ESTRUS SYNCHRONIZATION WITH MELENGESTROL ACETATE IN CYCLIC EWES - INSEMINATION WITH FRESH OR FROZEN-SEMEN DURING THE FIRST OR 2ND ESTRUS POST TREATMENT

A total of 540 cyclic ewes were randomly allocated to 1 of 6 groups ac cording to synchronization or not with melengestrol acetate (MGA), ins emination with fresh or frozen semen, and insemination during the firs t or second estrus post treatment. The MGA was effective in synchroniz ing estrus, since the percentage of ewes showing estrus during the fir st 6 days after treatment was significantly higher (P<0.05) for treate d (79.5%) than for nontreated ewes (33.5%); 74% of the treated ewes sh owed estrus during a 72-h period. Progesterone concentrations in plasm a samples obtained at random from 34 treated ewes on Day 8 post estrus revealed that 94% of them ovulated and formed a functional CL. Synchr onization was maintained during the second estrus post treatment, sinc e 71.9% of the ewes showed the second estrus during a 72-h period. Tre atment with 0.22 mg of MGA/head/d for 14 d had a detrimental effect on fertility when insemination was carried out during the first estrus p ost treatment. Delaying insemination until the second estrus post trea tment caused a less marked reduction in conception rates. Thus, MGA ca n be a useful alternative for estrus synchronization of a large number of ewes. Artificial insemination can be delayed until the second estr us post treatment, improving fertility without loosing the advantages of estrus synchronization.