Daily injection of 1,000 I.U. of human chorionic gonadotrophin (HCG) into five cycling heifers from the 15th to the 26th day of the estrous cycle maintained the corpora lutea and lengthened the estrous cycle from 17.7 to 32.4 days. Thirty-nine bred heifers which received HCG from the 15th to the 35 th day post-breeding did not show an increase in pregnancy. However, regression of the corpus luteum in bred heifers subsequently diagnosed open occurred earlier in untreated than in treated heifers. It would appear that the embryo dies first and this results in regression of the corpus luteum. Injections of HCG resulted in formation of accessory corpora lutea in 67% of heifers diagnosed pregnant, 42% of the bred heifers diagnosed open and in none of the cycling heifers.
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Wiltbank et al. (1961) studied this question.