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 pair] and placement). 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 mo 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 desirable to decrease production costs and is considered a good indicator of reproductive capability. Onset of puberty depends on a variety of factors, including genotype, liveweight, nutritional status, season, and management (including exposure to the boar). Exposure to a sexually mature boar, 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 number of senses stimulated by the boar decrease. Consequently, the boar effect is greatest with direct contact using a mature, sterile boar. Exposure of peripubertal gilts (5–6 mo old) to a mature boar for 10–15 min/day appears to provide an adequate stimulus. Along with the boar effect, other management tools to manipulate the onset of puberty include select crossbreeding, housing changes (eg, confinement to outside pens and vice versa), and forming new groups by mixing gilts from different pens of similar health status.
Strict culling criteria should be established for the gilt pool. Gilts in which first visible estrus does not occur by 136 kg body wt and 210 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 exogenous hormones to bring these reproductively inefficient gilts into estrus; if this is the case, the progeny should not be kept for breeding herd replacements. Gilts that have been serviced for two or three consecutive estrous cycles and do not conceive should also be culled.
Timing of estrus can be controlled by adding a progestagen to the feed (eg, altrenogest at 15–20 mg/day for 14–18 days). Estrus will be seen in gilts 4–9 days after the last feeding and with appropriate boar exposure. This allows estrus in gilts to be synchronized with that of a batch of weaned sows or formation of a group of gilts that will farrow together. Prostaglandins can also be used as an abortifacient to synchronize estrus when administered after day 12 and before day 55 of gestation; females generally come into heat 4–7 days later. The cost-benefit of these programs should be assessed before implementation.