Some alternatives to calving date and interval as measures of fertility in beef cattle

Records of 594 calving dates and 493 calving intervals collected from a herd of Retinta beef cows in southwest Spain were studied. Their efficacy as reproductive measures were compared when a long breeding season was utilized. Variables used were date of present calving (DOC), date of subsequent calving (DOSC), time from start of breeding season to calving (TBSC), interval to next calving (CI), two adjustments of calving interval by subtracting the time that bulls were not available to the cow from actual CI (ACI1 and ACI2), and three scores. Period score (SCOR) measured the 20-d increment of the 220-d calving season in which the cow calved. Pair score (PAIR) was computed by subtracting the deviation of calving interval from 365 d from the average SCOR of the two calvings involved. Adjusted pair score (APAIR) was calculated in the same way as PAIR, but using ACI1 instead of CI. Heritability estimates for DOC, SCOR, and DOSC and CI were .16, .14, .13 and 0, respectively. Heritabilities of all other measures ranged from .01 to .06. Repeatability estimates for DOC, SCOR, DOSC and CI were .29, .28, .34 and .14, respectively. Repeatability values were .28, .18, .18, .23 and .32 for TBSC, ACI1, ACI2, PAIR and APAIR, respectively. Genetic and phenotypic correlations among CI and its alternative measures were very high and favorable.(ABSTRACT TRUNCATED AT 250 WORDS)