Follicular development in heifers infused with follicle-stimulating hormone.

Follicle-stimulating hormone from different species has been used to increase ovulation and calving rates in cattle, injections proving to be most successful when given during the late follicular phase of the oestrous cycle or in combination with progestagens (Willett & Buckner, 1953; Bellows, Anderson & Short 1969). Some investigations have resulted in substantial increases in ovulation and calving rates, but there is usually a high degree of variation in ovarian response. Patterns of follicular growth and atresia have been studied in cattle (Rajakoski, 1960) and hypotheses have been suggested to explain how exogenous fsh stimulates follicle growth in relation to normal stages of growth and atresia (Schilling & Holm, 1963). There are no investigations on histometric evaluation of follicular development after fsh treatment. The purpose of this study was to determine follicular response after constant infusion of fsh from different species sources and to evaluate variation in follicular response among heifers when a constant level of exogenous fsh is maintained in the systemic circulation during late pro-oestrous. Twenty-one yearling Holstein heifers, ranging in weight from 236 to 454 kg, were used. They were checked twice daily for oestrus with vasectomized bulls. All heifers were observed in standing oestrus at least three consecutive oestrous periods before random assignment to the treatment groups. The heifers were infused with a bovine (LER-1306-2), ovine (nih-fsh-s8) or porcine (Armour FSH-P, Lot No. 14302) fsh preparation. The preparations, in a saline medium, were infused into the jugular vein at the rate of 0-382 ml/min for 72 hr. The heifers were restrained in a chute and the hormones were administered by a constant infusion pump. Total dosages of the bovine and ovine fsh were equivalent to 9-75 mg NiH-FSH-sl. Total dosages of the porcine fsh were 5-0, 7-5 or 10-0 mg equiv., but biological assay of fsh activity by the hcg augmentation method (Steelman & Pohley, 1953) revealed that these levels were equivalent to 1-82, 2-74 and 3-65 mg nih-fsh-s1. Infusion of fsh in these heifers was initiated 3 days before the expected day of oestrus. In three heifers, ovarian activity was suppressed by feeding 180 mg 6methyl-17-actoxyprogesterone (MAP)/day for 12 days. Corpus luteum re¬ gression was induced by an intramuscular injection of 5 mg oestradiol valerate