Sex-sorted bovine sperm must be reconcentrated by centrifugation[11] before they can be properly packaged in 0.25-ml French straws at doses of 1 to 6 x 106 sperm/ straw. This is contrasted to conventional AI procedures that use at least 20 x 106 sperm/straw making the insemination dose for sex-sorted bovine sperm about 1/20 to 1/3 that of a normal AI dose. This is necessary because it takes some time to sort each dose, even at the high sort rates now being achieved. Fortunately, sexed bovine sperm can be cryopreserved, thereby allowing efficient use in AI. Current sexing technology has been applied mainly in heifers due to their inherently higher fertility. Several regimens have been used successfully to synchronize estrus to optimize insemination with low doses of sexed sperm.[3] Pregnancy rates from the use of sexed, cryopreserved sperm have been about 60 80% of controls inseminated with 7 20 times more sperm or higher insemination dose.[3,4] With sex-sorted sperm, some differences in pregnancy rates have been noted among bulls, suggesting that only sperm from highly fertile bulls should be sorted for predetermining sex of their offspring. Until recently, field trials with sexed sperm were conducted by sorting the gametes in fluid pressurized to 50 psi. This pressure level, however, was found to be detrimental to sperm.[4] With the latest procedures, where the pressure has been lowered to 40 psi, pregnancy rates in heifers inseminated with sexed sperm have been 60 80% of controls, depending on the particular bull, inseminator skills, and management level of the herd.[4]

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