J. Anim Sci.
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J. Anim Sci. 1974. 38:1013-1017.
© 1974 American Society of Animal Science
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Embryonic and Fetal Development: Uterine Components and Influences1

L. C. Ulberg and G. B. Rampacek

North Carolina State University2, Raleigh 27607

Abstract

This discussion is to be concerned with the influence of the uterus on litter size, with special emphasis on its role in maintaining embryos during the first month of gestation. Although it is recognized that the loss of young occurs throughout the gestation period, it is generally agreed that most of the loss occurs within the first few weeks (Wrathall, 1971). Consequently, this discussion will deal primarily with that period of pregnancy.

There are but two ways in which to increase the number of pigs in a litter at term. There has to be either an increase in the number of fertilized ova (increased ovulation rate) or an increase in the number of fertilized ova which survive (decreased death loss) or some combination of the two. The number of ova can be increased by such techniques as flushing, super-ovulation or embryo transfer. The effects of these techniques on litter size have been reviewed by Zimmerman (1972) and will not be part of this discussion except to point out that, in general, increasing the number of embryos that start development does not greatly increase the number in the litter at term. Further, there is general agreement in the literature that only about two-thirds to three-quarters of those embryos which start development can be expected to actually reach term. It is also clearly established that the loss of some young occurs in nearly every litter. According to Perry and Rowlands (1962), most loss occurs some time during the very early part of the gestation period.

Footnotes

1 Presented at Symposium on Prenatal and Perinatal Development of Swine sponsored by the American Society of Animal Science and Shell Development Company, July 29, 1973, University of Nebraska, Lincoln.

2 Reproductive Physiology Research Laboratory, Department of Animal Science.






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