Variation in estrus-related odors in the cow and its dependency on the ovary.

Urine samples were collected from 10 cows during the estrous cycle (Day 0=day of observed estrus) and investigated for pheromone activity using a quantitative rat bioassay. Pheromone activity in this assay was given in impulses/45 sec. Progesterone was measured in milk fat to verify the stage of cycle. The maximal response of rats was found on Day -1 (20.0 +/- 3.5 impulses/45 sec; x +/- SEM), and impulse rates were clearly higher (P <or= 0.001) than in the diestrous urine samples (5.2 +/- 0.7 impulses/45 sec) from Day -2 to Day +2. A negative correlation (r = -0.34; P <or= 0.001) was found between progesterone concentrations and impulse rates. The dependence of pheromone activity on estradiol was investigated in urine samples from six ovariectomized cows collected before and after estradiol administration. In all cows estradiol led to clear estrous symptoms, but it did not induce pheromone activity in urine. It is concluded that the appearance of maximal pheromone activity one day before estrus signals the imminence of estrus to the bull. The presence of the ovary seems to be essential for the synthesis of the pheromone, and an estrogen-dependent source outside the ovary is unlikely.