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The reproductive efficiency of twenty-eight aged does, 49 to 72 months old, was compared with that of eighteen young does, 6 to 13 months old. Fertilization rate and development in vitro of fertilized ova from rabbits induced to superovulate were not influenced by the doe's age. Ovulation rates following natural mating were only slightly reduced with age. However, the number of embryos per doe was much greater in young than in old does at 12 and 24 days post coitum. All young does had viable embryos, whereas the percentages of aged does with detectable implantation sites and viable embryos were 80 and 40, respectively, at 12 days post coitum, and 77 and 44 at 24 days post coitum.
Aged female rabbits were given supplemental exogenous progesterone and/or oestradiol benzoate in an effort to increase reproductive efficiency. Progesterone treatment had no effect on the total number of young kindled but did prolong the gestation period, increase the birth weight and result in fewer live young kindled/doe. When administered on Days 3 to 29 of pregnancy, 4 μg/day of oestradiol alone or in combination with 2 and 4 mg progesterone completely blocked pregnancy in all does. Starting on Day 5 of pregnancy, oestradiol levels of 1 μg/day, with or without progesterone, had no effect.
Chromosomal analysis of fourteen embryos revealed eleven normal females (44,XX), one normal male (44,XY), one abnormal embryo (45,XX) with an extra acrocentric chromosome and one embryo with a modal number of forty-two chromosomes in 35% of the metaphases. Since most of the embryonic wastage in aged rabbits occurred during the first 12 days post coitum, chromosome studies of embryos younger than 12 days post coitum are indicated.
Most of the embryonic wastage could not be attributed to ovulation rate, fertilization rate, ovum potential, CL function, circulating levels of progesterone and oestrogen, or to chromosomal anomalies of the fetuses. It was concluded that uterine factors apparently limit reproductive performance in aged rabbits.