At What Age Should Gilts be Exposed to Boars?

Ronald O. Bates, State Swine Specialist at Michigan State University describes research from Brazil relating the age and weight of gilts when they are first exposed to the boar on the attainment of puberty. The article is published in MSU Pork Quarterly.
calendar icon 8 July 2009
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Much of the work determining age in puberty in gilts was completed decades ago. Since that time pigs have become leaner and later maturing. However, much of the research regarding attainment of puberty has not been repeated to determine how onset of puberty may have changed over time. Recently a large study (Amaral Filha et al., 2009) evaluated gilt age at initial boar exposure in relation to attainment of puberty.

Study Description

This study was completed during a start-up of a 2,400 sow farm in Brazil. Gilts were delivered to the farm in approximate 200 head lots, with 12 groups delivered to the farm. Eight of those groups, 1,486 gilts, were included in this study. Gilts were PIC Camborough 22® crossbreds. Ambient temperature during this study ranged from 86-97o F for day-time highs and 45-65o F for overnight lows. When temperature within the barn reached 77o F, a cooling regimen that included increased ventilation and misting was initiated. Within 3 days of delivery, gilts were weighted and backfat was measured. Gilt age at delivery was 147 ± 8.3 days. Estrous detection commenced within the first 5 days after delivery. The first observed estrus after delivery was designated as a gilt’s pubertal estrus. Estrous detection continued after gilts attained their first observed estrus to determine their recycling rate. Gilts were classified by age at delivery into two groups, Younger (130-149 days of age) and Older (150-170 days of age), and within each age group, growth rate, calculated as weight per day of age (WDA) was classified into three different groups (Table 1).


There was an age at puberty difference due to gilt age at initiation of boar exposure (Table 1). Gilts that began boar exposure at a younger age did have a younger age at puberty (162.2 days of age) than gilts that were older at first boar exposure (172.3 days). Within the younger age boar exposure group, there was a different in age at puberty dependent on the gilt’s growth rate classification. Gilts that were faster growing were approximately 3.5 days younger at puberty than gilts that were intermediate or slower growing. However age at puberty was not influenced by growth rate among gilts that were approximately 20 days older when boar exposure first began.

There was a difference in synchrony of estrous achievement dependent on age at when boar exposure began. Within the first 10 days of boar exposure, 44.5% of the gilts that were 150 to 170 days of age at first boar exposure were observed in estrus, compared to 30.9% of gilts that were 130 to 149 days old at first boar exposure. There was a difference in synchrony of estrus due to growth rate classification among gilts that were exposed to boars at 130-149 days of age. Among faster growing gilts, 38.1% were observed in heat compared 28.5% of those that were classified as intermediate or slow growing.

This difference in attainment of estrus persisted through 30 days of estrous detection. For gilts that were 150-170 days old at first boar exposure, 82.4% were observed in estrus after 30 days of boar exposure. However, for gilts which were 130-149 days of age when first exposed to boars, only 70.5% had an observed estrus within the first 30 days of heat detection. This discrepancy in percentage in estrus by age classification at first boar exposure was still persistent, somewhat at 40 days after first boar exposure (Table 1).


Once gilts did exhibit an initial estrus, 97% did achieve a second recorded estrus. Percentage of gilts recycling did not differ between the two groups that differed in age at first boar exposure. However, there were differences in recycling rate when comparing the three different growth rate groups. Low growth rate gilts (1.2-1.4 WDA) had a recycling rate of 94.3% whereas gilts that were classified as Intermediate (1.43-16.0 WDA) or high (1.61-1.83 WDA) growth rate recycled at a rate of 97.8%.


This study demonstrates that a high percentage of maternal gilts (92%) should achieve estrus, if boar exposure and estrous detection are provided adequately. Gilts that are older at first boar exposure will be more synchronous in the expression of first estrus than gilts that are younger at first boar exposure. If within the herd gilt introduction plan, the objective is to breed gilts at their second or later estrus, initiating boar exposure at a later age will allow for a more synchronous pubertal estrus. This should lead to more predictable percentages of gilts to be introduced into the breeding herd. If gilts are to be mated at a specified weight within the gilt introduction program, initiating boar exposure at a younger age may be more desirable so that gilts may have progressed through more estrous cycles before initial mating.

July 2009

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