U.S. pork producers replace over half of the sows in the breeding herd with replacement gilts each year. It is extremely important that a high proportion of selected replacement gilts reach puberty at an early age, conceive and easily enter sow groups at weaning.

P.G. 600 (Intervet/Schering-Plough Animal Health) is a combination of two hormones — equine chorionic gonadotropin (eCG; 400 I.U.) and human chorionic gonadotropin (hCG; 200 I.U.). P.G. 600 is a drug labeled for stimulating estrus in prepubertal, non-cycling gilts. It is not for stimulating or synchronizing estrus in gilts that have reached puberty and begun cycling.

Although P.G. 600 is an effective tool for advancing the onset of puberty, there is a great deal of variation in its efficacy amongst farms. Figure 1(A) shows the profile of blood progesterone in gilts during the estrous cycle. When gilts are in estrus, progesterone levels are low. Ovulation occurs during estrus, and corpora lutea (CL) that subsequently form on the ovaries secrete high levels of progesterone, suppressing follicular growth, ovulation and the signs of estrus.

At approximately Day 16 of the estrous cycle, progesterone levels begin to decline and gilts ultimately return to estrus. Thus, the inter-estrus interval is normally 20 to 21 days.

In the 1960s, researchers at the University of Missouri conducted an experiment during which mature, cycling gilts were treated with eCG on Day 5 or 6 of the estrous cycle, and 72 to 96 hours later the gilts received hCG. Although at the time of treatment the gilts were under the influence of progesterone secreted by the spontaneous CL, the eCG and hCG caused follicle growth and ovulation (with no display of estrus), and a new set of CL were induced. The original, spontaneous CL began to regress on Day 15 to 16 of the cycle, but the new, induced CL also functioned for a normal period of 15 to 16 days, thus extending the cycle and increasing the inter-estrus interval.

It was hypothesized that perhaps some of the erratic performance of P.G. 600 on some farms was due to treatment of gilts that were actually cycling and not prepubertal. And, depending on the day of the cycle at treatment, P.G. 600 may cause silent estrus and ovulation, formation of induced CL and a longer estrous cycle as diagramed in Figure 1(B).

To test this hypothesis, 80 prepubertal gilts were each placed in one of five treatment groups. Gilts received P.G. 600 (Treatment 1) or were allowed to display a natural first estrus, and then administered P.G. 600 on Day 6, Day 12 or Day 18 of the cycle (Treatments 2, 3 and 4, respectively). Another group of gilts received no P.G. 600 (Treatment 5). Gilts were checked daily for the return to estrus in the presence of a mature boar.

All gilts (100%) in Treatments 4 and 5 displayed normal estrous cycles of 17 to 24 days. Fewer gilts in the other treatment groups displayed normal estrous cycles, and the percentage was lowest (60%) for Treatment 3 (treated with P.G. 600 at Day 12).

Blood samples were collected and analyzed for progesterone on the day of first estrus and at 10-day intervals thereafter. Figure 2 shows the progesterone concentrations for gilts in Treatment 3 that had either normal estrus lengths (nine gilts, mean of 20.2 days) or abnormal (five gilts, mean of 32.5 days).

That circulating concentrations of progesterone remained elevated 20 days after estrus in the gilts displaying abnormal length estrous cycles suggests that P.G. 600 administered at Day 12 of the cycle caused ovulation and the formation of induced CL that resulted in an extended inter-estrous interval. The results of this experiment demonstrate the need to correctly classify replacement gilts as prepubertal or cycling before administering P.G. 600.

Researcher: Mark J. Estienne, Virginia Tech-Tidewater Agricultural Research and Extension Center. For additional information, contact Estienne by phone (757) 657-6450 or e-mail mestienn@vt.edu.