From large farms trying to adjust group size or time the introduction of additional females into the breeding herd; to small farms trying to have farrowing dates coincide with the demand for club pigs, there are instances when producers need to managing the timing of estrus or heat to meet the needs of the farm. Commercial products are available that assist managers with this process. To fully understand the application of these products it is helpful to be knowledgeable about the hormonal changes that take place during the normal gilt/sow estrous cycle.
Follicle Stimulating Hormone (FSH) is secreted by the anterior pituitary gland, a small gland at the base of the brain. FSH initiates the growth and development of follicles on the ovary, each developing follicle containing one maturing ovum (egg). As these follicles mature they secrete estrogen, which is responsible for typical signs of standing heat.
Luteinizing Hormone (LH) initiates the release of the ova from the ovary. Luteinizing hormone is also secreted by the anterior pituitary gland. Once an ovum or egg is released from a ruptured follicle, the remaining cells within the ruptured follicle continue to develop and form the Corpus Luteum. Collectively all corpus luteum are called the corpora lutea.
Considering that a normal sow/gilt estrous cycle is 21 days, the anterior pituitary gland will secret FSH late in the estrous cycle at approximately days 18-20. LH is then secreted on days 0 – 2 of the cycle to cause ovulation to occur.
The corpora lutea secrete progesterone. Progesterone is the primary hormone which maintains pregnancy. If the ova are fertilized and become viable embryos, they attach to the uterus. There is feedback to the corpora lutea that signals them to continue to produce progesterone and maintain pregnancy.
Prostaglandin is secreted by uterus. Prostaglandin fulfills two roles. First, if the ova are not fertilized, naturally there won’t be embryos to attach to the uterus. When there isn’t a pregnancy to maintain, at about day 14-17 the uterus will start to secrete prostaglandin into the blood stream. Prostaglandin causes the corpora lutea to regress ending the secretion of progesterone. As the corpora lutea regress, FSH will stimulate follicle growth on the ovary.
Prostaglandin’s second role is to initiate parturition. During pregnancy as the fetus reaches maturation, at the appropriate time for the conclusion of that pregnancy, the uterus will secrete prostaglandin. Again prostaglandin will cause the copora lutea to regress stopping the secretion of progesterone but in this instance the result is parturition.
Products for altering the normal estrus cycle
P.G. 600 ®1 is a combination of serum gonadotropin (Pregnant Mare Serum Gonadotropin or PMGS) and chorionic gonadotropin (Human Chorionic Gonadotropin, HCG). Pregnant mare serum and HCG mimic FSH and LH respectfully. In mature, pre-pubertal gilts, P.G. 600 will initiate estrus or heat in non-cycling pre-pubertal gilts that are nearing their natural initiation of puberty. Depending on the farm, gilts will reach maturity at about 170 – 175 days of age. Gilts experiencing estrus at this age are normally considered too young to breed. Skipping the first heat and breeding on the second or third estrus is the recommended management practice. P.G. 600 is not effective in cycling gilts. If the gilt is actively cycling and the corpora lutea are secreting progesterone, or the corpora lutea are in the initial stages of regression, P.G. 600 has no effect on initiating follicle development or egg release. P.G. 600 is very effective in synchronizing the initiation of first estrus in mature pre-pubertal gilts.
Research has shown that P.G. 600 will decrease the return to estrus interval in weaned sows but the economics of treating all weaned sows, including sows that would return to heat in 5 – 7 days with out treatment, is questionable. P.G. 600 is most effecting in treating sows with extended return to estrus intervals (7 – 10 days post weaning).
Matrix®2 is a 0.22% altrenogest solution, a synthetic progestagen (progesterone). When administered at the label rate Matrix acts like progesterone and will extend the time period to the next ovulation. Once Matrix is discontinued, the anterior pituitary gland will be signaled to secrete FSH, initiating follicle growth and estrus. The time period between discontinuing Matrix and standing heat is normally 5 – 7 days, similar to the return to estrus interval for weaned sows. Matrix must be fed daily, irregular feeding will result in poor results. Matrix is only labeled for gilts. If used for sows, producers must have a prescription for use from their veterinarian.
Prostamate®3 and Lutalyse®4, are products of various forms of the naturally occurring prostaglandin F2 alpha. These two products are the only prostaglandin products currently approved for use in swine in the U.S. If one of these products is administered at the labeled dose between day 21 and 42 of gestation, they will induce pregnant gilts/sows to abort and reabsorb the litter. Breeders wanting to synchronize groups of sows that are in random phases of their estrus cycle may breed females as the come in to heat, following up with a one time treatment of a prostaglandin F2 alpha product (i.e. Lutalyse or Prostamate) to induce the female to abort the litter. The sow/gilt may be bred when she returns to estrous, normally 5 – 7 days after treatment. Research has shown that there is no adverse affects on the subsequent litter.
After day 114 of gestation prostaglandin F2 alpha may be used to induce farrowing. Managers using prostaglandins in this manner must use caution. Late gestation is a time of final maturation of the developing fetus. Inducing farrowing prior to day 113 may result in the birth of weak, low viability pigs. Prior to treating pregnant sows with prostagladins to induce farrowing, breeding date must be recorded and expected farrowing dates must be accurately calculated. The thumb rule for using prostaglandins to induce farrowing is administration should be no sooner in gestation than one day before the average herd gestation length. For instance if the herd average for gestation length is 115, no sow should be induced to farrow before day 114 of gestation.