Gilts were tested daily or twice daily with a boar for the detec-
tion of estrus. Those exhibiting a synchronized estrus were insemi-
nated 12 and 24 hr. after the onset of estrus with 100 cc of diluted
semen collected on the day of insemination.

A1l gilts were slaughtered approximately 26 days after insemina-
tion. The number of corpora Tutea and the number of normal and
degenerating embryos were recorded.

Results. Estrus. Individual and group feeding were found to be
equally effective procedures for the administration of ICI 33828.
Estrus was blocked after the first 3 days of treatment in 98% of 93
gilts fed ICI 33828 (Table 1).

A11 gilts exhibited a post-treatment estrus. The onset of estrus
occurred 3 to 10 days after the last feeding of ICI 33828 in 94% of
the gilts. The time of the onset of estrus was influenced by the
injection of PMS after withdrawal of ICI 33828. The average interval
from last feeding of ICI 33828 to a synchronized estrus was decreased
from 5.4 to 4.4 days (P<.001) and the degree of synchronization of
estrus was improved by sequential treatment with ICI 33828 and PMS.

In Group I, 74% of the gilts showed estrus during a 2-day period;
whereas, 93% of Group II gilts were synchronized to a 2-day period
(Table 1). The average duration of estrus was not affected signifi-
cantly by treatment with PMS (Table 1).

Ovulation Rate. Average ovulation rate was increased signifi-
cantly (P<.00T) in gilts injected with PMS as determined by the
average number of corpora lutea of pregnant animals (Table 1). Gilts
in Group II had an average of 11.2 more corpora lutea than Group I
animals which was approximately a two-fold increase in ovulation rate.

The variation in ovulation rate was increased by PMS treatment.
Coefficients of variation of average number of corpora lTutea of Groups
I and II animals were 22.3% and 41.0%, respectively. Eight of 42
Group II gilts had less than 16 corpora lutea which indicates the lack
of detectable effect of PMS on ovulation rate in at least 19% of the
injected gilts.

The average conception rate of 87 gilts inseminated at a
controlled estrus was 80%. There was no difference between the
conception rates of Group I and Group II gilts.

Litter size on the 26th day of pregnancy was increased signifi-
cantly (P<.005) by the injection of PMS to induce superovulation.
Gilts treated with ICI 33828 and PMS had an average of 4 more embryos
per litter, which was a 43% increase in the average litter size of 9.3
embryos in Group I gilts. Litter size varied from 5 to 33 normal
embryos in gilts injected with PMS. The average number of degenerating
embryos in Group I gilts was 0.8 as compared to 1.5 in Group II
animals. The proportion of corpora lutea represented by normal embryos
was 19% lower in gilts in Group II than in Group I.

 

Discussion. The results from this study are in agreement with previous
reports which show that treatment with ICI 33828 provides an effective
method for the synchronization of estrus in swine. Even so, increased
precision in the prediction of estrus over that obtained with the basic

1420