Functional interrelationships between follicles greater than 4 mm and the follicle-stimulating hormone surge in heifers.

The interrelationships between the FSH surge that initiates a follicular wave and the follicles in the wave were examined in heifers. In experiment 1, > or = 5-mm follicles were ablated 5 days after ovulation and heifers (n = 6/group) received a total dosage of 0, 37.5, 75, or 150 units of porcine FSH. Half of the FSH dosage was administered 24 h after ablation followed by the other half 12 h later. Blood samples were taken after the initial FSH injection for FSH assay, and ovaries were examined daily with ultrasound to monitor follicle growth. There were progressively higher FSH concentrations at the mean peak (8 h after initial injection in all groups) as the dosage increased (interaction of dose and time; p < 0.001). Compared to values in controls, the highest dosage (150 units) approximately doubled the number of 5- and 6-mm follicles; this then progressed into a 4- to 7-fold increase in the number of 7- and 8-mm follicles. In experiment 2, either all (controls; n = 6), two (n = 11), one (n = 6), or zero (n = 6) follicles of the first wave of an estrous cycle were retained and the remaining were ablated upon reaching 5 mm. Scanning and blood sampling were performed every 8 h for 72 h after the initial ablation. Mean FSH concentrations during 0 to 72 h decreased (p < 0.004) as the number of retained follicles increased. In heifers in the one-follicle group, the randomly chosen 5-mm follicle developed the characteristics of a dominant follicle. The following conclusions were made: 1) the number of follicles that advanced into a follicular wave was increased by exaggerating the height of the FSH surge, 2) all > or = 5-mm follicles of a wave contributed to the declining portion of the FSH surge, and 3) any 5-mm follicles at the emergence of a wave were capable of becoming the dominant follicle.