Studies on embryo transplantation procedures using Booroola-Merino x Perendale ewes : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science at Massey University
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1987
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Massey University
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Abstract
A study was carried out to develop an embryo transfer programme
under farm conditions. For this embryos were collected from donor ewes
5-6 days after single-sire mating by rams of the same crossbreed as the
ewes. Then the embryos were transplanted soon after collection or
after they had been initially frozen and later on thawed before their
transfer. To enable this study to be carried out, one hundred and
sixty-one Booroola-Merino x Perendale ewes aged between 3 and 6 years
and of 34-69 kg live weight were examined. These animals were
classified as either carriers (F+) or non-carriers (++) of the
fecundity gene on the basis of the number of ovulations recorded at
laparoscopy carried out a few days after progestagen sponge treatment
to induce oestrus synchronisation, and in a few cases on the basis of
previous lambing data. The aims of the study therefore were also to
identify ewe carriers or non-carriers of the fecundity gene, and to
evaluate their ovulatory response to several doses of Pregnant Mare
Serum Gonadotrophin (Tl=O, T2=350, T3=700 and T4=1050 i.u. of PMSG)
given during the embryo transfer programme.
On the basis of the criteria used to classify F+ and ++ ewes, 76
ewes were considered as carriers and 85 as non-carriers of the
fecundity gene.
Immediately after progestagen synchronisation treatment the
incidence of oestrus and the distribution of onset of oestrus was
similar in both groups of ewes, and also following PMSG treatment given
in the succeeding cycle.
Ovulation rate after the progestagen sponge treatment was
significantly affected by genotype and age of the ewes, but it was not
associated with the live weight of the ewes. The least-squares means
for number of ovulations in F+ and ++ ewes and in 3 and 4-6 years old
animals were 3.02, 1.73, 2.14 and 2.45 respectively. Higher
sensitivity to PMSG treatment was observed from the right ovary than
from the left ovary (3.40 v. 2.82 ovulations respectively).
No significant effect of dose of PMSG was found on the percentage
of embryos recovered, the overall recovery rate being 66%. Of the ova
or potential embryos recovered 78% had been fertilised and had
developed to embryos. There were no significant differences between
the 3 rams in the fertilisation rate in the ewes compared on a per ewe
basis.
The reproductive performance of donor ewes which had been flushed,
was considerably influenced by the efficiency of the embryo recovery
procedures, since every embryo not recovered represents a potential
pregnancy and such a situation sometimes is not desirable. In this
study 30% of the ewes that were flushed, subsequently became pregnant
and produce lambs as a result of one or more embryos not being
recovered at surgery.
The number of ovulations after PMSG treatment was significantly
affected by the dose-level of PMSG and the genotype, age and live
weight of the ewes. F+ ewes recorded a significantly higher ovulation
rate than ++ ewes. Their respective least-squares means were 3.61 and
2.31 corpora lutea. No significant difference was found between
treatments 1 and 2 and between treatments 3 and 4. However, the
response from the last two treatments was significantly higher than
that from the first two treatments. Their respective least-squares
means were 2.10, 2.01, 3.74 and 4.37 corpora lutea. Ewes 4-6 years old
recorded a significantly higher number of ovulations than younger ewes
(3 years old). Small significant effect of live weight was found on
the ovulatory response of the ewes. Analysis within each genotype
showed a similar trend in both genotypes, but small significant effect
of weight was only detected in the ++ ewes.
The pregnancy rate that occurred after the transfer of two fresh
embryos (86%) was significantly affected by the genotype of the
recipient ewe (F+ ewes 75% v. ++ ewes 96%). The number of ovulations
in the recipients and whether or not they had received PMSG before
transfer had no effect on the incidence of pregnancy.
The pregnancy rate achieved after the transfer of frozen embryos
(35%) was significantly influenced by the number of embryos
transferred, but not by the ovulation rate in the recipient ewe or the
time elapsing from flushing until freezing. Pregnancy rate was
significantly higher after the transfer of 2 embryos compared to the
transfer of single embryos (43% v. 0%), but only 8 transfers were made
in the later category.
On the basis of the number of ovulations, it can be concluded that
ewe carriers of the fecundity gene recorded significantly higher
ovulation rate after oestrus synchronisation and were more sensitive to
PMSG stimulation than non-carrier ewes.
There was an encouraging pregnancy rate obtained with frozen
embryos, even although the small number of transfers carried out limits
conclusions that can be drawn. However, the eighty-six percent
pregnancy rate achieved after the transfer of two fresh embryos per
recipient, shows the feasibility of the embryo transfer programmme
under conditions where suitable recipients are available. Where
recipients are limited then additional embryos might be frozen and
stored until ready for transplantation.
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Keywords
Sheep, Reproduction, Embryo transplantation