Estimation of Population-Specific Genetic Parameters Important for Long-Term Optimum Contribution Selection—Case Study on a Dairy Istrian Sheep Breed

SIMPLE SUMMARY: Selection progress with minimal loss of genetic variability is a challenging task in small populations exposed to selection, such as the Istrian sheep breed. To achieve a balance between selection gain and loss of genetic variability, genomic optimum contribution selection (OCS) is emerging as the best long-term selection approach. However, investment in genomic OCS requires a deep knowledge of some specific population parameters such as effective population size ([Formula: see text]) and connectedness between flocks. The determined [Formula: see text] suggests recent loss of genetic variability, and low connectedness between flocks makes it difficult to rank animals (breeding values) from different flocks in an unbiased way. The former “calls” for the implementation of OCS to reduce the loss of genetic variability, and the latter for improvement of connectedness in order to conduct a fair genetic evaluation of animals belonging to different flocks. ABSTRACT: The Istrian sheep breed has been subjected to selection for dairy traits for more than two decades. However, a detailed study of some important population-specific parameters such as effective population size ([Formula: see text]) and connectedness between flocks has never been carried out. The aim of the study was to examine the above parameters in dairy Istrian sheep subjected to a national selection program. The [Formula: see text] was estimated as the mean rate of increase in coancestry, and connectedness was determined using four different statistics. The [Formula: see text] was estimated at 73 animals with pedigree constraints imposed on 4 equivalent generations and 3 full generations. Analysis of [Formula: see text] (“sliding window approach”) revealed a negative [Formula: see text] indicating a progressive loss of genetic variability ([Formula: see text] = −6.6, p < 0.01; [Formula: see text] = −4.9, p > 0.05). The overall connectedness ([Formula: see text] ~ 0.0001) was below the acceptable level for unbiased ranking of the animals belonging to different flocks ([Formula: see text] = 0.05). OCS appears to be the best option for the long-term survival (self-sufficiency) of the breed, but genetic links between flocks need to be strengthened to allow unbiased ranking of the animals based on the estimated breeding values.