Longevity is defined as the difference in days between the date of culling for culled cows (uncensored data) or the last recorded test-day for living cows (censored data), and the date of first calving.
The breeding value of a cow is calculated based on the estimated breeding values of the cow’s sire and dam’s sire, using the following expression:
BV_LONGEVITY = 100 + 0.5 * (sire BV – 100) + 0.25 * (dam’s sire BV – 100)
The breeding values of the sires (sire BV and dam’s sire BV) used in the above formula are estimated using the “survival analysis” method with a mixed Weibull model. The resulting estimates (relative risk of culling for daughters sired by the bull being evaluated) are transformed so that higher breeding values indicate greater cow longevity, and then standardized so that the mean breeding value of the animals forming the genetic reference population is 100, and the standard deviation is 10. The current genetic reference population consists of bulls born between 2009 and 2011, which have achieved a minimum reliability of 50% for their evaluation. (For more details on the methods of bull evaluation, see http://wycena.izoo.krakow.pl/index.php?item=2)