Embryos can be recovered from donor females following a normal estrous cycle that does not involve any hormonal stimulation. However, this limits the potential number of embryos available, especially in cattle. Consequently, in most cases, donors are superovulated, defined as treatment of a female with gonadotropins so that more ova than normal are ovulated. The most common gonadotropin employed is follicle-stimulating hormone (FSH), which is derived from either porcine or ovine pituitary glands. Regulation of the estrous cycle of the donor with prostaglandin F2a and progesterone usually is done concurrently with superovulation. Superovulated cattle usually are inseminated artificially and produce approximately 10 ova, of which 6 are viable embryos. The range of responses, however, is very large in all species; cattle on rare occasions produce as many as 100 ova, yielding 70 or more viable embryos. In contrast, the mare has proven quite difficult to superovulate, although recently a commercially available equine pituitary extract has proven moderately effective in increasing ovulation rates.

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