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Effects of oral administration of Melengestrol acetate (MGA) on the reproductive performance were investigated using 38 anestrous cows (Group A) and 7 sub-estrous heifers (Group B) of Holstein. MGA, 1.1 mg per animal, was fed daily for 14 days. The MGA treatment was started randomly for Group A and between day 4 and 8 of the estrous cycle for Group B. Changes of plasma progeste-rone were investigated by the radioimmunoassay method in two heifers with normal estrous cycle. Precision of the method was 427.0±36.50 pg (8.5% in coefficients of variation) of progesterone and the recovery rate was between 75 to 100% (mean 90.0%). The used antibody against progesterone-3-oxime-BSA did not cross-react with MGA. Following results were obtained.<BR>1) Seven of the 45 treated animals exhibited estrus during the treatment, and 28 became estrus following the treatment (62.2%). Of the latter group, 18 animals exhibited estrus 2 to 5 days after the treatment. Induction of estrus achieved in 23 of 28 animals (82.1%) and 12 of 17 animals (70.6%) for Groups A and B, respectively.<BR>2) Estrous symptom was intensified by the MGA treatment, so that 23 of 35 estrous animals were used for artificial insemination.<BR>3) Conception rate (N.R.: 60-90 days) by the insemination at the first estrus following the MGA treatment was 4/16 and 2/12 and that of the second estrus between day 15 and 28 after the first in-semination, was 2/14 and 3/5 for Groups A and B, respectively.<BR>4) Levels of plasma progesterone of the 2 heifers were decreased to approximately 1 ng/ml after 6 days of MGA treatment and remained low thereafter. These two heifers, exhibiting estrus 2 or 3 days after the termination of treatment, failed to conceive.<BR>5) Apparent effects of daily oral administration of MGA 1.1 mg for 14 days were not associated with improvement of fertility. Additional treatments by either estrogen or prostaglandin F<SUB>2α</SUB> would be required for the improvement of the fertility.
- 日本繁殖生物学会の論文
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