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Embryo Reduction Before Day 11 in Mares with Twin Conceptuses¹

University of Wisconsin, Madison 53706

Abstract

Incidence of embryo reduction (natural elimination of one member of a twin set) before d 11 was studied by comparing the number of ultrasonically detected conceptuses per ovulation between single and double ovulators. Effect of unilateral (n = 24) vs bilateral (n = 26) double ovulations on the incidence of embryo reduction also was considered. Each of 50 double-ovulating mares was matched with two single ovulators yielding 100 ovulations, or potential embryos, per group. Frequency with which an ovulation resulted in a conceptus was greater for single ovulators (85%, P < .01) and for bilateral double ovulators (77%, P < .05) than for unilateral double ovulators (60%). The difference between single and bilateral ovulators was not significant. Expected frequencies for none, one or two conceptuses per mare for double ovulators were calculated using the d-11 pregnancy rate for the single ovulators (85%). For unilateral ovulators, but not for bilateral ovulators, observed frequencies of none, one or two conceptuses were different (P < .05) from expected (observed and expected, respectively, for no conceptuses, 29% and 2%; one conceptus, 21% and 26%; two conceptuses, 50% and 72%). Results did not indicate the existence of an embryo reduction phenomenon before d 11 in bilateral ovulators; each ovum had the same chance (no significant difference) of developing into a d-11 conceptus as an ovum in single ovulators. In unilateral double ovulators, the lower d-11 pregnancy rate per ovum, compared with bilateral ovulators and single ovulators, was attributable to a greater frequency of mares with no embryonic vesicles rather than to a greater frequency of mares with one vesicle. There was no indication for existence of embryo reduction before d 11 in either unilateral or bilateral double ovulators.

¹ Supported by the College of Agric. and Life Sci., Univ. of Wisconsin, Madison, and by NIH grant 1 RO1 HD19771-O1A1. Technical assistance was provided by A. J. Ginther, L. Kulick and R. A. Pierson.

² Dept. of Vet. Sci.