Sow and Gilt Management

Gilt selection for genetic improvement and fertility, when possible, should be on the basis of dam measures for age at puberty, litter size, and average litter birth weight. Selection of the individual gilt should include litter size, birth weight, weaning weight, growth rate to 160 days old, lifetime health status, age at puberty, and days to estrus after start of boar exposure.

The reproductive history of the dam can also include her parity, wean-to-service interval, history for litter size in pigs born alive, milking ability, pigs weaned, body and leg structure/conformation, and underline (also called the milkline or mammary system, which includes teat number [7–9 pairs] and nipple structure and placement). Replacement gilts can be selected from primiparous or multiparous sows that have had large litters and short wean-to-first-service intervals and that have produced a litter to first service (see pig litter image).

Litter, pig

Image

Replacement gilts should be selected on the basis of dam's traits and current litter measures.

Courtesy of Robert Knox, University of Illinois.

Selection should be for average- to above-average-weight gilts within a large litter and not the heaviest gilts, which often are the result of smaller litter sizes. Of potential replacements, up to 30% can be culled, with most eliminated because of problems with slow growth, poor structure/conformation, teat issues, delayed puberty, and genital defects.

Prepubertal gilts are usually fed a sex-specific ration ad lib until they reach market weight (250–275 pounds [113–125 kg]) or are 5–6 months old. At that time, selected animals are moved into gilt development, where they are fed a diet formulated specifically for introduction into the breeding herd and are exposed to boars to stimulate estrus.

When group housing of animals in gestation is used, an electronic sow-feeding system requires specialized training and adjustment periods during gilt development. Some systems also use electronic estrus detection with sensors in the ear recording time spent near the boar.

Porcine reproductive and respiratory syndrome (PRRS), parvovirus, porcine circovirus type 2, porcine epidemic diarrhea virus, pseudorabies (Aujeszky disease), Japanese encephalitis, influenza, brucellosis, chlamydiosis, leptospirosis, and other infectious diseases can directly or indirectly affect reproductive performance depending on animal age at infection and stage of gestation.

The reproductive herd (gilts, sows, and boars) should be vaccinated, at a minimum, against leptospirosis, parvovirus, and erysipelas. Gilts purchased or brought in from outside the farm should be isolated and acclimated for a minimum of 45–60 days for visual observation and serial testing (ie, serological testing, oral fluids) to identify clinical signs of undesirable infectious diseases.

To minimize the number of days for introduction of gilts into the breeding herd, the latter portion of the isolation period can be used for acclimatization to the herd’s resident pathogens through exposure of the new gilts to cull sows, internal grow-finish hogs, and feces exchange and/or feedback. This natural exposure to endemic herd pathogens can provide essential protection against diseases such as PRRS, parvovirus, and influenza. During these periods, it is expected that gilts will be exposed to endemic pathogens and develop a healthy immune response. However, they should not become overwhelmed by infectious pathogens to the point they show clinical signs of severe illness.

Early puberty is considered a good indicator of reproductive capability and is associated with increased lifetime fertility and decreased production costs. Gilts that respond to boar exposure within the first 4 weeks tend to have greater lifetime productivity and longevity in the herd.

Onset of puberty depends on factors including genotype, growth rate, body weight and backfat (indicative of body condition) at 160 days old, diet formulation and feed allowance, housing, season, environmental management, and puberty induction methods.

The effect that exposure to a sexually mature boar has on a gilt, also known as the boar effect, is the most influential of all management factors in inducing puberty in well-developed gilts. The boar effect is strongest when females are exposed once or twice daily to the sight, sound, touch, and smell of a mature boar, and the effect decreases as the intensity of each of these stimuli is limited. Continuous exposure is not recommended, as this decreases the stimulation effect when compared to novel introduction and can lead to increased refractory behavior.

The boar effect is greatest when providing direct or close fenceline contact with a mature boar. When using direct contact, the boar should be sterilized, and if fenceline exposure is chosen alone or in combination with direct contact, close contact and time of exposure should be optimized. There are available data that suggest that a dedicated boar exposure area that can provide both fenceline and physical exposure can have advantages for early puberty induction (1).

Regardless of the most practical method chosen for boar exposure, close interaction of the boar with the females will produce the greatest effects on puberty advancement and estrus expression. Exposure of prepubertal gilts (5.5–6 months old) to a mature boar for 10–15 minutes per day and continuing daily for 4 weeks will provide adequate stimulus (2). This also requires that most of the gilts be in proximity to the boar. Most farms begin boar exposure at 150-170 days old and continue exposure for the next several weeks.

Along with the boar effect, other factors that advance the onset of puberty include use of crossbreeding, gilt regrouping, and relocation to a new pen with start of boar exposure. However, the remixing approach should be limited, as repeated mixing of gilts can induce excessive stress and injury from fighting while they establish a new social order.

Strict culling criteria should be established for the gilt pool. Gilts in which the first estrus is not detected by 140 kg body weight and 230 days old after various intervention strategies should be culled. Initial estrus in gilts might be weak or last only one day, so a robust estrus detection program with experienced personnel and use of best procedures is essential. Some producers elect to use approved exogenous gonadotropin hormones to induce a synchronized estrus in gilts or to induce estrus in delayed-puberty gilts.

On most pig breeding farms, without an observed estrus, producers must assume that their gilts are still prepubertal, that the animals have cycled and the farm failed to identify them, or they cycled without displaying estrus (silent estrus or heat).

Producers can choose to induce gilts, and if the gilts are less than 200 days old, the likelihood is high that the animals are still prepubertal and will display estrus within 5 days. As gilts age past 210 days, the likelihood that they have cycled without expressing estrus increases. Treatment of these gilts is still possible, and if they are still prepubertal, they will express estrus in 5 days. However, if the gilts are cycling, the treatment will induce follicle growth without ovulation, which will likely only delay estrus for 1-3 weeks.

Using PMSG-hCG hormones to induce puberty and estrus within 5 days is most effective when gilts have not been detected in estrus after 3-4 weeks (3). This is most effective in young gilts less than 190 days old. However, using PMSG-hCG in anestrus gilts after 3-4 weeks of boar exposure has been shown to be quite effective in inducing gilts less than 200 days old. After this age, and following 5-7 weeks of boar exposure, the likelihood that the gilt is cycling increases, and treatment will likely delay estrus for some weeks. In this case, another approach is to use prostaglandin and check estrus for one week, then give another prostaglandin treatment for the remaining anestrus gilts, and give PMSG-hCG the next day.

If the gilts were cycling, the first or second prostaglandin injection could induce luteolysis in the corpora lutea, and the PMSG-hCG treatment will initiate a new follicle phase and estrus in 5 days. However, the female progeny of delayed-puberty gilts should not be kept for breeding herd replacements. Gilts that have been serviced for two consecutive estrous cycles and still do not conceive should also be culled.

The timing of estrus can be synchronized in a group of mature cycling gilts or in an individual gilt by adding an approved synthetic progestagen to the feed (eg, altrenogest at 15–20 mg/gilt/day for 14–18 days) (4, 5). Ensuring that the females consume their dose each day is required for effectiveness, and the product will only work if the gilts have previously shown estrus.

Estrus typically occurs 5–8 days after the last feeding of altrenogest, when daily boar exposure is provided. This hormone can synchronize estrus in a group of replacement gilts to coincide with the breeding of the weaned sow group to create a uniform group that will farrow within a few days of each other.

Although typically used only when there are excess numbers of pregnant females, prostaglandins can also be used as an abortifacient to synchronize estrus when administered after day 12 and before day 55 of gestation. With this procedure, the females generally come into heat 4–7 days after prostaglandin administration. However, although females in later stages of gestation will respond, they require another cycle before their uterus is suitable for breeding. The cost-benefit ratio of these programs should be assessed before implementation.

• Kraeling RR, Webel SK. Current strategies for reproductive management of gilts and sows in North America. J Anim Sci Biotechnol. 2015;6:3(1):1-14.

• Knox RV, Arend LS, Buerkley AL, Patterson JL, Foxcroft GR. Effect of physical or fenceline boar exposure and exogenous gonadotropins on puberty induction and subsequent fertility in gilts. J Anim Sci. 2021:99(12):skab348.

1. Patterson J, Foxcroft G. Gilt management for fertility and longevity. Animals. 2019;(7):434. doi:10.3390/ani9070434

2. Patterson JL, Willis HJ, Kirkwood RN, Foxcroft GR. Impact of boar exposure on puberty attainment and breeding outcomes in gilts. Theriogenology. 2002;57(8)2015-2025. doi:10.1016/s0093-691x(02)00674-x

3. Patterson J, Triemert E, Gustafson B, et al. Validation of the use of exogenous gonadotropins (PG600) to increase the efficiency of gilt development programs without affecting lifetime productivity in the breeding herd. J Anim Sci. 2016;94(2):805-815. doi:10.2527/jas.2015-970510.2527/jas2015-9705

4. Wang Z, Liu BS, Wang XY, Wei QH, Tian H, Wang LQ. Effects of altrenogest on reproductive performance of gilts and sows: a meta-analysis. Anim Reprod Sci. 2018;197:10-21. doi:10.1016/j.anireprosci.2018.08.035

5. De Rensis F, Mazzoni C, Saleri R, Scollo A, Plush KJ, Kirkwood RN. Effect of duration of altrenogest treatment on farrowing rate and litter size of gilts. Anim Prod Sci. 2018;58(11):2029-2031. doi:10.1071/AN17114