The objective of the paper was to evaluate the impact of the use of reference sires (RS) in insemination on the estimation of breeding value and its reliability by means of stochastic simulation. Simulation was divided into three variants according to the connectedness calves through RS (variant 1 ? unrelated calves, variant 2 calves related between HYS but unrelated within HYS, variant 3 ? calves related within and between HYS). The degree of connectedness was determined by computation of genetic driftvariance (GDV). The use of a higher number of RS in insemination with only one offspring in each of the connected herds appeared as the most suitable approach. A marked influence of herd connectedness on the estimation of breeding value and its reliability, and on the result of breeding, is to be expected after RS have been used in several successive generations.