This study provides “proof-of-principle” that use of a purpose-built boar exposure area (BEAR) to induce a natural first heat in “select” gilts, and the use of low-dose exogenous gonadotropins to induce first heat in non-cyclic “opportunity” gilts, are essential strategies for meeting gilt breeding targets.

In addition, the use of oral progestagens to synchronize breeding weeks helps to focus barn staff at the time of breeding, and maximizes first-parity farrowing rate, litter size and lactation length.

Improvements in gilt development programs can lead to major increases in breeding herd efficiency by meeting replacement targets from smaller pools of truly select gilts, which also have improved lifetime breeding performance.

This would ultimately reduce annual replacement rates (target for top 30% of breeding herds is less than 50%), improve sow fitness, decrease involuntary culling of sows and non-productive days (NPD), increase labor efficiency and achieve a flow of service-ready gilts within the design specifications of the gilt facility.

Exposure of gilts around 160 days or more of age to the “primer” pheromones secreted in the foamy saliva of high-libido boars triggers the gilts to increase their secretion of endogenous gonadotropins, induces ovarian estrogen secretion from large pre-ovulatory follicles, and induces natural first estrus and ovulation.

Injecting low doses of exogenous gonadotropins (PG600 from Intervet USA, Inc.) induces heat in prepubertal gilts that have adequate ovarian maturity but have not yet shown a natural first estrus; these are defined as “opportunity” gilts.

The progestagen analog, altrenogest (Matrix from Intervet USA, Inc.), mimics the effect of naturally produced progesterone during the luteal phase of mature, cyclic gilts. Progestagen treatment stops the development of mature ovarian follicles, regardless of the day of the cycle when treatment starts, and blocks the onset of behavioral estrus. After withdrawal of the progestagen, gilts show a synchronized heat within 4-9 days.

The objective of this collaborative study was to demonstrate that efficient management in the gilt development unit (GDU) would improve production efficiency of a 3,200-sow, farrow-to-wean farm. The overall production targets set for the GDU were:

  • 80% heat-no-serve (HNS) gilts within a 28-day selection window (85-90%, including opportunity gilts);

  • 100% of gilts bred at second or third estrus;

  • 100% of gilts bred within a target weight range of 300-350 lb.

A 28-day stimulation program was managed as follows:

Day 1-13: Direct and fenceline contact with high libido, vasectomized boars in the BEAR. Gilts with recorded estrus designated as NatHNS (natural heat-no-service);

Day 14: Mixing and repenning of remaining non-cycling gilts;

Day 23: Treat all opportunity gilts (no recorded heat but at target weight) with 400 iu of eCG (equine chorionic gonadotropin) and 200 iu of hCG (human chorionic gonadotropin), followed by daily heat detection in the BEAR; treated gilts with recorded estrus designated as PGHNS (gonadotropin induced heat-no-service);

Day 28: All eligible gilts are identified and all gilts not classified as heat-no-service (HNS) are culled.

Results of this study indicate variable responses to boar exposure between Day 1 and Day 23, due to variable health status (17% to 74% response range). However, the controlled use of exogenous gonadotropins was an effective tool for inducing estrus in opportunity (known non-cyclic) gilts. Of the 1,124 opportunity gilts treated with exogenous gonadotropins, 91.5% were recorded as PGHNS and 83.7% were recorded as PGHNS within seven days of treatment (Figure 1).

Both NatHNS and PGHNS gilts within a designated weight range were considered “service eligible” and were individually housed for daily progestagen administration in the stall section of the GDU. Altrenogest was given at a rate of 15 mg (6.8 ml of oil-based product)/head/day using the supplied dosing gun and applied directly onto the feed in the drop boxes each day for 14 consecutive days.

During the 14-day treatment period, all gilts received daily fenceline contact with a mature, vasectomized boar for at least one hour.

After treatment, gilts were checked for estrus daily by placing active, mature boars in front of the females. Breeding was carried out in gilt stalls using standardized artificial insemination (AI) protocols.

For analysis, gilts were classified as NatHNSMAT (natural boar induced estrus) or PGHNSMAT (gonadotropin-induced estrus) prior to progestagen treatment.

Progestagen treatment effectively synchronized estrus in all gilts (Figure 2). However, considering only gilts bred within 10 days of progestagen withdrawal, PGHNSMAT was slower to return to estrus than NatHNSMAT gilts (6.4 days vs. 6.2 days after withdrawal), respectively.

All gilts bred showed excellent productivity (Table 1), although the percentage of gilts bred within 10 days of progestagen withdrawal was higher in NatHNS (88.1%) compared to PGHNS (81.5%) gilts.

The benefits of using progestagen treatment for reducing average age of semen used for insemination are illustrated in Figure 3.

These management tools can help facilitate the introduction of gilts into the breeding herd in a timely, productive manner. When applied effectively, these tools can also ensure fresh semen is used and improve labor efficiency.

Researchers: J. Patterson, G. Foxcroft and E. Beltranena, University of Alberta; W. Wilson and C. Francisco, Intervet, USA; N. Williams, PIC, USA; Gordon Spronk, DVM, Pipestone Veterinary Clinic, Pipestone, MN. Contact Foxcroft by phone, 780-492-7661 or e-mail: george.foxcroft@ualberta.ca.

Table 1. Productivity of All NatHNSMAT and PGHNSMAT Gilts Served Within 10 Days After Progestagen Withdrawal

NatHNSMAT PGHNSMAT
No. with HNS* 814 661
Served, % 88.1 81.5
Farrow rate, % 93.3 94.2
Total born 13.2 13.0
Born alive 12.0 11.7
*HNS=Heat-no-service