for the two drugs. The pregnancy rate data demonstrate that estrus
synchronization does have some practical potential and continuing
attempts will be made for improvement.

Pregnancy Rates (Treated:Control (a)
26 or 28(¢)
Two Services(b) Day Period
Repromix
Upjohn Studies 88:89 76:69
University Trials 63 :69 ==
Field Trials - A Ali=50 -—

Field Trials - B 62:65 S 23
MGA

Upjohn Studies =eg 501208

Field Trials Ee 51:45

(a) Percent of entire group that was pregnant.
(b) Two services or inseminations regardless of
time of occurrence.
(c) First 26 (Repromix) or 28 (MGA) days of breeding.

MODE OF ACTION: In the normal cycling animal upon regression of
the corpus luteum almost all animals receiving MGA developed a
mature size follicle which persisted for varying periods of time.
‘Persistence of the follicle appears to be associated with effects
of estrogenic activity which have been described in detail 615)%
These include swelling of external genitalia, mucous discharge,
typical estrogenic ferning pattern of cervical mucus and increased
adrenal weights. This has led to its use in .feedlot heifers with
improvements in weight gain and feed efficiency. At an adequate
level, estrus is inhibited during the feeding phase. The results
of the first 25 trials indicated that MGA improved growth rate
and feed conversion by 11.2% and 7.6% over untreated controls and
6.9% and 6.3% over diethylstilbestrol treated heifers. Improve-
ments in rate of growth and feed efficiency did not occur in
spayed heifers or steers.

Studies on pituitary gonadotrophins have demonstrated essen-
tially no effect on FSH content, but a possible increase in the
LH concentration during MGA treatment in some situations. Folli-
cular weights in the presence of the CL of pregnancy are not in-
creased, indicating that the increased follicular size following
CL regression is a permissive action of MGA and not a stimulatory
action.

OVULATION INDUCTION DURING AND FOLLOWING MGA: The following agents
have been shown to induce ovulation during treatment with MGA:
estradiol cypionate, 5-20 mg., oil, IV; estradiol-17-beta, 10 mg.,
0il, IM; human chorionic gonadotropin (HCG), 2500 i.u., IV;
pituitary luteinizing hormone (PLH-Armour), 50-100 mg., IV. A

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