FUNCTIONAL INTERRELATIONSHIPS BETWEEN FOLLICLES GREATER-THAN 4 MM ANDTHE FOLLICLE-STIMULATING-HORMONE SURGE IN HEIFERS

The interrelationships between the FSH surge that initiates a follicul ar wave and the follicles in the wave were examined in heifers. In exp eriment 1, greater than or equal to 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 a dministered 24 h after ablation followed by the other half 12 h later. Blood samples were taken after the initial FSH injection for FSH assa y, and ovaries were examined daily with ultrasound to monitor follicle growth. There were progressively higher FSH concentrations at the mea n peak (8 h after initial injection in all groups) as the dosage incre ased (interaction of dose and time; p < 0.001). Compared to values in controls, the highest dosage (150 units) approximately doubled the num ber 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, eit her 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 FS H 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 exa ggerating the height of the FSH surge, 2) all greater than or equal to 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 c apable of becoming the dominant follicle.