GnRh Or Estrogenprogestinprostaglandin F2a Combinations

Creating persistent follicles is a particular problem when using progestins. Persistence occurs because the exogenous dose of progestin is inadequate to inhibit normal LH pulse secretion as effectively as progesterone secreted by the CL or because greater rates of metabolism reduce biologically available progestin in lactating dairy cattle with high feed intakes. Gonadotropins or GnRH address the problem of follicle persistence by pharmacologically releasing sufficient LH to induce ovulation of LH-depend-ent follicles that are present at the onset of progestin treatment. Asa consequence, a new follicular wave emerges and a mature follicle ovulates when progestin is withdrawn and the CL is lysed by PGF2a.[2]

A common regimen in cattle includes an injection of GnRH at the onset of progestin treatment and the injection of PGF2 a either one day before or on the day of progestin withdrawal.1-3-1 The initial GnRH injection may produce the first postpartum ovulation, resulting in a shortened luteal phase or short cycle. The progestin primes the uterus of the anestrous cow, so when ovulation occurs, the life span of the first CL is nearly normal.[2-

For cows undergoing normal estrous cycles, the progestin is likely unnecessary and can be deleted.[4] Subsequent to luteolysis induced by PGF2a, ovulation occurs spontaneously or can be induced by an injection of estrogen or a second injection of GnRH to accommodate insemination by appointment (timed AI [TAI]).

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