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  • Title: Interaction of stage of the breeding season and intrauterine devices on estrous cycle length in the ewe.
    Author: Warren JE, Hawk HW, Williams WF.
    Journal: J Anim Sci; 1974 Feb; 38(2):363-8. PubMed ID: 4855848.
    Abstract:
    To determine whether the presence of IUDs would reduce estrous-cycle length throughout a breeding season, 51 and 58 ewes, respectively, were studied in 2 experiments. The estrous cycles of ewes with an IUD adjacent to each ovary averaged 9.6 days in length from September through November, and nearly 20 days from December through February. The same ewes showed a similar pattern in a second September-to-February breeding season. In the third breeding season, ewes with an IUD in each uterine horn were laparotomized if they had not returned to heat within 9 days after a designated estrous period. Of 54 ewes laparotomized during the course of the breeding season, corpora lutea 1) had regressed or were regressing in 49 (91%) by Day 9, and 2) had regressed in the other 5 ewes by Day 18. It is concluded that cycles longer than 9 days were usually caused by 1 or more successive ovulations without estrus, and that the increased mean cycle length for IUD-bearing ewes during the second half of the breeding season was due primarily to a greater frequency of ovulation without estrus.
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