J. Anim. Sci. 2004. 82:3088-3093
© 2004 American Society of Animal Science
Effect of housing system and boar exposure on estrus expression in weaned sows1,2
R. V. Knox3,
S. M. Breen,
K. L. Willenburg,
S. Roth,
G. M. Miller,
K. M. Ruggiero and
S. L. Rodriguez-Zas
Department of Animal Sciences, University of Illinois, Urbana 61801
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Abstract
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Reproductive efficiency depends on detection of estrus, which may be influenced by housing and boar exposure. This experiment investigated the effects of housing system and boar contact on measures of estrus in weaned sows. Mixed-parity sows were randomly assigned to be weaned into gestation crates away from boars (AWC, n = 45), into pens away from boars (AWP, n = 42), or into pens adjacent to a mature boar (ADJ, n = 46). Estrus detection was initiated at approximately 0700 (0 h) and again at 0.25-, 0.5-, 1-, 2-, 4-, and 8-h intervals beginning on d 4 and continuing through d 7 following weaning. Estrus detection involved observation of the standing response after application of nose-to-nose boar exposure, backpressure, and side rubbing. For the AWC sows, a mature boar was moved to the front of the crates for a 10-min period and then removed. Sows housed in AWP were moved approximately 15 m to an empty pen adjacent to a mature boar for a 10-min period, and then returned to their pen. Sows housed ADJ were not moved and estrus detection was performed in their home pen for a 10-min period. The proportion of sows expressing estrus within 7 d from weaning was lowest for ADJ (80%, 37/46) compared with AWP (98%, 41/42) and AWC (96%, 43/45; P < 0.05). There was an effect of interval from weaning to estrus on the percentage of sows expressing estrus, but there was no interaction with treatment. Sows in AWC and AWP (4.7 d) had decreased (P = 0.01) intervals from weaning to estrus compared with ADJ (5.2 d). The duration of estrus was also shorter (P < 0.001) for ADJ (45 h) compared with AWC (58 h) or AWP (62 h). There was a treatment x interval x day of estrus effect for the percentage of sows expressing estrus. After detection of the first standing response on the first day of estrus, only 62 to 82% of sows were detected standing over the next 2 h for all treatments. However, at 4 to 8 h, this increased to 85 to 98% for the AWC and AWP sows, but <73% of the ADJ sows were detected during this period. On the second day of estrus, estrus expression was not influenced by interval for the AWC and AWP sows and was between 90 to 100% during the 8-h period, whereas ADJ sow detection rates were between 68 to 88%. These data suggest that housing sows adjacent to boars negatively affects estrus expression and detection. In addition, refractory behavior occurs in approximately 30 to 40% of sows and is influenced by housing relative to the boar, day of estrus, and interval from last boar exposure.
Key Words: Boar Exposure • Crates • Estrus • Housing • Pens • Sows
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Introduction
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Detection of estrus is essential for timing inseminations (Soede et al., 1995
), and factors that decrease accurate detection could decrease the percentage of sows mated within optimal times in the 7 d after weaning. Estrus may be influenced by housing proximity to boars. In crate housing, sows are typically isolated from boars, except during estrus detection. In pen housing, some sows may be housed adjacent, whereas others are housed away from boars. Each system has advantages and disadvantages, but their effect on estrus detection remains unclear.
There are conflicting reports for the effects of boar housing on estrus in sows. One study reported that continuous exposure had no effect on interval to estrus, but did decrease expression and duration of estrus (Dyck, 1988). Another observed that continuous exposure had no effect on sows mated after weaning, but estrus duration was shortened and single services increased by 8% (Hemsworth and Hansen, 1990). However, other reports suggest that adjacent boar housing improved return to estrus in weaned sows (Walton, 1986; Pearce and Pearce, 1992). The reasons for the conflicting results are uncertain, but they could be due to differences in the intensity and frequency of boar exposure. For example, increasing the intensity of boar exposure did not affect estrus expression or the interval from weaning to estrus, but did decrease the duration of estrus (Langendijk et al., 2000). In addition, in cases where estrus expression was >90% in weaned sows, once-daily exposure improved estrus expression compared with twice-daily exposure (Knox et al., 2002).
The effects of boar housing relative to sows on characteristics of estrus remain uncertain. We hypothesized that estrus measures are altered by housing sows adjacent to boars. This experiment investigated the effects of housing weaned sows in pens adjacent to boars or away from boars in either crates or pens on measures of estrus within the 4 to 7 d following weaning.
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Materials and Methods
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This experiment was conducted in eight replicates during January, February, May, and June (two replicates) of 2001, and March, April, and May of 2002. Between 12 to 21 mixed-parity sows within a farrowing group were randomly assigned by parity (1, 2, and 
3), lactation length (18 to 24 d), and genetics (Large White-Landrace, Duroc, and their crosses) to be weaned into gestation crates away from boars (AWC, n = 45), into pens that were >15 m from a boar (AWP, n = 42), or into pens that were adjacent to a mature boar (ADJ, n = 46). The crates and pens were located in separate rooms within a confinement facility, with environmental control provided from natural air inlets, mechanical ventilation, and space heaters. For the ADJ treatment, sows were housed in groups of two to five in 2.1 x 4.7 m pens to provide an average of approximately 3.0 m2 pen space per sow. For the AWP treatment, the sows were housed in groups of two to four in 1.8 x 3.7 m pens to provide an average of approximately 2.0 m2 of pen space per sow. The sows in the AWC treatment were housed sequentially in the same row in individual 0.6 x 2.0 m crates. The side and front gates of the pens had 1.9-cm vertical bars spaced at 14.6-cm intervals. The vertical crate bars were approximately 1.2 cm in diameter and spaced at 10-cm intervals.
Estrus detection was initiated at approximately 0700 (0 h) and performed again at 0715 (0.25 h), 0730 (0.5 h), 0800 (1 h), 0900 (2 h), 1100 (4 h), and 1500 (8 h), beginning on d 4 and continuing until d 7 following weaning. During the experiment, six boars, ranging in age from 1.5 to 2.5 yr, were chosen randomly within a replicate for assignment to one treatment. For all treatments, estrus detection involved positioning the boar to approximate the nose-to-nose position with sows, while a technician applied backpressure and side rubbing stimuli during the 10-min observation period. For sows housed in crates, a mature boar was moved to the front of the crates for a 10-min period and then removed. Sows housed in pens away from the boar were moved approximately 15 m to an empty pen adjacent to a mature boar for a 10-min period and then returned to their pen. Sows housed adjacent to a boar were not moved and estrous detection was performed in their pen for a 10-min period. The estrous response was characterized by a rigid standing response with little or no vocalization from the female while the technician applied the stimuli. Estrus duration was defined in 12-h increments by the first (a.m.) and last (p.m.) estrus detection periods within a day. The duration of estrus was defined by 24 h-intervals, and included the period from the first standing response (0700 in the a.m. on first day of estrus, 0 h) to the first negative response at the last estrus detection of the day (p.m.) or the first check of the next day. End of estrus was confirmed with continued interval observation for symptoms of estrus in the presence of a boar.
The overall proportion of sows expressing estrus within 7 d of weaning and the proportion of sows expressing estrus of various durations were analyzed using logistical modeling procedures of SAS (CATMOD). Mixed-model procedures (PROC MIXED) of SAS (SAS Inst., Inc., Cary, NC) were used to analyze estrous responses: interval from weaning to estrus (days), duration of estrus (hours), percentage expressing estrus at intervals (days) from weaning, and at intervals (hours) within a day of estrus, and interactions. For interval from weaning to estrus and for duration of estrus, the data included a single data point for each sow, and the models included treatment (ADJ, AWC, AWP), replicate (Replicates 1 to 8 as random effects), and interval from weaning (days, where appropriate). The percentage of sows expressing estrus at discrete intervals (days) after weaning included only sows that were standing at the first estrus-detection interval from each day (d 4 to 7). The model included treatment, interval from weaning (days), replicate (1 to 8), and interactions with sow as a random variable. The effect of treatment on the percentage of sows standing at intervals between 0 and 8 h, included treatment, day of estrus (first, second, or third day), replicate, and interactions, with sow as a random variable.
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Results
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Overall, the proportion of sows expressing estrus within the 7 d from weaning was lowest for sows housed adjacent to boars (80%, 37/46) compared with sows housed away in either pens (98%, 41/42) or crates (96%, 43/45; P < 0.05). There was also an effect of day from weaning on the percentage of sows expressing estrus (P < 0.05), but there was no interaction with treatment (Figure 1). The effect of day resulted from more sows expressing estrus on d 5 and 6 compared with d 4 and 7. Sows housed away from boars in crates or pens had reduced (P = 0.01) intervals from weaning to estrus compared with sows housed adjacent to boars (Table 1). The duration of estrus was also shorter (P < 0.01) for sows housed adjacent to boars compared with those housed away in crates or pens (Table 1). The percentage of sows showing estrus for various durations indicated a treatment x duration interaction (P < 0.001; Figure 2).
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Figure 1. Means (±SE) for the effect of treatment (P < 0.05) and day (P < 0.05) on percentage of sows expressing estrus following weaning in response to housing weaned sows in pens adjacent to boars (ADJ, n = 37), in crates away from boars (AWC, n = 43), or in pens away from boars (AWP, n = 41). Within a day, means with different superscripts differ, P < 0.05.
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Table 1. Means (±SE) for weaning-to-estrus interval (WEI) and duration of estrus in response to housing weaned sows in pens adjacent to boars (ADJ), in crates away from boars (AWC), or in pens away from boars (AWP)
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Figure 2. Percentage of sows expressing estrus for various durations following weaning in response to housing weaned sows in pens adjacent to boars (ADJ, n = 37), in crates away from boars (AWC, n = 43), or in pens away from boars (AWP, n = 41). Within an estrous duration, means with different superscripts differ, P < 0.05.
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The estrous response of sows when tested at sequential intervals on each day of estrus indicated a treatment x interval x day of estrus interaction (P < 0.001; Figure 3). After detection of the first standing response on the first day of estrus, only 62 to 82% were detected standing over the next 2 h for all treatments. However, at 4 to 8 h, this increased to 85 to 98% for the AWC and AWP sows, but ADJ remained <73% (P < 0.05; Figure 3a). On the second day of estrus, estrus expression was not influenced by interval for AWC and AWP and was 90 to 100% during the 8-h period, whereas detection rates for ADJ sows were between 68 to 88% and lower at 1-, 4-, and 8-h intervals (Figure 3b). Data for the third and fourth days of estrus are not shown because few ADJ sows contributed data for comparison.
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Figure 3. Means (pooled SE ± 7%) for the percentages of sows standing at sequential estrus detection intervals on the first (Panel a) and second (Panel b) day of estrus in response to housing weaned sows in pens adjacent to boars (ADJ, n = 37), in crates away from boars (AWC, n = 43), or in pens away from boars (AWP, n = 41). Means within an interval with different superscripts differ, P < 0.05; treatment x interval x day of estrus = P < 0.001.
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Discussion
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Failure to detect estrus and breed sows soon after weaning can result in an increase in nonproductive days, lower farrowing rates, and reduced litter sizes (Dial et al., 1992; Dewey et al., 1994; Tantasuparuk et al., 2000). The results of the current study indicate that continuous housing of boars adjacent to weaned sows was associated with decreased estrus detection rates and shorter duration of estrus. There was at least a 14% decrease in estrus detection rate within 7 d of weaning, and duration of estrus was 12 h less when sows were housed adjacent to boars. The effects of housing sows adjacent to boars was also evident in the interval from weaning to estrus, which was shortened by at least 9 h, and on the percentage of sows returning to estrus on each day after weaning. The interval from weaning to estrus has important implications for timing AI, as the interval from weaning to estrus affects the interval from estrus to ovulation (Soede and Kemp 1997; Knox et al., 2002). Specifically, estrus of shorter duration has been reported to increase the incidence of single matings (Hemsworth and Hansen, 1990)
For all treatments, the reasons for 12 sows failing to express estrus within the week following weaning were not identified, but they could have involved limited follicle growth after weaning, estrus and ovulation occurring during lactation, or failure to express behavioral symptoms of estrus when large follicles were present on the ovary. Although not assessed in the current study, if any of these conditions existed, then real-time ultrasound or endocrine measures for estrogen or progesterone concentrations in blood could have helped to determine the involvement of the ovary in the failure to express estrus. However, despite the inability to identify the reason for the failure, the difference in the proportion of sows expressing estrus can most likely be explained as a result of habituation to boar stimuli. Habituation to the boar has been reported in both gilts (Hemsworth and Barnett, 1990) and sows (Dyck, 1988; Hemsworth and Hansen, 1990). The lowered estrus response was likely due to boar housing and not sow space because sows housed in the AWP and AWC both had less space than sows in the ADJ, but both had higher rates of estrus expression. Similarly, group sizes of two to five sows per pen also had little effect on estrus. This was not unexpected because it has previously been reported that when sows are given approximately 2 m2, and when housed in groups of two to eight females, no detrimental affect on estrus expression was observed (Hemsworth and Barnett, 1990). In contrast, boar exposure is implicated in estrus failure since Langendijk et al. (2000) reported that sows receiving excessive boar contact were less likely to show estrus in response to lower stimulus levels. The authors observed that refractory responses changed during estrus and depended on the level of boar stimuli provided. It has also been reported that estrous response changed by interval and day of estrus for gilts (Levis and Hemsworth, 1995). In that study, the estrous response of ovariectomized, estrogen-treated gilts was determined following continuous boar exposure during a 20-min period over two consecutive days. Between the first and last time points, there was a 35% decrease in gilts expressing estrus, and this response differed by day of estrus. This effect was also evident in the current study because sows showed differences in the percentage expressing estrus at various intervals on the first and second day of estrus. For all sows, approximately 20 to 40% of sows failed to show estrus within 4 h following first standing response on the first day of estrus. This was clearly influenced by housing, and at 4 h, refractory behavior was not evident in sows housed away from boars but was still evident in the adjacent housing treatment. On the second day of estrus, refractory behavior was not evident in sows housed away from boars but was apparent with adjacent housing. Collectively, this would suggest that refractory behavior may be possible anytime during an 8-h period when housing boars adjacent to sows, but for sows housed away from boars, a 4- to 8-h period may be needed for sows to recover from refractory behavior on the first day of estrus.
Our observation that housing sows adjacent to boars results in a shorter estrus is similar to the results of Dyck (1988) and Hemsworth and Hansen (1990). Similarly, when delivering more intense boar exposure, Langendijk et al. (2000) also observed a shorter duration of estrus. However, Knox et al. (2002) reported that estrus duration was actually longer when boar exposure was delivered at more frequent intervals (every 8 or 12 h) when compared with every 24 h. The difference between the studies may result from altered intensity of boar contact; however, in the latter study, it would seem that the longer estrus observed in the more frequent exposure groups was due to more accurate detection of both onset and end of estrus because these sows were not housed adjacent to boars. Wean-to-estrus interval was decreased in the current study by <10 h for sows housed adjacent to boars. This finding is similar to observations by Pearce and Pearce (1992); however, other studies reported that different levels of boar exposure did not influence the interval from weaning to estrus (Dyck, 1988; Hemsworth and Hansen, 1990; Langendijk et al., 2000; Knox et al., 2002). Although the reason for the differences between studies is unclear, it is possible that the results could be related to how the measures for estrus were recorded and the type of stimulus provided. For example, in the current study, and in the studies of Dyck (1988) and Hemsworth and Hansen (1990), detection of estrus occurred once daily, whereas Walton (1986) and Pearce and Pearce (1992) used twice-daily detection, and Langendijk et al. (2000) and Knox et al. (2002) used up to three times daily detection. Other differences could be related to the type of housing and number of sows per pen. Dyck (1988) and Hemsworth and Hansen (1990) used two sows per pen, Pearce and Pearce (1992) and the current study reported two to five sows per pen, Langendijk et al. (2000) reported individual housing groups of four sows, and Knox et al. (2002) reported the results from only individual housing. Variation in how measures of estrus were recorded, such as the percentage of sows expressing estrus within 7 (present study), 10 (Pearce and Pearce, 1992), or 21 d (Hemsworth and Hansen, 1990), may affect the interpretation of whether a statistical difference is observed in response to boar housing.
Hughes (1982) and Hughes et al. (1990) reviewed the effects of the boar, which influenced the ability to induce puberty and estrus. There was some evidence that boar age, libido, duration of exposure, proximity (fence-line vs. physical contact), and frequency of exposure altered the estrous response. Hughes et al. (1997) observed that increasing boar exposure to three times each day, for a period of 40 d, decreased age at puberty by approximately 6 d compared with once-daily exposure. Paterson et al. (1989) evaluated the effect of 2- to 30-min daily boar contact and observed no effect on the proportion of gilts reaching puberty, but noted that age at puberty was reduced as duration of boar exposure increased. It is possible that many of these exposure factors alter pheromones that are transferred from the boar to the gilt or sow. Implication of pheromonal transfer is indicated because estrus expression is markedly decreased when the boar is housed 1 m away compared with fence-line or physical contact for both gilts and sows (Hemsworth and Barnett, 1990; Pearce and Paterson, 1992). Further evidence implicating pheromonal signals originates from studies in which spraying hormone (androstenes) into the nose of the females induced estrus (Melrose et al., 1971), whereas Perry et al. (1980) reported that submaxillary gland removal altered the ability of boars to elicit normal estrous response in females.
Based on physiological data, it seems that the introduction of the boar produces measurable endocrine responses in female pigs. In prepubertal gilts (Kingsbury and Rawlings, 1993) and anestrus sows (van de Wiel and Booman, 1993), boar introduction increases the frequency of LH pulses within a 6-h period. Others have shown that within minutes of gilt exposure to a boar, cortisol (Pearce and Hughes, 1987) and prolactin (Prunier et al., 1987) release occur. Langendijk et al. (2000) has reported that boar contact stimulates ovarian activity and induces ovulation. If the presence of the boar stimulates gonadotropin and hormone responses, then it is likely that this mechanism might explain enhanced ovarian activity. However, in cases of adjacent housing, sows appear to become refractory to the pheromonal signals that induce estrous behavior, yet it is not clear whether sows also become refractory to the pheromonal signals, which induce endocrine responses that stimulate ovarian activity. Collectively, data from Walton (1986), Dyck (1988), Hemsworth and Hansen (1990), Pearce and Pearce (1992), and the current study support the observation that behavioral problems associated with detection of estrus occur when sows are housed adjacent to boars, but it remains unclear whether ovarian activity is influenced by this type of boar exposure.
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Footnotes
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1 This research was supported in part by the Illinois Council on Food and Agric. Research (C-FAR) and the Dept. of Anim. Sci., Univ. of Illinois. The Animal Care and Use Committee of the Univ. of Illinois approved Protocol No. 00197 for use of animals in this experiment. 
2 The authors thank the Univ. of Illinois swine farm personnel for their assistance in animal management and collection of research data.
3 Correspondence: 360 Animal Sciences Laboratory, 1207 W. Gregory Dr. (phone: 217-244-5177; fax: 217-333-8286; e-mail: rknox{at}uiuc.edu).
Received for publication February 17, 2004.
Accepted for publication June 15, 2004.
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Abstract
Introduction
