SYNCHRONIZATION OF ESTRUS IN GILTS
Introduction
Gilt development and management represent integral components of reproductive performance
in the breeding herd. The availability of cycling gilts often determines the culling
practices on many farms. Without gilts, producers are forced to retain sows of questionable
reproductive value. Under most circumstances, producers receive gilts when they are in a
peri-pubertal stage of sexual development. If these animals are the appropriate age and
body weight, their first estrus should occur within one month after arrival at the farm.
The decision to breed gilts on their first estrus or wait until the second or third estrus
is usually decided on a farm-by-farm basis.
Control of the onset of estrus in gilts has been approached along three lines;
This paper will review the advantages and problems associated with each method of estrus
synchronization.
Transport-Induced and Boar-Induced Estrus
Several reports indicate that gilts normally attain puberty at approximately 200-210 days
of age. However, the natural attainment of puberty is influenced by many intrinsic and
extrinsic factors, such as genotype, environment and boar contact. Many producers indicate
that the first estrus is commonly observed when gilts are six months of age. The onset of
estrus often coincides with relocation or transport of animals from the gilt developer to
the sow farm. Undoubtedly, the best-known stress factor in pigs is that of transportation.
If the age of gilts at the time of transport is close to the normal onset of puberty,
approximately 25-35% of gilts will display estrus within one week after transport. This
transport-induced estrus serves to synchronize a proportion of gilts.
Although transport may induce estrus, it is evident that boar contact is a potent form of
puberty stimulation. The major factor controlling the efficiency of boar contact as a
puberty stimulus is the age of the gilt at the time of boar introduction. When boar contact
is initiated when gilts are 4 months of age, the pubertal response is minimal. It was
suggested that the young gilt may become habituated to the boar stimulus at a stage in
development when she is too young to respond. Conversely, when boar introduction is delayed
until the immediate prepubertal period (6 months of age and above), the response is again
limited for a different reason. By virtue of the relatively old ages, ie. 6 months, of gilts
at introduction, the actual pubertal ages of these gilts are not much reduced below those
of unstimulated animals. When boar introduction occurs at gilt ages in the region of 160
days, both the interval from first boar contact to puberty and gilt age at puberty are
minimized, while maximum synchronization of the pubertal estrus occurs. These observations
may elucidate the common problem of delayed puberty in replacement gilts. Due to the age at
arrival on the sow farm, perhaps boar contact is initiated too late to stimulate puberty.
Thus, the only function of boar contact is to identify animals in estrus and not to
stimulate the onset of puberty.
Oral Progestins
This approach to estrus synchronization has utilized suppression of ovarian activity
through the administration of oral-active progesterone or synthetic progestins. Feeding
cyclic gilts individually or in groups at a rate of 15-20 mg RegumateR/pig/day
for 14 to 18 consecutive days produces a synchronous onset of estrus between 2 and 8 days
after the last progestin feeding. This product is not licensed for use in the
pig.
eCG/hCG (PG600R)
Presently, the most common exogenous hormone combination for induction of follicle growth
and ovulation in acyclic females is a combination of equine chorionic gonadotropin (eCG,
formerly called pregnant mare's serum gonadotropin or PMSG), and human chorionic
gonadotropin (hCG). The product PG-600R contains 400 iu PMSG and 200 iu hCG.
This hormone combination is cost-effective for the induction of estrus and ovulation in
acyclic pigs. Gilts usually show estrus 3-6 days after treatment and the time of ovulation
is approximately 110-120 hours. Conception rates, farrowing rates and litter sizes of
treated animals are comparable to animals with naturally occurring estrous cycles.
The response rate is enhanced if gilts are given daily boar contact, beginning at the time
of treatment. Since PG-600R is not effective in animals with corpora lutea,
ie, animals that already cycled, producers must be aware of the approximate age that gilts
achieve puberty and commence ovarian cyclicity. Treatment failures occur when gilts have
been a gilt pool for several weeks and natural estrus has been overlooked.
Prostaglandins
The porcine corpora lutea (CL) remain unresponsive to the acute administration of
PGF2" or its analogs until days 12 to 14 after ovulation. As the porcine
CL typically commence natural regression due to endogenous prostaglandins at days 14-17,
there is only a transient period when a single treatment with exogenous prostaglandins
hastens the onset of estrus. Repeated PGF2" injections between days 5 and
10 will reduce the duration of the estrous cycle; however, the labor and cost preclude this
technique from practical application.
In contrast to the relative unresponsiveness of the CL to PGF2" during
diestrus, PGF2" is effective for inducing luteolysis, abortion, and a
prompt return to estrus in pregnant (and pseudopregnant) gilts beyond the second week of
pregnancy. One method for synchronization is to pen-mate gilts for three weeks and then,
treat with PGF2" two weeks later. This technique is effective, but the
increases in age at first farrowing and nonproductive sow days are often difficult to
justify.
Conclusions
Presently, PG600 is the only licensed hormone preparation for the induction of estrus in
prepubertal gilts. Although exogenous hormones may be beneficial for the induction of
estrus, sound management techniques, including boar exposure, are essential for the
detection of estrus on any farm.
Take-Home Message
