Delayed puberty in replacement gilts lowers reproductive performance and creates overcrowding in the gilt pool.

Some breeding herd managers have turned to the estrus-inducing drug, PG600. The drug is commonly used in gilts that have already failed to express estrus within a normal period and, therefore, results are often disappointing.

However, when PG600 is used as a production tool for stimulating early puberty, it has been consistent for inducing a fertile estrus in 65% or more of prepubertal gilts. It is still unclear why the response rate is less than optimal, and whether this may be due to lack of physiological maturity in some of the replacement gilts.

Boar Exposure

Boar exposure is one of the most effective stimulants for inducing estrus in gilts and advancing age at puberty. It is not known whether combining the stimulatory effects of boar exposure with PG600 could enhance the estrus induction response in gilts.

The first experiment examined whether physical or fence-line boar exposure was more effective for inducing puberty when used in conjunction with PG600.

Fifty-six prepubertal gilts, 150 days of age, were allotted to receive either fence-line boar exposure (FBE) or physical contact boar exposure (PBE) for a period of 30 minutes, once a day for 19 days. At 169 days of age, all gilts received an injection of PG600. Gilts were checked for estrus for seven days and reproductive tracts were collected 12 days later.

The percentage of gilts showing estrus within seven days (77%) and ovulating (68%) was not affected by boar exposure. There was no effect of treatment on age at puberty (169 and 172 days, respectively), interval from PG600 to estrus (3.9 and 4.3 days) or ovulation rate (11.2 and 13.7 corpa lutea, for the PBE and FBE, respectively). Twenty-five percent of all treated gilts expressed estrus before it could be synchronized with PG600.

In experiment 2, since some of the gilts expressed estrus before estrus could be synchronized with PG600, the effect of short-term priming boar exposure was evaluated in two gilt age groups.

One hundred and sixty eight prepubertal gilts, either 160-165 or 180-185 days of age, received either no boar exposure (NBE) or four days of boar exposure (BE) for 30 minutes once a day prior to injection of PG600. There was no interaction of age group and boar exposure on any measure except ovulation rate, which indicated that BE at a younger age (165 days) had a lower ovulation rate (9.7) when compared to the ovulation rate (12.7) following exposure and PG600 at an older age (185).

There was no effect of age group on the percentage of gilts expressing estrus, ovulating or cystic within seven days of injection (Figure 1). Boar exposure improved the percentage of gilts expressing estrus (65%) compared to NBE (47%) and also shortened the interval from PG600 to estrus (3.6 and 4.3 days, respectively), but did not influence the percentage ovulating (72%), age at puberty (181 days) or percentage of cysts (Figure 1).

Collectively, these results indicate that physical contact may not be necessary when used in conjunction with PG600 to induce early puberty. In addition, long-term boar exposure in young gilts does not facilitate synchronizing estrus in gilts with PG600.

Short-term boar exposure in 165-185-day old gilts improves the estrus response to PG600, but priming BE in younger gilts reduced ovulation rate. Therefore, fertility following puberty induction may be improved by exposing 180-day-old gilts to four days of boar exposure prior to PG600, and greater synchrony may be obtained by limiting the amount of boar exposure before PG600.

Researchers: S. Breen, K. Farris, K. Ruggiero, and R. Knox, Department of Animal Sciences, University of Illinois at Urbana-Champaign.