Responses to sire selection for dystocia

Abstract

The hypothesis that low-risk bulls for dystocia sire small calves, which develop into small heifers and which calve with more difficulty was tested in a two-way sire-selection trial with Holstein (HF), Dutch Red and White (DRW) and Dutch Friesian (DF) cattle in pure breeding. Low-risk (L) and high-risk (H) bulls were selected from sire summaries to sire F 1 calves. Birth traits (ease of birth, gestation length, weight and dimensions) of F 1 calves, and body development of F 1 females, were monitored. Within sire groups, F 1 females were in turn bred to low-risk (L ∗) and high-risk (H ∗) sires. Birth traits of F 2 calves were recorded. Quantitative and categorical traits were analysed by ordinary leastsquares and generalized linear model procedures, respectively. Differences between sire groups for ease of birth and gestation length of F 1 calves were significant ( P < 0.10) in the DRW breed only; for calf weight and dimensions, there was no such breed-by-sire group interaction. F 1 females sired by L and H bulls showed only minor differences in body development during rearing and after first calving. Effects of both maternal grandsire group (L against H) and sire group (L ∗ against H ∗) were significant for birth traits of F 2 calves, and additive. Maternal grandsire group effect on ease of birth was not significant in the DF breed, however. In the DRW breed, the equality of signs of sire group contrast in F 1 and maternal grandsire group contrast in F 2 calves supports the conclusion from studies on field data that direct and maternal grandsire genetic merits for ease of birth are correlated positively. This conclusion was neither supported nor contradicted by the results from the Black and White breeds. The results indicate that the hypothesis stated above has to be rejected, and it is conjectured that the maternal grandsire effect on ease of birth is dominated by direct inheritance of calf size.