Breeding for polledness in Holstein cattle

Currently almost all dairy cattle are dehorned as calf to avoid injuries later in life. A welfare friendly alternative to dehorning is to breed polled cattle. This paper explores the potential to breed for polledness in the Holstein breed, and investigates the genetic merit and relatedness of both polled and horned bulls. In 2009 there were 33 polled bulls available for artificial insemination (AI), two of them being homozygous for polledness. In 2014 more than 150 bulls were available, 31 of them being homozygous. Breeding values for the total merit index (NVI-Dutch Flemish Index) have increased considerably for polled bulls. In 2009 the difference in average Estimated Breeding Value (EBV) for NVI between polled bulls (available for AI) and the top 100 horned AI bulls was 180 points, equivalent to about 18 years of selection at that time. In 2014 the difference was reduced to 149 points, equivalent to about 5 years of (genomic) selection. Genomic selection has made an important contribution to this reduced difference between polled and horned bulls. Polled bulls in 2009 were more inbred (F=0.045) than horned bulls (F=0.037) but less related to cows born at Dutch farms in that year (r=0.070 vs. 0.089). Using optimal contributions and a combination of polled and horned bulls, a next generation of animals can be bred that combines a high genetic merit with a relatively low relatedness and higher frequency of polledness. However, homozygous polled bulls born in 2012–2014 had a relatively high average inbreeding level (F=0.079) and almost all originated from the same two polled founder bulls. This may form a potential risk for lethal alleles showing up with inbreeding. Overall, breeding high genetic merit polled Holstein cows has become a realistic perspective, but care must be taken to avoid high relatedness and inbreeding levels.