Association between Plasma and Milk Urea on the Insemination Day and Pregnancy Rate in Early Lactation Dairy Cows

Lactating dairy cows (n=177) fed with grass and corn silage ad libitum kept in pasture, were randomly assigned to evaluate how urea nitrogen in plasma and milk can be related to their pregnancy rate. blood and milk samples were collected on the artificial insemination (ai) day to evaluate plasma urea nitrogen (pun) and milk urea nitrogen (mun) as well as progesterone levels, excluding cows with progesterone higher than 0.5 ng/ml. cows were considered pregnant if six weeks after artificial insemination did not return to estrus. concentrations of pun or mun greater than the average (16 mg/dl) were associated with decreased pregnancy rates (13% and 14%, respectively) (p<0.05) compared with the cows with urea levels less than this value on the insemination day. as pun and mun increased to greater than 16 mg/dl, the likelihood ratio for pregnancy decreased. there was a high correlation between pun and mun concentrations (r2= 0.97, p?0.001). the results of this study indicate that an increase in pun or mun can exert direct or indirect effects in reproduction, impairing the conception of grazing dairy cows