Genetic variability of the length of postpartum anoestrus in Charolais cows and its relationship with age at puberty

Fertility records (n = 1802) were collected from 615 Charolais primiparous and multiparous cows managed in an experimental herd over an Ii-year period . The objectives of the study were to describe the genetic variability of t he re-establishment of postpartum reproductive activity and the relationshi p with body weight (BW) and body condition score (BCS) at calving and age a t puberty. The length of postpartum anoestrus was estimated based on weekly blood progesterone assays and on twice daily detection of oestrus behaviou r. The first oestrus behaviour was observed 69 days (+/-25 days s.d.) post- calving and the first positive progesterone measurement ( > 1 ng . mL(-1)) was observed at 66 days (+/-22 days s.d.) for the group of easy-calving mul tiparous suckling cows. Estimates of heritability and repeatability were h( 2) = 0.12 and r = 0.38 respectively, for the interval from calving to first oestrus (ICO). Corresponding values were h(2) = 0.35 and r = 0.60 for the interval from calving to the first positive progesterone test (ICP). The ge netic correlation between both criteria was high (r(g) = 0.98). The genetic relationships between postpartum intervals and BW and BCS of the female at calving were negative: the genetic aptitude to be heavier at calving and t o have high body reserves was related to shorter postpartum intervals. A fa vourable genetic correlation between age at puberty and postpartum interval s was found (r(g) between 0.45 and 0.70). The heifers which were geneticall y younger at puberty also had shorter postpartum intervals.