Genetic aspects of some reproductive, udder health and energy status traits in Holstein cows
Introduction
Higher incidences of reproductive problems and diseases in dairy cattle cause increased culling rates and animal welfare issues in the recent years. It has been demonstrated that genetic selection for milk yield had unfavorable repercussion for functional traits such as reproductive performance and health traits [[1], [2], [3]], because profitability of dairy farming is determined not only by milk yield but also by functional traits such as reproductive performance. Therefore, inclusion of fertility and health traits in breeding goals (other than production traits) is necessary to minimize the detrimental effect of selection for production traits on functional traits. Generally, the functional traits are lowly heritable. However, the selection for these traits can result in cumulative gains [[4], [5], [6]]. Mastitis is one of the most common diseases in dairy cattle, with the incidence rate of over 26% in the first lactation of Holstein cows in Iran [7]. Also, the results of a previous study [8] showed that infertility and mastitis were the first and second reasons for culling in Iranian Holstein cows, respectively.
There is often considerable debate on the compensation of added costs due to increased incidence rates of diseases and increased culling rate by greater returns from milk production in dairy herds. Bertrand et al. [9] reported that, although daughters of sires with high predicted difference for milk had higher health and feed costs, these costs were counterbalanced by the additional income received from extra milk production. Though, Simianer et al. [1] declared that continued selection for production traits may not necessarily be the best way to achieve maximum revenue.
Genetic parameters of the traits are needed for construction of selection indices. Bayesian methods have become more popular for the estimation of genetic parameters during the last years. One of the advantages of this approach is that it allows obtaining credible intervals for genetic parameters [10]. Moreover, having precise starting values to estimate genetic parameters is not a necessity in this approach. Some studies have shown that threshold models are more suitable than linear models for the prediction of breeding values for categorical traits since their estimates have higher reliability [11,12]. However, several studies [[13], [14], [15]] found little difference between ranking of sires based on threshold models compared with linear models. Therefore, the aim of this study was to estimate repeatability, heritability as well as genetic and environmental relationships between some reproductive traits with mastitis, different measures of somatic cell count (SCC) and fat to protein ratio (F:P) in the first lactation of Holstein cows of Iran using linear and threshold animal and sire models. Also, a repeatability model analysis of reproductive traits was conducted using the first to ninth lactation records of Holstein cows.
Section snippets
Data set
Records of 33851 first lactation of Iranian Holstein cows related to five herds with calving dates from March 2002 to September 2014 were used in this study. Information of sire, dam, parity, birth date, calving date, insemination date, daily somatic cell count and date of mastitis occurrence were available. These records were extracted from on-farm software which was used for herd management. Farm technicians were responsible to register records of reproductive traits on software. However, for
Descriptive statistics of reproduction and udder health traits
Estimated mean and standard deviations of traits are shown in Table 1. The mean and median of DFH were 46.85 and 40.00 days, respectively. Reports on DFH are rare in the literature. Vessies et al. [20] reported the mean of DFH for Dutch Holstein cows was 55 days. In contrast, investigating the effect of mastitis on ovarian function, Huszenicza et al. [21] reported much longer time between calving and first postpartum estrus (83.77 days). The mean of DFS was calculated as 70.49 days which was
Conflicts of interest
None.
Acknowledgements
The authors are grateful to Pars Agricultural and Animal Husbandry holding for providing data. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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This research is a part of Ph.D. thesis of the first author.