The purpose of this article is to highlight the key components of a commercial off-site gilt development unit designed and managed to consistently meet the target of identifying 70% of gilts entered as “select” and breeding eligible.

“Select” in this context indicates that gilts have a recorded heat-no-serve event in the GDU and a known weight at HNS. Using this information, gilts can be shipped to the downstream sow farm on a weekly basis, with the expectation that all gilts will undergo a 14-day acclimation period before being bred at the next observed heat.

The detailed information on which this more practical summary is based can be found in the following publication: Patterson et al., 2016. J. Anim. Sci., 94, 1–11. Collectively, the information accumulated from this large commercial study indicates that effective GDU management and rigorous selection of breeding eligible gilts removes some of the key constraints to gilt and sow retention in the breeding herd.

Management and GDU facility
The results summarized here are based on the performance of more than 4,000 pre-pubertal gilts from approximately 160 days of age until culling or rebreeding at fourth parity. Weaned gilts with acceptable growth and conformation were relocated to a single nursery and housed in pens of approximately 25 gilts. Floor space allowance was 0.2 meter squared per gilt until four to five weeks of age and 0.25 meter squared until 10 to 11 weeks of age. At approximately 70 days of age, gilts were relocated to a single gilt grower/finisher barn until approximately 135 days of age and housed in pens of 25 with a space allowance of 0.7 meter squared per gilt. Starting at approximately 135 days of age, a preselection process identified successive batches of approximately 288 potential replacement gilts for relocation to a single off-site GDU facility.

Typical recorded reasons for “non-selection” for entry to the GDU were small size for age, lameness, structure and health. At the GDU, gilts were housed in pen groups of 14 on partially slatted floors and provided at least 1.2 meters squared space per gilt throughout the gilt development and puberty stimulation phases. All gilts were allowed ad libitum access to water and to diets from a single two-space feeder in each pen.

Each half of the off-site GDU provided adequate penning for successive cohorts of 288 gilts for a six-week period. During the first two weeks of acclimation to the GDU, routine management practices included vaccinations and unique ear tagging of gilts. A boar exposure area was located on each side of the GDU and was used for the stimulation and detection of estrus. The BEAR consisted of a row of eight stalls in which an ongoing rotation of mature boars was individually and continuously housed. The stimulation pens located on either side of the boar stalls each allowed adequate space (3.4 by 4.9 meters) for effective direct contact between a single boar and a group of gilts. This enables effective “soliciting” (head-to-head contact that allows gilts to access the salivary pheromones produced by the boar) of boars by the gilts when a gilt-to-boar ratio of no greater than 15:1 per side was maintained during stimulation.

Natural protocols

Starting at approximately 160 days of age, groups of gilts were taken daily to the BEAR and received at least 15 minutes of direct exposure to a single boar, as well as fence-line exposure to additional boars facing that particular stimulation area. Physical signs of pending estrus in gilts, such as redness, degree of swelling and mucosal discharge from the vulva, were recorded daily as an aid to the successful detection of pubertal estrus and identification of truly non-cyclic “opportunity” gilts that were candidates for PG600 treatment. Only the observation of full standing estrus in the presence of a boar was accepted as a record of a natural pubertal estrus. Regrouping of non-estrous gilts at Day 14 after the start of stimulation was used as an additional “stress” stimulus for triggering a natural HNS event.

PG600-induced estrus
If the number of gilts with a natural HNS record did not meet selection targets by Day 23, a PG600 injection was used to stimulate pubertal estrus in available opportunity gilts. PG600 use was strictly managed, and only gilts with no evidence of previous ovarian activity (no vulval scores recorded in the first 23 days of the stimulation program) were considered to be truly pre-pubertal and eligible for treatment. Intensive contact with boars in the BEAR continued until days 28-30, after which groups of noncyclic gilts received daily boar contact until they were either culled or recorded in estrus and shipped to the sow farm on Day 35. The weight of all gilts was recorded at natural or induced pubertal estrus, using either a scale or a purpose-designed weight tape.

Sow management on delivery
During the present study, gilts were delivered to a single off-site breeding farm every three weeks and were considered breeding eligible. The prebreeding protocol required a minimum of 14 days of crate acclimation and full feeding until gilts were bred by standardized artificial insemination procedures using boars with proven fertility. At least one to two days before farrowing, all gilts were transferred to a designated downstream gilt farrowing barn.

After weaning, Parity 1 sows were transferred to off-site breeding farms, with NAT and PG600 gilts equally distributed to these farms, although sow farm staff had no knowledge of the origin of the sows they received (NAT or PG600-induced gilts).

This protocol addresses three key issues that dictate the effectiveness of gilt-selection programs. Firstly, limiting the time from the start of gilt stimulation to a recorded HNS event (30 days starting at 160-170 days of age) usually allows between 40% and 60% of gilts to display a boar-induced natural heat in the first three weeks of the daily stimulation program, taking advantage of the reported link between earlier sexual maturity in gilts and improved sow lifetime productivity.

Secondly, given the excellent growth performance of gilts under the ad libitum feeding systems typical of the North American industry, identifying select and breeding eligible gilts by 200 to 210 days of age allows gilts to be bred at second estrus without exceeding a targeted breeding weight of 135 to 160 kilograms (300 to 350 pounds).

Thirdly, the GDU takes total responsibility for identifying and shipping breeding-eligible gilts to the sow farm, with an almost 100% expectation that these gilts will be bred (see Table 2). This limits gilt nonproductive days by strictly controlling the entry-to-service interval.

Performance and retention
Overall, some 78% (n = 3,475) of gilts exhibited standing estrus (NAT = 2,654; PG600 = 821) within 35 days of starting the puberty stimulation program, varying from 64% to 90% among successive groups (Figure 2).

The overall percentage of gilts responding to boar stimulation and remixing alone with a spontaneous “natural” pubertal estrus was 59%, but ranged from 37% to 73%. As a normal management strategy, the potential shortfall in breeding-eligible gilts with a natural HNS was compensated by treating a variable number of gilts with PG600, and overall, 25% of all gilts received PG600 injections, with a range of 9% to 49% in successive groups. Overall, 77% of gilts selected for treatment with PG600 had a recorded HNS event. After removal of gilts for issues such as health, injury, missing records and death, some 72.0% (n = 2,374 NAT; n = 741 PG600) of gilts entering the GDU were actually delivered to the sow farm as breeding-eligible females with a recorded HNS, meeting the 70% selection target for this commercial GDU.

Mean age at puberty and days to puberty were less for NAT than for PG600 gilts (Table 1). PG600 gilts were heavier at puberty detection, but grew slower than NAT gilts and the timing of PG600 injection relative to the GDU exit date dictated that the days from puberty to GDU exit were fewer for PG600 gilts.

As a consequence of the protocol implemented, PG600 gilts were older at puberty and variance in age was smaller than that of NAT gilts. All gilts without a recorded natural puberty event, or those treated with PG600 and not responding within 10 days of treatment, were considered “non-select,” and protocol dictated that they were not transferred to the sow farm.

The use of a purpose-designed BEAR system maximizes the components of the “boar effect” known to be effective in the stimulation of puberty, with full physical contact between the boar and the pen of gilts, a gilt-to-boar ratio of <15:1, and fence-line contact with additional boars during the daily stimulation periods. Furthermore, as can be seen in Figure 2, the remixing “stress” imposed in non-pubertal gilts at Day 14 is marginally effective in inducing the rate of HNS records between Day 14 and Day 23.

Because of the three-times-weekly delivery schedule of gilts from the GDU to the downstream sow farm during this study period, and the requirement for a 14-day acclimation period before breeding, more PG600-treated gilts were bred at their second estrus than natural gilts, and the entry-to-service interval was also lower for PG600 than NAT gilts. However, gilts consistently returned to their next heat at approximately 21-day intervals, regardless of whether they were bred at second, third or fourth heat, suggesting that a well-managed GDU program with reliable records essentially removes any issue with “hard-headed” gilts that are sometimes reported as a problem in the industry.

Following completion of the study summarized here, gilt shipments to the sow farm have become weekly events, ensuring that essentially all gilts are now bred at second estrus and incurring minimal non-productive days, even with a 14-day acclimation program in place. However, as shown in Figure 3, delaying breeding to third estrus when the GDU-to-sow farm delivery schedule was not ideal carried the high risk of gilts being too heavy (>160 kilograms or 350 pounds) at breeding. In turn, this carries the risk of being culled as mature sows because of issues with lameness and other mobility issues.

As most gilt selection programs in the North American industry involve gilts reared under ad libitum feeding regimens, inefficiencies in identifying select gilts, and then flowing them through an adequate crate-acclimation program before breeding, will be associated with an increasing proportion of gilts being overweight when bred. Gilts being too heavy at breeding due to inefficient GDU protocols is considered to be a much more serious issue for today’s industry than a concern that gilt growth performance might be limiting effective boar-induced stimulation of a pubertal heat.

However, even using this optimized boar stimulation program in a well-managed GDU, large differences in the accumulated percentage of naturally induced estrus were observed by Day 23 in the successive cohorts of gilts. It is impossible retrospectively to identify the cause of this variation, but variable growth rate, season, health status and unknown litter-of-origin effects may be involved. Given this variation in natural HNS responses, PG600 treatment of a variable number of opportunity gilts seems to be an essential part of an efficient all-in/all-out GDU program. It should be emphasized that the good responses to PG600 treatment are partly the result of targeting treatment at truly non-pubertal gilts. Additionally, these PG600 treatments followed 23 days of boar stimulation and based on the dynamics of the pubertal responses shown in Figure 4, were given to gilts that would have been expected to reach a natural HNS within the next 20 days with continued boar stimulation taking place.

Finally, delaying PG600 treatment to Day 23 of the stimulation protocol ensures that any gilts that had reached pubertal estrus before the start of the stimulation program would be recorded in second estrus before intervening with PG600 treatment.

Considering all gilts delivered to the sow farm, a greater proportion of NAT than PG600 gilts were successfully served (Table 2), associated with a greater proportion of PG600 (3.7%) than that of NAT (1.9%) gilts being culled for failing to show estrus. As a proportion of all gilts delivered to the sow farm, pregnancy and farrowing rates to first service and overall farrowing rate (including gilts that returned and were rebred) were also greater for NAT than for PG600 gilts. Farrowing rate at second and third parity was similar between NAT and PG600 gilts. However, by fourth parity, a greater proportion of NAT gilts successfully farrowed.

Considering only the gilts actually served, there was no difference in the proportion of NAT and PG600 gilts farrowing a third litter. However, a greater proportion of NAT than PG600 gilts farrowed their fourth litter. There was no difference between NAT and PG600 gilts for litter size at Parity 1 through 4 or total pigs born over four parities. Over their lifetime, more PG600 gilts were culled for locomotion and condition problems.

Interestingly, in Parity 1-weaned sows, the weaning-to-estrus interval was shorter, and the percentage of sows bred within seven days was greater in NAT than in PG600 gilts. WEI was also longer in weaned Parity 1 than in Parity 2 and 3 sows. We interpret these differences as indicating intrinsic differences in the rate of sexual maturation in the PG600-treated gilts and decreased sensitivity of the brain-pituitary-ovarian to endogenous (after weaning the first litter) and exogenous (PG600 to induce puberty) gonadotropic stimulation. In commercial practice, special attention should be paid to optimizing the stimulation and estrus checking protocols in Parity 1 sows after PG600 induction of pubertal estrus.

Final discussion
The results summarized here provide a solid basis for suggesting that an investment in effective GDU management sets up a supply of high-quality breeding-eligible gilts that lays the foundation for good breeding herd performance. Various benchmarks of excellent gilt performance have been suggested, including farrowing rates to first service >80%, >12.5 total born in the first litter, 85% of gilts entered farrowing a first litter, >70% of gilts initially served farrowing a third litter, no “second-parity dip” and >50 pigs weaned per sow lifetime.

It is also generally accepted that a sow needs to farrow three litters to cover her costs as a replacement female. The gilts delivered by the GDU selection program implemented in the present study met or exceeded these expectations.