A total of 112 heifers was used in an egg transfer study to examine some of the factors affecting egg survival and twinning rates following bilateral transfers. Seventy-two recipient heifers were involved and pregnancy rates of 82\m=.\0%,66\m=.\7% and 61\m=.\5% were recorded for recipients in oestrus on the same day as (Day 0), 1 day before (Day +1) and 1 day after (Day \m=-\1) the donor respectively. Short-term (10-day) intravaginal progesterone, or intramuscular prostaglandin treatments for synchronization of oestrus in recipient animals did not depress pregnancy rate following egg transfer. One egg was transferred to each uterine horn and twinning rates of 55\m=.\6%,44\m=.\4% and 20\m=.\0% were recorded for transfers on Days 0, + 1 and \m=-\1,respectively. Of the recipients becoming pregnant following transfers on Day 0, 74\m=.\1% had twins. Embryo survival was higher (69\m=.\2%) in the uterine horn adjacent to the ovary containing the CL than in the opposite horn (30\m=.\8%). The storage period in vitro in TC 199 (37\m=+-\1\s=deg\C) affected subsequent egg survival rates following transfer, with fewer eggs surviving the longer storage periods. The pH of the medium used for egg collection and storage did not affect the pregnancy rate between pH values of 7\m=.\0to 8\m=.\6. Day of transfer (Days 3 to 7) did not seem to affect the pregnancy rate. The high pregnancy and twinning rates already reported following closely synchronized transfers were confirmed.
data are available on pregnancy rates following the surgical transfer of fertilized cow eggs. Rowson et al. (1969) recorded a pregnancy rate of 91 % in a group of eleven heifers, each of which received either two or three fertilized eggs transferred to the uterine horn adjacent to the ovary carrying the corpus luteum. No incidence of twinning was recorded following these transfers. Rowson et al. (1971) reported a twinning rate of 50% following the transfer of one fertilized egg to each uterine horn in eighteen recipients. The total egg survival in this study was 61\m=.\1 %. When both the donor and recipient were in oestrus on the same day, however, the corresponding rates were 71\m=.\4% and 78\m=.\6%. The necessity for close synchronization to achieve high pregnancy rates was again shown by Rowson et al. (1972) when they reported a pregnancy rate of 91\m=.\1% following transfers where the donor and recipient came into oestrus on the same day. This pregnancy rate fell to just over 50% when the donors and recipients were out of phase by 1 day. Studies are in progress to determine some of the factors which affect the outcome following the surgical recovery of fertilized eggs from donor animals after superovulation and the transfer, both surgical and non-surgical, of one fertilized egg to each uterine horn in recipient animals. This communication deals with pregnancy rate and egg-survival, following surgical transfers in heifers.
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