The hormone control of ovulation in the calf

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Abstract
1. Unlike many other mammals, the calf is capable of responding to pituitary injections almost from birth. Age has little effect on the ability of the follicles to develop during a series of a.p.h. injections, the follicular response generally being as good 3 weeks after birth as in calves 6–8 months old.2. Ovulation can occur spontaneously in the a.p.h. treated calf before the end of the first month of post-natal life.3. Ova obtained from such ovulations are capable of fertilization, although the percentage becoming fertilized is very low. This is thought to be due to the infantile state of the Fallopian tube and the difficulty in inseminating into the uterus, rather than faulty maturation of ova.4. Although many follicles (10–60) can develop during a series of a.p.h. injections, superovulation was not consistently obtained either by intravenous injections ofa.p.h., l.h.orp.u., or combinations of any of these three.5. Superovulation can generally be obtained, with the formation of many corpora lutea, if the a.p.h. treatment is repeated a second time after a period of luteal activity.6. The inhibitory effect of a corpus luteum on ovulation in cattle can be overridden in tho a.p.h. treated animal if suitable quantities ofa.p.h., l.h.orp.u.are intravenously injected when the newly formed follicles are approaching maturity.7. Ovulation in the calf at all ages up to 8 or 9 months of age, either in the presence or absence of luteal tissue, is unaccompanied by oestrus. Oestrus, when occurring during a series of a.p.h. injections, commenced after 90–100 hr. of treatment. In such cases slaughter has always revealed the presence of luteal tissue, indicating that ovulation during a ‘silent heat’ had occurred before the commencement of pituitary treatment.8. Great individual variation exists in the ability of the ovary to respond to pituitary injections. This insensitivity to gonadotropins is not confined to young calves and has been observed in calves 4–6 months of age.