High- milking sheep have a lower ovulation rate and tend to yield fewer embryos in response to superovulation and intrauterine artificial insemination

Antagonistic relationship between milk yield and reproduction is reported in several
livestock species. This study aimed to investigate whether genetic merit for milk production
in dairy sheep affects responses to superovulation, embryo yield and quality.
A total of 21 cross-bred
Sarda x Lacaune ewes homogeneous for age, parity and stage
of lactation were included. The ewes were stratified as high-producing
or low-producing
based on their genetic merit for milk production estimated by a pentatrait
repeatability animal model. Oestrus was synchronized using an intravaginal progesterone
pessary inserted on Day 0 and removed on Day 14. Superovulatory treatment
consisted of 350 I.U. of porcine FSH administered in eight decreasing intramuscular
doses every 12 hr with a total dose of 10 ml of solution starting 12 days after insertion
of sponges. Laparoscopic artificial insemination (AI) was performed 48 hr after pessary
removal. Surgical embryo recovery was performed at Day 8 after pessary removal.
Correlation between breeding value for milk production and the number of corpora
lutea (CL) was significantly different from zero (−0.49). High-producing
ewes had a
lower number of CL than low-producing
counterparts (7.6 ± 2.50 vs 12.1 ± 5.16 respectively;
p < .02). Furthermore, there was a tendency for high-producing
ewes to
yield fewer embryos than low-producing
females (5.3 ± 3.46 vs 9.18 ± 5.11; p = .09).
No differences were observed between ewes in both genetic groups with regard to
the number of embryos of grades 1, 2 and 3. To our knowledge, this is the first report
highlighting an antagonism between genetic merit for milk production and the ability
to produce embryos in sheep. These results deserve to be considered in sheep breeding
programmes.