Estrus induction and maintenance of cycles in gilts with PG-600 and boar exposure

The ability to induce puberty in gilts at an earlier and predictable age can facilitate introduction into the breeding herd. One method to initiate puberty (PG-600; 400 IU of PMSG and 200 IU of hCG, Intervet, Millsboro, DE), elicits estrus in a majority of gilts. However, a proportion of those gilts do not recycle normally. This study looked at the efficacy of PG-600 and boar exposure, alone and in combination, to induce and maintain regular cycles. Two replicates of 160 gilts each (182 days old) were conducted on a commercial farm in June and July, 2006. Replicates were combined for analysis. Gilts were presumed prepubertal on arrival from the multiplier and randomly assigned to one of four treatments in a 2 x 2 factorial arrangement: PG-600, weekday 10 min of full physical contact boar exposure (BE), PG-600 + BE, and neither PG-600 nor BE (control). Initial weight and backfat measurements were taken. Detection of estrus was performed during BE or during two min of fence-line boar exposure. Gilts were considered in estrus when they stood to be mounted. A total of six gilts were removed from the experiment for health reasons. The PG-600 and PG-600 + BE gilts had a higher (P < 0.0001) percentage in estrus within seven days than the BE and control treatments (69.4 ± 7.04 %, 80.8 ± 7.04 % versus 25.1 ± 7.04 %, 12.5 ± 7.04 %, respectively). Eighty-five of 146 ix gilts in estrus within seven days returned to estrus 18 to 23 days later. Although the BE and control groups had fewer gilts respond within seven days, a larger proportion recycled within 18 to 23 days; (100 ± 9.74 % of BE versus 78 ± 9.74 % control and, 55.8 ± 9.11 % and 50 ± 9.11 % for PG-600 and PG-600 + BE respectively). Half of control gilts showed estrus within 30 days regardless of initial weight. PG-600 and BE gilts with heavier initial weights had a higher probability of coming into estrus. Heavier gilts in the PG-600 + BE group had a lower probability of cycling. Similar probabilities were seen for initial age. Gilts that expressed estrus within 30 days in all treatment groups did so regardless of back-fat thickness. There was no significant difference among treatments regarding first farrowing rate and litter size. The greatest response was to PG-600. Response to BE is dependent on many factors, and in this study was lower than expected. Addition of daily boar exposure to PG-600 did not result in dramatic increase in proportion maintaining cycles and may not be warranted.