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Effect of age and physical or fence-line boar exposure on estrus and ovulation response in prepubertal gilts administered PG600^1,2

Department of Animal Sciences, University of Illinois, Urbana 61801-3939

	Abstract

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Boar exposure has been used for estrus induction of prepubertal gilts, but has limited effect on estrus synchronization within 7 d of introduction. In contrast, PG600 (400 IU of PMSG and 200 IU of hCG; Intervet, Millsboro, DE) is effective for induction of synchronized estrus, but the response is often variable. It is unknown whether boar exposure before PG600 administration might improve the efficiency of estrus induction of prepubertal gilts. In Exp. 1, physical or fence-line boar contact for 19 d was evaluated for inducing puberty in gilts before administration of i.m. PG600. Exp. 2 investigated whether 4-d boar exposure and gilt age influenced response to PG600. In Exp. 1, 150-d-old prepubertal gilts were randomly allotted to receive fence-line (n = 27, FBE) or physical (n = 29, PBE) boar exposure. Gilts were provided exposure to a mature boar for 30 min daily. All gilts received PG600 at 169 d of age. Estrous detection continued for 20 d after injection. In Exp. 2, prepubertal gilts were allotted by age group (160 or 180 d) to receive no boar exposure (NBE) or 4 d of fence-line boar exposure (BE) for 30 min daily before receiving PG600 either i.m. or s.c. Following PG600 administration, detection for estrus occurred twice-daily using fence-line boar exposure for 7 d. Results of Exp. 1 indicated no differences between FBE and PBE on estrus (77%), age at puberty (170 d), interval from PG600 to estrus (4 d), gilts ovulating (67%), or ovulation rate (12 corpora lutea, CL). Results from Exp. 2 indicated no effect of age group on estrus (55%) and days from PG600 to estrus (4 d). A greater (P < 0.05) proportion of BE gilts expressed estrus (65 vs. 47%), had a shorter (P < 0.05) interval from PG600 to estrus (3.6 vs. 4.3 d), and had decreased (P < 0.05) age at estrus (174 vs. 189 d) compared with NBE. Ovulation rate was greater (P < 0.05) in the BE group for the 180-d-old gilts (12.7 vs. 11.9 CL) compared with the NBE group. However, age group had no effect on ovulation (77%) or ovulation rate (12 CL). Collectively, these results indicate that physical boar contact may not be necessary when used in conjunction with PG600 to induce early puberty. The administration of PG600 to 180-d-old gilts in conjunction with 4 d prior fence-line boar exposure may improve induction of estrus, ovulation, and decrease age at puberty.

Key Words: Boar Exposure • Estrus • Gilts • Ovulation • PG600 • Puberty

	Introduction

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Early induction of puberty can improve fertility of replacement gilts (Christenson et al., 1985; Koketsu et al., 1999). Synchronizing estrus is important for decreasing costs associated with large gilt pools and for allowing efficient replacement of sows following weaning (Christenson, 1986; Faust et al., 1993). Boar exposure and PG600 (400 IU of PMSG and 200 IU of hCG; Intervet, Millsboro, DE) are production tools that are commonly used for inducing puberty, but neither has produced the desired level of effect.

Boar exposure decreased age at first estrus in gilts (Pearce and Hughes, 1985; Paterson et al., 1989); however, efficiency of induction ranges from 10 to 80% and synchrony of estrus may range from 5 to 90 d (Caton et al., 1986; Hughes et al., 1990; Pearce and Paterson, 1992). Method of boar exposure may affect efficacy because physical boar exposure is more effective than fence-line for inducing precocious puberty (Deligeorgis et al., 1984; Paterson et al., 2002). At the same time, physical boar exposure has not been shown to influence ovulation or ovulation rate.

Administration of PG600 induced puberty in 45 (Paterson et al., 1984) to 100% of prepubertal gilts (Guthrie, 1977). It is unclear why there is such a large variation in response rate and whether this may be due to lack of physiological maturity. Ovaries are first responsive to exogenous gonadotropins at 120 to 180 d of age (Christenson et al., 1985), but the optimal time to induce puberty has not been established. There is no evidence to suggest whether boar exposure before administration of PG600 might improve the induction response to PG600. Therefore, the objectives of this study were to establish whether physical boar contact was more effective than fence-line contact for inducing puberty in gilts receiving PG600, and whether short-term boar exposure before PG600 interacts with age and route of PG600 administration on measures of estrus and ovulation.

	Materials and Methods

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Exp. 1. Effect of Physical vs. Fence-Line Boar Exposure
The experiment was conducted in two replicates during July 2001 and June 2002. Prepubertal gilts 150 ± 2 d of age were blocked by age, BW, and litter, and allotted randomly to one of two treatments, fence-line boar exposure (FBE, n = 27) or physical boar exposure (PBE, n = 29). The gilts were crossbred and purebred maternal (Yorkshire or Landrace) and terminal (Berkshire or Duroc) line genotypes, with an average BW of 100.6 ± 1.8 kg at allotment. The gilts were group housed in 3.7 m x3.7 m pens with seven to 10 gilts per pen. A mature boar (>18 mo old) was housed at least 12 m away and downwind from the fan-controlled air movement and also from the gilt pens. Boar exposure was initiated at 150 d and continued for 19 d, with exposure occurring 30 min once daily. During this time, the gilts were checked for signs of estrus. Gilts were considered to be in estrus when standing in response to the backpressure test during boar exposure. Gilts that expressed estrus during the 19-d boar exposure period were removed from the study. At 169 ± 2 d of age, all remaining gilts were restrained and injected s.c. with PG600 in the flank using a 38.1-mm, 20-gauge needle. Following injection, detection for estrus continued twice daily for the next 20 d. The duration of estrus was defined as the interval from the first standing response to the first negative standing response. Gilts were killed on d 20 after PG600, and reproductive tracts were collected. Ovaries were examined for the presence and number of corpora lutea, corpora albicantia, and cystic follicles (>12 mm in diameter).

Exp. 2. Effect of Short-Term Boar Exposure and Age at PG600
This experiment was conducted in three replicates (August 2001, November 2002, and April 2003) at three different farms of the University of Illinois. Prepubertal gilts that were either 160 ± 2 or 180 ± 2 d of age were blocked by age group, BW, and litter, and allotted randomly to receive no boar exposure (NBE, n = 85) or fence-line boar exposure (BE, n = 87) for 4 d before PG600. The gilts were crossbred and purebred maternal (Yorkshire, Chester White, and Landrace) and terminal (Berkshire, Duroc, Hampshire, and C-22; PIC USA, Franklin, KY) line genotypes with an average BW of 108.6 ± 1.4 kg at allotment. The gilts in the BE group were exposed to a mature boar for 30 min once daily for 4 d. All gilts were assigned by age and boar exposure to receive PG600 either by i.m. or s.c. injection using a 38.1-mm, 20-gauge needle. Gilts receiving an i.m. injection were restrained and given PG600 in the neck behind the ear, whereas gilts receiving a s.c. injection were snared and given PG600 in the flank between the fat and the muscle layers just anterior to the rear leg and lateral to the nipple line. All gilts were checked for estrus twice-daily using fence-line boar contact for 7 d in conjunction with the backpressure test. Between 17 and 25 d after injection, all gilts were killed, and reproductive tracts were collected. Ovaries were examined for the presence of corpora lutea, corpora albicantia, and cystic follicles.

Statistical Analyses
In Exp. 1, the continuous response variables, days from PG600 to estrus, length of estrus, and ovulation rate (number of corpora lutea) were analyzed using linear effects modeling procedures (PROC MIXED) of SAS (SAS Inst., Inc., Cary, NC). The model included the explanatory variables: treatment (PBE or FBE), BW (covariate), replicate (1 or 2), genetics (maternal or terminal line), and their interactions where appropriate. The discrete response variables included proportion of gilts expressing estrus, ovulating, and cystic (1 cyst). All discrete variables were analyzed using categorical modeling procedures (PROC CATMOD) of SAS because these variables are binomially distributed. For Exp. 2, continuous response variables (same as those from Exp. 1) also were analyzed using linear models. The explanatory variables included age group (160 or 180 d of age), boar exposure (BE or NBE), route of PG600 (i.m. or s.c.), replicate (1, 2, or 3), farm (1, 2, or 3), BW (covariate), and their interactions where appropriate. The discrete response variables were also the same as those from Exp. 1 and were analyzed using categorical modeling procedures.

	Results

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Exp. 1. Effect of Physical vs. Fence-Line Boar Exposure
From the initiation of boar exposure at the start of the experiment, five gilts from the FBE and seven gilts from the PBE treatment group expressed estrus before d 169, and these gilts were subsequently removed from the study. Three additional gilts were removed from the study due to illness not related to treatments. There was no effect of PBE compared with FBE on percentage of gilts expressing estrus (77%), age at puberty, days from PG600 to estrus (4.1 d), or on duration of estrus (2.3 d; Tables 1 and 2). Method of boar exposure did not influence the proportion of gilts ovulating (67%), ovulation rate (12 corpora lutea), or the proportion of gilts with follicular cysts (Tables 1 and 2).

	Discussion

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The results of Exp. 1 indicate that PBE for 19 d before PG600 did not improve estrus or induction of ovulation in prepubertal gilts and suggest that FBE may be just as effective as PBE when PG600 is used. The results of Exp. 2 provide evidence that short-term boar exposure for as little as 4 d before PG600 can improve the estrous and ovulation responses in prepubertal gilts. This short-term boar exposure was more effective in 180-d-old gilts than in 160-d-old gilts because it seemed to compromise ovulation rate in the 160-d-old gilts. Collectively, these data suggest that short-term boar exposure before PG600 for gilts between 160 to 180 d of age can be used to improve the estrous induction response.

Several studies have reported estrous (approximately 78%) induction rates following PG600, similar to those observed in Exp. 1 (Schilling and Cerne, 1972; Varley et al., 1989; Knox et al., 2000). In addition, boar exposure alone advanced age at puberty in gilts (Hughes et al., 1990). It has been suggested that PBE is more effective in decreasing the age to puberty than FBE (Deligeorgis et al. 1984; Paterson et al. 2002). Paterson et al. (2002) exposed prepubertal gilts to either PBE or FBE for 10 to 15 min daily and reported that PBE decreased the days to estrus from 32 to 24 d and decreased age at puberty from 191 to 184 d compared with FBE. Previous experiments have demonstrated that daily boar exposure for 30 min during the period of 80 to 210 d of age also decreased age of puberty (Kirkwood and Hughes, 1979; Paterson and Lindsay, 1981; Paterson et al., 1989). However, in Exp. 1 there was no effect of PBE vs. FBE when given before PG600. This result was unexpected, but may be explained by the fact that even with physical boar exposure as the sole stimulus for puberty induction, variation in the estrous response is observed. It is likely that any difference between PBE and FBE is negated when exogenous gonadotropins are administered. Other studies have shown considerable variation in the interval from boar exposure to pubertal estrus and age at puberty, and these intervals range from 7 to 72 d in gilts that were between 167 to 237 d of age when exposed to a boar (Hughes et al., 1990). The lack of an observed effect of PBE vs. FBE also may be related to the short duration of the exposure because in the previously mentioned studies, the average duration of exposure was approximately 30 d. However, because shorter exposure was effective in enhancing the estrous induction response to PG600 in Exp. 2, it is doubtful that exposure of gilts for periods longer than 19 d would have enhanced the response. Without the use of PG600, gilts exposed to PBE or FBE between 160 to 180 d of age typically do not express estrus as early as those observed with PG600. In the studies by Caton et al. (1986) and Hughes et al. (1990, 1997), approximately 50% of the gilts expressed estrus by 210 d following 30 d of boar exposure. This would indicate that PG600 given at 165 d was effective for inducing earlier and more synchronized estrus compared with long-term boar exposure alone.

The ovulation responses were not influenced by boar exposure. In Exp. 1, the proportion of gilts ovulating was similar between PBE and FBE, but it was approximately 20% lower than ovulation responses observed in previous studies using PG600 (Schilling and Cerne, 1972; Paterson and Lindsay, 1981; Knox et al., 2000). The low proportion of gilts ovulating could be due to differences in management, genetics, season, and hormone preparation. Nonetheless, it is worthy noting that Exp. 1 was performed in the summer months when reproductive performance is often compromised (Hughes et al., 1990; Paterson et al, 1991). However, Phillip and Hughes (1995) reported that exposure of gilts to boars in the summer may override the adverse effects of season on gilt fertility and in providing boar exposure, observed no difference in days to puberty (33 d) and age at estrus (193 d) among seasons (fall, winter, spring, and summer). It is unclear as to why the ovulation response in Exp. 1 was less than optimal. Variation in the boar influence on ovulation could be related to the intensity of boar exposure. It is possible that the boar’s stimulatory value may also be influenced by season, and that the boar may produce less-stimulating pheromones during the summer months (Claus and Weiler, 1985).

In Exp. 2, short-term FBE for 4 d before PG600 improved the induction response by 17% and decreased the interval from PG600 to estrus by 1.4 d, compared with no boar exposure before PG600. This finding is consistent with previous reports comparing boar exposure to no boar exposure as the sole stimulus for prepubertal gilt induction (Hughes et al., 1990). Unexpectedly, there was no interaction for the effect of boar exposure and age group on expression of estrus. However, we noted that the 180-d-old gilts showed a numerical, but nonsignificant increase in estrous expression compared with the 160-d-old gilts. Dziuk (1991) suggested that gilts induced closer to their natural age of puberty have improved reproductive responses.

There was no effect of gilt age or boar exposure on percentage of gilts that ovulated, which averaged 72%; however, boar exposure numerically increased the percentage of gilts ovulating by 13% compared with no boar exposure. This finding indicates that females between 160 to 180 d of age can respond to PG600, but other sources of variation, such as season or boar exposure may influence the effectiveness of the response. The interaction of age and boar exposure on ovulation rate is interesting, in that boar exposure has a positive effect in the older gilts but a negative response in the younger gilts. In contrast, no boar exposure has a negative effect on older gilts and little effect on the younger gilts. The reason for this age x boar exposure effect may be related to the follicular status of the ovaries (Bolamba et al., 1992) at the time of boar exposure and the effect of the boar on endocrine responses in the gilt (Hughes et al., 1990).

The effect of route of PG600 administration was included in Exp. 2 because it has been reported that route of PG600 influenced the estrous and ovulatory responses (Knox et al., 2000). However, estrus induction rates were not influenced by route and were similar to those reported using s.c. (45 to 100% Schilling and Cerne, 1972; Baker and Rajamahendran, 1973; Paterson et al., 1984) and i.m. (50 to 73%, Britt et al., 1989; Nephew et al., 1994; Tilton et al., 1995) routes of administration. The route of PG600 administration also did not influence the proportion of gilts ovulating, but ovulation rate was actually increased with s.c. (13.0 corpora lutea) compared with i.m. (10.8 corpora lutea) administration. This is opposite the pattern that Knox et al. (2000) observed for the effect of route of PG600, and it suggests a possible genotype effect or possibly an effect of prior boar exposure. The Knox et al. (2000) study reported a higher percentage of gilts ovulating (86%) and even higher ovulation rates (17.9 corpora lutea) in animals of a single genetic source. In the present study, variation in the responses due to hormone preparation, genetics, season, and boar exposure may have also prevented detection of an effect of route on estrous induction.

	Footnotes

 
¹ This research was supported by Intervet Inc. (Millsboro, DE), the Illinois Council on Food and Agricultural Research (C-FAR), and the Dept. of Anim. Sci., Univ. of Illinois. The animal care and use committee of the Univ. of Illinois approved the protocol (No. 99359).

² The authors would thank the Univ. of Illinois Farm staff for technical assistance and care of the animals during this study.

³ Correspondence: 360 Animal Science Laboratory, 1207 W. Gregory Dr. (phone: 217-244-5177; fax: 217-333-8286; e-mail: rknox{at}uiuc.edu).

Received for publication June 23, 2004. Accepted for publication November 29, 2004.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Breen, S. M. Articles by Knox, R. V. Search for Related Content PubMed PubMed Citation Articles by Breen, S. M. Articles by Knox, R. V.