Age at puberty in replacement gilts and wean to estrus interval in sows are each linked to litter productivity and retention in the breeding herd. Induction is a procedure used to advance estrus by days, weeks or even months. In gilts, boar exposure can accelerate brain maturation to help breed at targeted age, weight, and maturity. Boar contact for 15 minutes each day starting 3-4 weeks before expected puberty advances and tightens heat in batches of gilts.
Exposure too early will not achieve good synchrony and starting too late will not prevent delayed puberty or breeding older and heavier gilts. Optimal induction responses can be obtained using once daily physical boar contact with a pen of gilts. Proper boar:gilt ratio (≤1:15) is important, and if the ratio increases, extend the duration of daily exposure or the number of boars. Physical exposure using an intact or vasectomized boar provides the full range of stimuli. Control of high libido boars with panels, leashes, or mechanical moving devices is important to avoid stress and feet and leg injuries in the gilts.
Crossbred Meishan boars are popular since they are smaller, more docile, but still display a high level of libido. Fenceline boar contact is also effective when moving gilts or the boar to an exposure pen. Once, twice, or continuous daily fenceline exposure can each advance puberty, with better responses resulting from more frequent contact. For fenceline exposure to be effective, all gilts in the pen must have the space for close interaction to see, smell and hear the boar at the fence. Designed boar exposure areas can optimize fenceline exposure using multiple boars and pens. Sows also respond to induction after weaning or in late lactation if weaning is extended beyond 3 weeks. Daily fenceline exposure for 1 to 2 minutes for sows weaned into stalls, increases the proportion in estrus by days 4 to 6, and improves estrus in primiparous sows and periods of seasonal infertility.
Hormonal injection in prepubertal gilts and sows with a combination of eCG and hCG induces rapid follicle growth and estrus in 4 to 5 days. In gilts, treatment a few weeks ahead of puberty or in those that have failed to respond to 3-4 weeks of boar exposure can also be effective. Treatment of sows at weaning is sometimes used to minimize the risks of delayed estrus. Optimal responses to hormone treatment depend upon the animal receiving the full dose at the correct site and proper depth, and that dose leakage from the injection site does not occur.
Detection of estrus is characterized by the immobility reflex of the gilt or sow most often in the presence of a boar and with or without application of backpressure. When ovary follicles mature and release estrogen, it reaches the brain and allows behavioral expression of heat. The intensity and time it takes the female to stand when introduced to a boar depends on the stage of estrus. The quickest and strongest immobility reflex occurs during mid-estrus compared to just after the start or before the end of estrus. For detecting heat in gilts after moving to a new area, allow more time for them to explore before detection tests.
When checking sows for estrus in stalls, sometimes older sows and certain genetic lines may not show the obvious positive or negative reactions to the boar and backpressure test. In this case, use of back and flank rubbing, and noting assessing absence of vocalization, fixed head position, and a base ear reflex can help with proper identification. Consistent detection at a set interval and using the same procedures each day is important for accuracy, especially for gilts with short heats lasting only 1 to 2 days. In weaned sows, estrus is longer, but accurate detection for onset and end of estrus can be used for strategic timing of the first service and also prevent a late insemination after the end of estrus. Lastly, gilts and sows in estrus do stand continuously, and show refractory behavior lasting from only minutes to several hours. Refractory behavior becomes a problems when estrus cannot be detected or detection is delayed and often occurs with continuous boar exposure, housing boars too close or upwind, and when detection and breeding occur at the same time. This approach allows extended boar exposure times for females downwind and in adjacent rows well before they are ever checked for heat.