Haugen, Arnold O. 1974. Reproduction in the Plains Bison. Iowa State Journal of Research 49.
Observations on female reproduction in Bison bison were made under range conditions in South Dakota and Nebraska in 1965, 1967, 1968, and 1971. Counts were obtained for both corpora lutea and embryos.
Only one of the 17, 1½-year-old cows observed had ovulated, but she was not visibly pregnant when sacrificed Oct. 10. The ovaries of all 39 2½-year-old cows had corpora lutes and 34 were pregnant as indicated by the presence of an embryo or fetus. Of 23 cows, 3½ to 4½ years old examined, 15 had embryos, and an additional four had only corpora lutea. Seventy-one (88%) of 81 cows 5 years old and older were pregnant.
Approximate ages of bison embryos were estimated through use of a rump-crown curve for domesitic cattle. The sex ratio for 101 fetuses was 119.4 males: 100 females. Of 154 corpora lutea observed, 51.9% were in the right ovary.
Breed dates were calculated from data on age of the embryo and the date the cow was sacrificed. Results indicate that most bison cows conceived between July 21 and August 19, with peak about August 1. The earliest concecption was July 4, and the latest October 2, which indicates that the peak of calving occurred between May 7 and 21 after a gestation period of about 285 days.