The successful development of replacement gilts requires the integration of several factors including health, nutrition, genetics, housing and management practices.

The facilities these potential sow herd replacements are housed in must meet several criteria:

* Enhance estrous stimulation;

* Maximize the percentage of gilts cycling at an appropriate age;

* Maximize the percentage of gilts showing regular estrous cycles;

* Optimize soundness of feet and legs;

* Optimize body condition and backfat;

* Prevent gilts from infecting the existing sow herd at time of entry;

* Enhance adequate exposure of gilts to farm microorganisms before breeding;

* Optimize the utilization of labor and,

* Optimize construction cost of facilities.

Because of the Porcine Reproductive and Respiratory Syndrome (PRRS) virus, numerous management practices and building schemes have been set up to manage the gilt pool to control PRRS virus shedding in the breeding herd.

Common Design Questions Regardless of the production system used to rear gilts (Figure 1), pork producers most often ask the following questions:

* Floor space - Although the minimum floor space needed to prevent a delay in reaching puberty has not been established, the floor space required at various weights is listed in Table 1. These space provisions should allow young gilts to mature properly.

* Gilts per pen - Scientists at the U.S. Meat Animal Research Center allotted gilts at 4.5 to 5 months of age to groups of 3, 9, 17 or 27 gilts/pen. The gilts were kept indoors and given 11.6 sq. ft./gilt. Gilts were fed daily 3.5 to 4 lb. of a 13% protein diet after 5 months of age.

Daily estrous checks (10 min./pen) with a mature boar were initiated when the oldest gilt reached 7 months of age and continued until the youngest gilt reached 9 months of age.

Gilts in pens of three had 12% to 16% fewer regular estrous cycles than gilts in pens of 9, 17 or 27 (Figure 2). There was no difference in the percentage of gilts showing regular estrous cycles as the number of gilts/pen increased from 9 to 27.

* Synchronization of estrus - Cycling gilts should remain in their pen and not be removed. Australian research found the synchrony of puberty in response to boar contact was significantly better when pubertal gilts remained in their pens for 5-15 days after exhibiting either first or second estrous periods, than when they were removed at the first sign of puberty. The differences were measured in terms of days to puberty and age at puberty. Boar exposure started when the gilts were 160 days of age.

* Photoperiod - The influence of photoperiod on age of puberty and proportion of gilts reaching puberty remains controversial.

Many scientific studies evaluating the effect of photoperiod on attainment of puberty used boar exposure to detect estrus; consequently, the true effect of photoperiod can't be determined.

Table 2 shows the proportion of gilts reaching puberty was greatest when gilts were exposed to mature boars, regardless of whether duration of daylight is increasing or decreasing. Neither the type of lighting (fluorescent or incandescent) nor intensity of light (lux) significantly influences puberty attainment or ovulation rate in gilts.

Moreover, it is most economical to maintain developing gilts under cool, white fluorescent light (270-500 lux at eye level of gilt) for 10 to 12 hours/day. Complete darkness delays puberty as compared to 11 hours/day of natural light.

* Ambient temperature - Elevated ambient temperatures have detrimental effects on puberty in gilts.

When the University of Missouri allotted crossbred gilts at 140 days of age to either a chronic heat stress (92 degrees F, 35% relative humidity, 12 hours light: 12 hours dark) or "optimal" environment (60 degrees F, 35% relative humidity, 12 hours light: 12 hours dark), more gilts reached puberty by 230 days of age under optimal conditions (90%) than under heat-stress conditions (20%). Average age at puberty was not statistically different (optimal, 204 days; heat-stress, 213 days).

The number of eggs ovulated at puberty were numerically greater in optimal conditions (12.1) than in heat-stress conditions (9.3).

All gilts not reaching puberty by 230 days of age, on both treatment regimes, received 1,000 IU of pregnant mare serum gonadotrophin (PMSG).

Gilts that were heat-stressed had a greater incidence of cystic follicles than those housed in optimal conditions (50% vs. 0%). Feed intake and average daily gain on a pen basis were not different between heat-stress and optimal conditions.

The effect of heat stress on behavioral estrus, ovulation rate and length of estrous cycle in post-pubertal gilts is controversial. In view of this controversy, it is advisable to protect replacement gilts from high environmental temperature (>85 degrees F) by providing adequate shelter and supplemental cooling to prevent severe stress.

The proportion of gilts cycling during the summer months is less than other times of the year (Table 3).

Therefore, the need to have more gilts for breeding during summer months can cause housing problems. To help increase the proportion of gilts cycling during the summer months, be sure to provide boar exposure.

* Air quality - The rearing of gilts indoors and exposing them to a gaseous environment may cause delayed puberty.

Purdue University found that 33% of gilts reared in an environment having 5-10 ppm ammonia reached puberty by 203 days of age, compared to 12% for gilts reared in an environment containing 20-35 ppm ammonia.

However, the proportion of gilts attaining puberty by 240 days of age was similar, as was their average age at puberty.

A study at the University of Nebraska evaluated the effect of a "dirty" environment (20 ppm ammonia gas concentration) versus a "clean" environment (< 10 ppm ammonia gas). The proportion of gilts showing first estrus by 240 days of age was 96.7% in the clean environment and 92.9% in the dirty environment. The dirty environment tended to delay age at puberty (200.5 vs. 192.8 days).

* Housing of boars - The exposure of prepubertal gilts to a mature, sexually aggressive boar will greatly reduce the age of the gilt at first puberty.

However, stock people need to be sure estrous gilts are not "refractory" to boar stimuli at the time of estrous detection. Gilts in estrus show a rigid and immobile response when they receive boar stimuli (sound and smell), but after a few minutes they need to relax their muscles (Table 4). Relaxation can start to occur at five to 10 minutes after receiving boar stimuli (the refractory stage). Once relaxed, it is much more difficult to detect estrus. Thus, to maximize the efficiency of detecting gilts in estrus, boars should be housed away from gilts to ensure gilts do not receive continuous boar stimuli prior to estrous detection.

* Housing gilts with sows - Although boar exposure is generally more effective to stimulate puberty in gilts than mature sow exposure, research shows that contact (continuous or 20 minutes/day) with mature sows can stimulate and synchronize the onset of puberty in gilts.

An Australian study evaluated the effect of exposing crossbred gilts (starting at 159 days of age) for 20 minutes/day to either an estrous gilt, estrous sow, anestrous gilt or anestrous sow on puberty attainment. The sexual status of the exposure female had no effect on the proportion of gilts reaching puberty.

However, the proportion of gilts reaching puberty was greater for gilts exposed to sows than those exposed to non-farm-raised gilts (Figure 3).

A similar study in Australia found that the percentage of gilts attaining puberty within 55 days of the puberty-inducing stimuli was greater for gilts exposed to estrous sows (80%) than anestrous sows (40%).

Research has not been conducted to determine whether the combination of boar and estrous sow exposure have an additive effect on puberty attainment.

* Estrous stimulation. To maximize the boar effect on stimulating puberty in gilts, a mature boar (10-11 months of age) with a high level of sexual behavior (mounting, chomping, nosing, chanting), should be placed in a pen of gilts two or three times daily. Attainment of puberty is 14 to 30 days quicker when gilts are exposed at 160 days of age to boars with a high degree of sexual behavior compared to low sexual behavior boars.

* Estrous detection - The facility in which gilts are provided boar exposure should be designed for easy movement of people and animals. Provide quick and easy-to-use latches, gates that cut off alleys when open and non-slick floor surface of alleys.

Research at the University of Nebraska found that the rate of estrous detection was not different when gilts are heat checked in stalls (67%) or pens (68%) with fenceline boar exposure. However, the rate of heat detection (fenceline boar exposure) was higher when gilts in pens were moved to the boar room (81%) than in gilts where heat detection occurred in their stall or pen.

Also, the immobilization response occurred more rapidly in gilts relocated to the boar room (1.7 min.) for heat detection than gilts heat checked in their stall (2.5 min.) or pen (2.0 min.). The University of Nebraska evaluated the accuracy of estrous detection of cycling gilts in response to either physical or fenceline contact with a boar in the boar room.

On the first day of estrus, 100% of the gilts receiving fenceline boar exposure were detected in estrus during the first five minutes, compared to 83.8% of gilts receiving physical boar exposure. But, within 15 minutes, 100% of the gilts receiving physical boar exposure were detected in estrus. And, interestingly, in gilts detected in heat with physical boar exposure, estrus was expressed about .6 days longer than gilts in which heat was detected with fenceline boar exposure.

* Maintaining estrous cyclicity - Australian research has suggested that boar contact is required during the post-pubertal period to maintain normal estrous cycles in gilts. The proportion of gilts found cycling over three cycles was 97% for gilts receiving daily boar exposure versus 66% for gilts not exposed to boars.

* Gilt housing - Replacement gilt facilities need to be designed so cycling and acclimating gilts can be easily exposed to mature boars. The facility in Figure 4, specifically designed for replacement gilts, provides numerous advantages. All gilts entering this facility must first go through an isolation protocol.

* One end houses acclimating gilts and gilts eligible for mating, the other end houses bred gilts in stalls. Boars and mating pens are placed between the two sections. When artificial insemination is used, old, sexually aggressive boars are housed in the boar room. Eligible gilts are moved into the boar room which contains a high level of boar stimuli. When in the boar room, gilts can receive either fenceline or physical boar contact.

Acclimating gilts are provided boar stimulation to induce puberty by either taking a boar to their pen or moving the gilts to the boar room. Boars can be easily taken to the bred gilt section to check for recycling. Because the boar room is totally enclosed, the proper ambient temperature for natural service boars can be maintained year around.