Gilt selection should be based on 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), conformation, and underline (including teat number and placement). Of potential replacements, up to 30–40% may be culled because of problems such as delayed puberty, failure to conceive, defective teats, locomotor problems, or vulval abnormalities (indicative of intersexuality or genital hypoplasia). Prepubertal gilts are usually fed a grower-finisher ration ad lib until they reach 150–200 lb (68–90 kg) or are 3–5 mo old. At that time, they are usually separated and placed in gilt development, where they are fed a diet formulated specifically for replacement gilts.
Gilts selected for breeding should have exhibited estrus cyclicity by 136 kg body wt. They should not have excessively straight legs or muscling. Their external genitalia should be well developed by 5 mo of age, and their udders and milk pads should be well developed with at least 6 pair of evenly spaced teats.
Porcine reproductive and respiratory syndrome (PRRS), parvovirus, porcine circovirus type 2 infection, pseudorabies, influenza, brucellosis, chlamydiosis, leptospirosis, and other infectious diseases can affect reproductive performance. 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 serologic testing 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, market 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. First estrus usually is seen between 5–8 mo of age depending on 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 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 (5–8 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 for manipulating the onset of puberty include crossbreeding, changes in housing (eg, confinement to outside pens and vice versa), and forming new groups by mixing gilts from different pens. Under some management systems, gilts are not served until their second or third estrus to allow for uterine development and aid in an optimal ovulation rate and litter size.
Strict culling criteria should be established for the gilt pool. Gilts in which first visible estrus does not occur by 136 kg body wt should be culled. Some producers may elect to use injectable gonadotropins 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 3 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. 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 longterm implementation.
Last full review/revision July 2011 by Gary C. Althouse, BS, DVM, MS, PhD, DACT