Kalvningsintervall hos svenska köttkor : finns det genetisk variation som kan användas i avelsarbetet?

Abstract: The Swedish beef cattle population is growing when the dairy cattle become fewer. The fertility of the beef cows is important for the profitability in beef cattle production. Today there is no genetic evaluation for female fertility in Swedish beef cows. The purpose of this study was to see if calving interval can be used in the genetic evaluation as a measure of female fertility in beef cows. For this purpose the variation in calving interval for beef cows of different breeds and ages registred in KAP was studied. Genetic parameters were calculated for charolais and hereford. Data from Svensk Mjölk were used. In the analyses purebred cows that calved between the years 1987 and 2005 were included. The cows were divided into six different age classes, depending on age at the first calving in the interval. To seperate animals that calved for the first time from older cows it was asumed that calvings that happened when the animal was between 19 and 30 months old was the first calving of that animal. Mean values for the length of the calving intervals were calculated for Angus, Blonde, Charolais, Hereford, Highland, Limousin and Simmental to see if there were any differences between them. The mean value was as a rule longer for animals that calved for the first time than for older cows. The calving interval then becomes shorter until age class 3-4. Afterwards it became somewhat longer again. There was variation between the breeds in the length of the calving intervals such that Angus and Hereford had the shortest intervals, whereas Highland and Blonde had the longest. The variance components for calving interval in charolais and hereford were calculated with DMU package. A univariat analysis with repeated observations was done, where calving interval was seen as the same trait in each age class. A bivariate analysis was also done where age class 1 was seen as one trait and for the higher age classes it was seen as another trait. The heritabilities for calving interval in charolais and hereford were low, 0.01 for charolais and 0.02 for hereford in the univariate analysis. In the bivariate analysis the result was somewhat better for charolais in age class 1 (0.03), whereas the heritabilities for charolais in age class 2 6 and for hereford in all age classes were the same as in the univariate analysis. The estimated correlations between calving interval in age class 1 and calving interval in age class 2 6 were 0.76 for Charolais and 0.28 for Hereford. The heritabilities in this study were low compared to earlier studies where the heritabilities ranged between 0.0125 and 0.10. The correlations between calving interval in age class 1 and age class 2 6 were below unit and calving intervals recorded both for younger and older cows should be taken into consideration in a genetic evaluation. The next step in the evaluation of a good measurement of fertility for beef cattle is to estimate heritabilities and correlations for calving day. Then comparisons between calving day and calving interval need to be done to see which trait/traits are the best for Swedish purposes. More studies on correlations between fertility traits and other production traits are also needed.