Dietary urea for dairy cattle IV. effect on reproductive hormones

Effects of continuous free-choice feeding of urea in blended rations on reproductive hormones were compared during three consecutive reproductive periods. Urea nitrogen replaced plant nitrogen as follows for Groups 1, 2 and 3, respectively: 0, 22 and 45% from insemination to mid pregnancy, 0, 8 and 16% thereafter until 2 weeks before first calving, and 0, 18 and 36% precalving and during lactations. Dietary urea was consistently associated with significantly increased progesterone in blood plasma at all periods. In contrast, estrogen was significantly lower in urine of Group 3 during Days 1 to 18 after first insemination as heifers, but not at second calving or during Days 1 to 18 after insemination for third pregnancy. Plasma estrogen was significantly lower in Group 3 than Group 1 only during the first pregnancy. Group 2 was intermediate. Length of first gestation was subnormal in Groups 1 and 2. It was concluded that dietary urea caused differences in reproductive hormones which were improtant only during the first pregnancy as heifers.