Heritability of hypocalcemia at first parturition in Norwegian cattle: genetic correlations with yield and weight.

Minimum plasma calcium level around parturition and its relation to BW at parturition, milk yield, and weight change in mo 1 of lactation were studied in 334 first lactation Norwegian cows. Minimum plasma calcium level was reached 18 to 30 h postpartum. Single observations for all animals were fitted with a multitrait animal model including all genetic relationships. Heritabilities of the traits were calcium postpartum, .11; milk yield, .35; BW, .65; and weight change, .17 with standard errors about .10. Genetic correlations between calcium postpartum and the other traits were milk yield, -.49; BW, -.66; and weight change, .42 with standard errors about .25. Environmental factors with detectable effect on hypocalcemia were age at parturition and calving season. Regression of calcium postpartum on age was -2.66 x 10(-4) mmol/L per d. Calvings shortly after the pasture season were related to higher calcium postpartum than calvings after 3 to 4 mo with indoor feeding.