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Relationships between Ovulation and Lambing Rates¹

Oklahoma State University, Stillwater²

Abstract

Two mathematical relationships were developed showing the expectations in terms of frequency of ewes lambing to first estrus matings, frequency of ewes bearing twins and frequency of ewes bearing triplets when ovulation rates varied and when the frequency of fertilization plus survival varied. Under Condition 1 ova fertilization and embryo survival were on the basis of the individual ovum or embryo. Under Condition 2 ova fertilization and embryo survival were on the basis of the ewe, i.e., all-or-none. Tables showing the expectations of ewes lambing and ewes twinning to first estrus matings were developed and are quite different.

Mating and lambing records from two groups of ewes involving 541 and 1994 first estrus matings were applied to the tables. The estimates of ovulation rate from table 1 of Condition 1 were unrealistic, because of a high frequency of twins. The estimates from table 2 of Condition 2 were more realistic, suggesting that Condition 2 is probably more nearly correct than Condition 1 under the assumptions made and the experimental conditions involved. These data do not provide strong evidence, however, for the truth of Condition 2, because the method involves predicting independent variates from dependent variates and necessary assumptions cannot be proved. If true, however, estimates of ovulation rate are directly reflected in the ratio of single, twin and triplet births observed and the frequency of lambing to first estrus matings is the frequency of ova fertilization plus survival.

A suggestion is made that attempts to confirm the relationships suggested herein or to prove them false are needed. If one of these conditions is true, adequate data from field trials would be more interpretable in terms of whether treatment or breed differences in reproductive rate were due to differences in ovulation rate or to differences in the rate of fertilization of ova and survival of embryos.

¹ Approved by the Director as Journal Series Paper No.1279

² In cooperation with AHRD, ARS, U. S. Department of Agriculture.