Sow and Gilt Management

Gilt selection for genetic improvement should be indexed based on such categories as growth rate, disease status, sexual development, reproductive history (including dam’s performance as to wean-to-service and wean-to-estrus intervals, litter size, milking ability, and pigs weaned), structure/conformation, and underline (including teat number [7–8 pairs] and placement). Replacement gilts can be selected from dams, either from primiparous sows that have had large litters or from multiparous sows that have had large litters and short wean-to-first-service intervals, and that have produced a litter to that service. Selection should be for average- to above-average-weight gilts within a large litter and not heavy gilts from a small litter. Of potential replacements, up to 30%–40% may be culled, with most eliminated because of problems with structure/conformation, teat issues, and genital defects. Prepubertal gilts are usually fed a sex-specific ration ad lib until they reach market weight (250–275 lb [113–125 kg]) or are 5–6 months old. At that time, selected animals are then 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 cyclicity.

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. Brought-in gilts should be isolated for a minimum of 45–60 days, during which visual observation and serial testing (ie, serology, oral fluids) for exposure to undesirable infectious diseases should be done. To minimize the number of days for introduction of these gilts into the breeding herd, the latter portion of the isolation period can be used for acclimatization to the herd’s resident pathogens through the introduction of cull sows, internal grow-finish hogs, and manure exchange and/or feedback. This natural exposure to endemic herd pathogens can provide essential protection against diseases such as PRRS, parvovirus, and influenza.

Early puberty is considered a good indicator of reproductive capability, associated with increased lifetime fertility and decreased production costs. Onset of puberty depends on a variety of factors, including genotype, liveweight, nutritional status, season, and management (including exposure to the boar). The effect that exposure to a sexually mature boar has on a sow or gilt, also known as the boar effect, is the most influential of all management factors. The boar effect is strongest when females are exposed to the sight, sound, touch, and smell of a mature boar, and it decreases as the intensity of each of these stimuli decreases. Consequently, the boar effect is greatest with direct contact using a mature, sterile boar; however, it can also be effective when using proper fence-line exposure. Regardless of the method chosen as most practical for the operation, close interaction of the boar with the females for the appropriate duration of time will produce the greatest effects on puberty advancement and estrus expression. Exposure of peripubertal gilts (5–6 months old) to a mature boar for 10–15 minutes/day appears to provide an adequate stimulus. Along with the boar effect, other management tools that can advance the onset of puberty include crossbreeding, gilt relocation to novel housing (eg, confinement to outside pens or to another inside pen), and forming new groups by mixing gilts from different pens of similar health status. However, repeated mixing of gilts can induce excessive stress and injury from fighting; thus, the practice should be minimized.

Strict culling criteria should be established for the gilt pool. Gilts in which the first visible estrus does not occur by 136 kg body wt and 220 days of age should be culled. Initial estrus in gilts may be weak, so a robust estrus detection program with experienced personnel is essential. Some producers may elect to use approved exogenous gonadotropin hormones to induce a synchronized estrus in gilts or to induce estrus in delayed-puberty gilts. 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 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/day for 14–18 days). Estrus typically occurs 4–9 days after the last feeding of the hormone, when daily boar exposure is provided. This technique allows estrus and breeding in gilts to be synchronized with that of a batch of weaned sows and is used to form a group of gilts that will farrow together. 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, females in later stages of gestation will require another cycle before their uterus is suitable for breeding. The cost-benefit ratio of these programs should be assessed before implementation.