The use of foreign versus domestic sires to breed cows and bulls and the overlap in the use of sires between countries was documented for five dairy breeds (Ayrshire, Brown Swiss, Guernsey, Holstein and Jersey). Across-country selection has been practiced most in the Holstein and least in Ayrshire breed group. For Holstein bull sires on average 30 sons were progeny tested, high compared with the other breed groups. The extreme use of certain popular Holstein bull sires has been tempered lately. The percentage of bulls graduating from progeny test ranged between 8% and 24%. Intensity of selection of sires of sons was higher than that of sires of daughters, with the exception of the Ayrshire breed group. There was a decreasing trend over time in selection differentials for sires of sons for protein yield in favour of milk somatic cells. However, selection differentials for somatic cells were unfavorable for several breed groups.

W. F Fikse, J. H Jakobsen, C. Gustafsson

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume , , 01.40, 2006
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