in response to administration of GnRH. This response is a measure of the ability of the pituitary to react to GnRH, which was found to be heritable in heifers at 5 months of age and to be genetically correlated with fertility in adulthood (Royal, 1999). This approach provides the means for pre-pubertal prediction of fertility and since the same genes control fertility in both males and females, so the pre-pubertal response in bull calves would also be expected to be correlated to their daughters’ fertility. Recently, measures of somatotrophic hormones at six month of age have also been suggested to provide an indication to cow’s performance over the first three lactations; due to the involvement of somatotrophic axis in the regulation of both fertility and milk production. It has been found that higher growth hormone pulse amplitude and lower insulin like growth factor-I (IGF-I) were associated with delayed ovulation in the first lactation, thus offering a new juvenile predictor for fertility in dairy cattle (Wathes et al., 2008)

1.5.3.3 Molecular markers

Molecular markers for fertility traits are considered to be another method for improving fertility index as they are highly heritable (h²=1) and predictable (Hastings et al., 2006). These molecular markers are parts of the animal’s DNA which are close to (linked to) the genes controlling fertility and in some cases they may be in the genes themselves. As reviewed above, there are many single nucleotide polymorphisms causing important effects on prolificacy and ovulation rate. Polymorphisms have also been found in several human genes to be associated with reproductive pathologies, such as mutations in the luteinising hormone receptor (Huhtaniemi, 2002). To identify mutations affecting fertility, the fertility trait PTAs of the sires in the fertility index can be tested for association with genotypes of candidate genes in their daughters. This method will provide an opportunity to

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improve fertility by identifying sires carrying genes of particular importance for fertility before the sires are bred. This approach is the basis for a large part of the work carried out in this thesis (see Chapters 4 and 5). The impact of molecular markers on breeding program will be dicussed in in more details in the secetion 1.6.