Reproduction & Genetics

Plan Helps Avoid Second-Parity Slump Breeding Parity 1 sows at their second postweaning estrus has been shown to increase the number of live embryos detected at Day 30 of gestation. This increase in embryo viability may negate the effects of the second-parity dip in reproductive performance. Recent work at the University of Alberta has shown excellent reproductive performance of contemporary dam-line,

‘Skip-a-Heat’ Plan Helps Avoid Second-Parity Slump

Breeding Parity 1 sows at their second postweaning estrus has been shown to increase the number of live embryos detected at Day 30 of gestation. This increase in embryo viability may negate the effects of the second-parity dip in reproductive performance.

Recent work at the University of Alberta has shown excellent reproductive performance of contemporary dam-line, first-parity sows. Even when these sows lose large amounts of body tissues through imposed feed restriction during peak lactation, there is a relative lack of effect on many measures of postweaning fertility. Still, a second-parity “dip” - or lack of an increase - in the size of the second litter is often observed.

In earlier studies, first- and second- parity sows subjected to “skip-a-heat” breeding (bred at the second estrus after weaning) averaged two pigs/litter more than sows bred at first post-weaning estrus. However, this breeding procedure also accumulated 21 non-productive days (NPD), and resulted in a 9% chance that sows would not stand to be bred on the second estrus.

The data on the changing reproductive characteristics of contemporary commercial sows suggests that the response to skip-a-heat breeding in first-parity sows needs to be reevaluated.

A trial was initiated to reassess the effect of breeding commercial sows at first vs. second postweaning estrus on follicular development, size of the largest ovulatory follicle, ovulation rate and embryonic survival to Day 30 of gestation.

Fifty-five, Parity 1 sows were allocated to the study at farrowing. Within 24 hours after farrowing, all litters were standardized to 10-12 pigs.

Sow body weight, backfat depth, loin muscle depth and area, and individual piglet weights were measured on Days 1, 7, 14 and 21 of lactation, at breeding and at Day 30 of gestation.

All sows were fed a standard lactation diet “to appetite” for the 21-day lactation.

Sows were relocated to the breeding area and, from the day after weaning, were allowed twice-daily fenceline contact with mature boars for detection of estrus. All sows were bred using established artificial insemination (AI) protocols to insure insemination within 24 hour of ovulation. Semen from boars of known fertility was pooled and used within three days of collection.

Sows were divided into two treatments:

  1. Bred at their first postweaning estrus (PE1), or
  2. First postweaning estrus skipped and bred on second postweaning estrus (PE2).

Sows were paired on the basis of similar wean-to-first postweaning estrus intervals and weight, and randomly allocated within pairs to treatment.

PE1 sows averaged 111.7 hours to first postweaning estrus at an average weight of 392 lb. PE2 sows averaged 113.6 hours to first postweaning estrus and averaged 396 lb.

The study showed PE2 sows were heavier at breeding and had a greater positive weight change between weaning and breeding compared to the PE1 sows (Table 1).

Real-time ultrasonography (RTU) was used to estimate the size of the largest pre-ovulatory follicle at PE1 and PE2. At PE1, the size of the largest detected pre-ovulatory follicle in PE1 and PE2 sows was essentially the same (Table 2). However, at breeding, the size of the largest pre-ovulatory follicle detected in PE2 sows (8.2 mm) was greater than in PE1 sows (7.2 mm).

Of all sows weaned, 95% returned to estrus within 10 days. All PE2 sows exhibited a second postweaning estrus at an average of 22 days after their first postweaning estrus (range 19.1 to 28.4 days). All PE1 sows and 96% of PE2 sows were bred. There were no differences between PE1 and PE2 sows for pregnancy rates, which exceeded 90%.

Although there was no difference in ovulation rates between treatments, PE2 sows had greater numbers of live embryos and higher embryonic survival at Day 30 of gestation compared to PE1 sows (Table 3).

These results confirm that under good management practices, high production performance is achievable.

The study confirms that breeding first-parity weaned sows at second postweaning estrus will negate the effects of the second-parity dip. Consistent with a previous study, 22 NPDs were accumulated and an additional 2.3 live embryos at Day 30 were detected. However, in this study, all “skipped” sows were detected in heat and were successfully bred.

Current expectations are that one additional pig born/litter will cover the additional cost of 21 NPDs ($2/day × 21 days = $42). If the use of this management practice increases the number of pigs born by more than one, a positive cost:benefit ratio would be anticipated.

Alternative methods to increase second-parity litter size, without the accumulation of the NPDs associated with skip-a-heat breeding, merits further investigation.

Researchers: J.L. Patterson, P.R. Zimmerman, M.K. Dyck and George R. Foxcroft, University of Alberta, Edmonton, Alberta, Canada. Contact Foxcroft by phone (780) 492-7661 or e-mail:

Table 1. Weight at First Postweaning Estrus and Breeding, and Weight Change from Weaning to Breeding for PE1 and PE2 Sows
PE1* PE2*
Number 27 25
Weight at PE1, lb. 375.3 376.0
Breed weight, lb. 375.3 422.6
Weight change (wean-breed), lb. -15.8 26.2
*PE1=sows bred at first postweaning estrus; PE2=sows “skipped” and bred on second postweaning estrus.
Table 2. Largest Detected Pre-Ovulatory Follicle size (mm) in PE1 and PE2 Sows
Number 27 25
1st postweaning estrus (PE1) 7.2 7.1
Range 4.7 - 9.1 4.9 - 9.1
2nd postweaning estrus (PE2) - 8.2
Range - 4.8 - 9.9
Table 3. Effect of Breeding at First (PE1) or Second (PE2) Postweaning Estrus on Reproductive Performance
Number 25 22
Ovulation rate 19.0 19.6
Number of live embryos 12.9 15.2
Embryo survival to Day 30, % 68.1 77.4

Management Tools Help Improve Reproductive Efficiencies

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:

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

Test Tube Pigs are Now a Reality

The ability to produce pig embryos in vitro (in the laboratory environment) has been mastered at Mississippi State University.

Pig oocytes (immature egg cells) are matured in the laboratory and fertilized. The resulting embryos develop from a single-cell stage to 2-, 4-, 8-cell and until the blastocyst, the most advanced stage, before being transferred to a recipient sow or gilt.

The benefits of this technology are multi-fold — it increases reproduction efficiency, accelerates genetic improvement, offers certain pig health advantages and provides important means for biomedical research.

To accomplish in vitro production (IVP) of embryos, oocytes were first aspirated from follicles 2-6 mm. in diameter from ovaries harvested at a slaughterhouse. Only oocytes with several layers of cumulus cells were collected under a microscope. After three washes in a special media, the oocytes were matured under 5% CO2, at 39°C (102°F) in a humidified tissue culture incubator for 44 hours.

The matured oocytes were then placed into fertilization media and in vitro fertilization was performed using percoll separated live sperm from freshly collected boars. Fifty oocytes in 500 microliters of fertilization medium were incubated with 500,000 spermatozoa for five hours.

Once the in vitro fertilization was completed, cumulus cells around the oocytes and the spermatozoa were removed from the fertilized oocytes, now called zygotes. The zygotes were then placed into embryo culture media for six days with the addition of 10% fetal bovine serum on Day 4 after fertilization.

Researchers tested two of the most widely used commercial embryo culture media (PZM-3 and NCSU-23) for their ability to support embryo development to blastocyst stage. The experiments were repeated more than four times with about 900 oocytes for each media.

Results showed that PZM-3 media supported higher embryo development to blastocyst stage than did the NCSU-23 media (20% vs. 6%).

These in vitro-produced embryos can be successfully transferred to hormonally prepared recipient sows or gilts, although successful pregnancy rates are less efficient than natural breeding or artificial insemination.

The IVP of pig embryos make it possible to propagate genetically superior sows and boars much faster. The value of these genetically superior pigs can be extended through cloning, followed by in vitro culture of the embryos and embryo transfer. Animal genomes can be modified (genes added or removed), making them valuable for biomedical research, production of biopharmaceutical proteins in pigs and, possibly, for the production of organs for human transplantation.

Researchers: Hongfeng Wang, Tribetta Spires and Erdogan Memili, Mississippi State University. Contact Memili by phone (662) 325-2937 or e-mail:

Don't Mix Sows 10-14 Days after Breeding

As pork producers take up the challenge to house gestating sows in groups vs. individual stalls, questions arise about the impact that mixing and grouping will have on farrowing rates and litter sizes.

In a joint Michigan State University/University of Guelph study, 617 newly bred, mixed-parity sows were assigned to individual stalls or grouped 15/pen. Each group of 15 unfamiliar sows was comprised of three sows at each of 2, 7, 14, 21 and 28 days after breeding. Sows were fed 5.5 lb. of standard gestation ration once a day, on the floor. Of the total, the 122 sows housed in individual stalls after breeding served as “controls” (Table 1).

Sows spent five weeks in groups and then were moved to individual stalls until they farrowed.

Neither the impact of grouping, nor the day of gestation they were grouped appeared to affect subsequent litter size. However, farrowing rate tended to be lower for sows mixed at 14 days of gestation.

“Mixing of strange sows into groups can be done without adversely affecting fertility,” researchers noted. “However, the period about the time of first signal for maternal recognition of pregnancy (10-14 days) is more sensitive to the stress of grouping strange sows and therefore should be avoided.”

The National Pork Board and the University of Guelph-Ontario Ministry of Agriculture, Food and Rural Development Animal Research Program funded the study.

Researchers: Glen Cassar, Monica Seguin, Tina Widowski, Abdolvahab Farzan, Adroaldo Zanella, and Robert Friendship, University of Guelph; and Roy Kirkwood, DVM, Michigan State University. Contact Kirkwood by phone, (517) 432-5198 or e-mail:

Table 1. Effect of Days after Breeding on Farrowing Rate and Litter Size when Sows are Mixed
Days after breeding No. of sows Farrowing, % Litter size (total) Litter size (live)
Control* 122 82.0a 11.6 ± 0.3 10.6 ± 0.3
2 98 77.5a 11.0 ± 0.4 10.2 ± 0.4
7 97 75.3a 11.2 ± 0.4 10.3 ± 0.4
14 101 72.3b 11.6 ± 0.4 10.7 ± 0.3
21 101 83.2a 11.4 ± 0.4 10.4 ± 0.3
28 98 82.6a 11.5 ± 0.3 10.6 ± 0.3
*Sows maintained in individual stalls.
abValues with different superscripts within a column tend to differ from control (P < .10; Chi square analysis)

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