The maintenance of genetic diversity in a local cattle breed through optimum contribution selection

The present study aimed to evaluate the effects of optimum contribution selection (OCS) in a small native cattle breed. In practical animal breeding, the genetic improvement is often accompanied by an increase of inbreeding level due to the preferential use of closely related animals, particularly in small populations. This may lead to a reduction of genetic variability and to detrimental effects on some traits. The OCS maximizes the genetic merit of newborns while putting a restriction on the average relationship of the current generation. Despite the benefits, OCS has not been widely applied in practical breeding plans yet. This study considered the effects of OCS in the dual-purpose Rendena cattle, by applying different penalties to the average relationship of current generation (from 0 to -100,000). The OCS was applied on the candidate bull-dams and bull-sires for the years 2014 to 2019, and compared with simulations of random mating, traditional selection and mating system used by the breeders association. Considering the mating of 2014 and 2015, OCS allowed to obtain a predicted offspring with lower genetic merit than in traditional selection, but also with a lower inbreeding. When OCS was routinely introduced in the breed, in 2016, a reduction in genetic merit but also a consistent reduction in the average relatedness and inbreeding rate were observed. Subsequent years showed the actual effects of the OCS program: after the introduction of the optimization, the inbreeding rate did not increase over years. Moreover, the traditional mating system results were suboptimal in respect to OCS simulations. The study confirmed the benefit of OCS as effective tool for long-term preservation of small local breeds under selection, which is important for biodiversity and sustainable use of the genetic resources.