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Attainment of puberty in female pigs: Influence of boar stimulation
Animal Reproduction Science, 4 (1981/1982) 313--319
313
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ATTAINMENT OF PUBERTY IN FEMALE PIGS: INFLUENCE OF BOAR STIMULATION
INGRID KARLBOM
Department o f Obstetrics and Gynaecology and Department o f Clinical Chemistry, Swedish University o f Agricultural Sciences, S-750 07 Uppsala (Sweden) (Accepted 26 October 1981)
ABSTRACT Karlbom, I., 1982. Attainment of puberty in female pigs: influence of boar stimulation. Anita. Reprod. Sci., 4: 313--319. The object of the present study was to investigate whether the presence of a boar is of importance for the age at puberty and the expression of external heat symptoms in female pigs. Altogether 60 crossbred gilts were purchased at an age of 12 weeks and grouped, three or four per pen, in three separate barns (treatment groups A, B and C). Treatment group A: Boars were kept in adjacent pens to the gilts during the whole period. One vasectomized boar was penned with the gilts daily for 20 rain during the experimental period, which started at a mean age of 142 days. Treatment group B: Boars were kept in adjacent pens to the gilts during the whole period. Treatment group C: No boars were kept in the barn at any time. The study was performed six times. Daily heat control was carried out during the experimental period, which was terminated when the gilts had passed at least two oestrous cycles. Blood samples for progesterone analysis were taken once a week. Laparoscopy was performed in all gilts after their first observed heat. The gilts were slaughtered after their third or fourth heat. The effects of treatment group were estimated by the method of least squares analysis. All gilts except one showed external heat symptoms at regular intervals during the experimental period. A few gilts did not stand for the boar at each oestrous period, regardless of oestrous number. Gilts belonging to group A showed their first oestrus on average 20 days earlier than gilts in group B and 35 days earlier than gilts in group C. These differences were highly significant (P < 0.0Ol). The stimulatory effect of the boar on the age at puberty varied between the six experimental groups and the interaction between treatment and experimental groups was significant (P < 0.01).
INTRODUCTION
Recently studies on p u b e r t y in gilts have shown that gilts of Swedish crossbreeds are sexually mature at an age of approximately 185 days and a b o d y weight of approximately 100 kg (Andersson and Einarsson, 1980; Karlbom et al., 1982). Several factors influence the age at puberty. Gilts born in spring attain p u b e r t y later than those born in autumn (e.g. Wiggins
0378-4320/82/0000--0000/$02.75
© 1982 Elsevier Scientific Publishing Company
314
et al., 1950; Bane et al., 1976). The group size of gilts in confinement is also considered a factor of importance (Mavrogenis and Robison, 1976). In several studies carried o u t during the last decade it has been shown that gilts exposed to a boar attained p u b e r t y at an earlier age than unexposed gilts (e.g. Brooks and Cole, 1970; Thompson and Savage, 1978). The efficacy of the boar stimulatory effect is to some extent related to the age of the gilts at exposure (Brooks and Cole, 1970; Bourn et al., 1974; Zimmerman et al., 1974). According to Mavrogenis and Robison (1976) and Alliston et al. (1974) there is a higher incidence of silent heats among gilts exposed to boars than among unexposed gilts. The control of the sexual functions of gilts in the above-mentioned studies was effected by checking for heat with or without the aid of a boar. The ovarian activity was n o t followed directly by laparoscopic examination or indirectly by measurement of the blood plasma progesterone levels. The results from the aforementioned studies must therefore be judged with caution. The object of the present study was to investigate whether the presence of a boar is of importance for the age at p u b e r t y and the expression of external heat s y m p t o m s in female pigs. M A T E R I A L AND METHODS
The present study, carried o u t during 1977--1979, involved a total of 60 crossbred gilts (Swedish Landrace X Swedish Yorkshire). The gilts were purchased from different herds at an age of approximately 12 weeks. On arrival they were grouped three or four per pen in three separate barns (treatment groups A, B and C). Three gilts selected from each of 14 litters and six gilts from each of three other litters were allocated to the three treatment groups. The gilts were fed according to Swedish breeding-stock standards (Eriksson et al., 1972}. The allowance was based on mean b o d y weight per animal in each pen. The gilts were weighed once a week until their first heat, and thereafter every 14th day. Treatment group A. Boars were kept in adjacent pens to the gilts during the whole period. One vasectomized boar was penned with the gilts daily for 20 min during the experimental period until all pen-mates had shown their first heat. Treatment group B. Boars were kept in adjacent pens to the gilts during the whole period, b u t were never penned with the gilts. Treatment group C. No boars were kept in the barn at any time during the entire period. This study was performed six times (see Table I). Daily heat control was carried out during the whole experimental period. It comprised inspection of the vulva (swelling and reddening) and test of standing reflex. In treatment groups A and B the standing reflex was tested in the presence of a boar. The gilts belonging to treatment group C were checked without the assistance of a boar. In the case of a weak response to the "riding test" the gilt was taken
315 TABLE I Birth date, age and body weight of the gilts at the start of the experiment in the different experimental groups Experiment
Number of gilts
Birth date
I
12
August 1977
II
12
September 1977
III
9
April 1978
IV
9
April 1978
V
9
August 1979
VI
9
August 1979
Total
60
Start of experiment Mean age (days)
Mean weight (kg)
146 (143--148) 134 (132--140) 141 (137--146) 141 (137--146) 148 (141--155) 139 (136--141) 142
61 (49--75) 68 (57--78) 65 (53--89) 65 (59--79) 66 (61--74) 67 (62--73) 65
o u t o f the barn and checked in the presence of a boar. The experimental period ter min ate d when the gilts had passed at least t w o oestrous cycles. Blood samples were taken by p u n c t u r e o f an ear vein once a week during the whole experimental period and collected in heparinized tubes. After centrifugation, the plasma was r e m oved and stored in plastic tubes at --20°C until assayed. Levels o f plasma progesterone were determined using a radioimmunoassay system (Bosu et al., 1976). Laparoscopy, according to Wildt et al. (1973), was p e r f o r m e d in all gilts after their first observed heat. The ovaries were inspected for ovulations and f o r m a t i o n of corpora lutea. The occurrence o f older corpora lutea was also carefully recorded. The gilts were slaughtered after their third or fourth heat. The genital organs were removed and examined macroscopically, especially in respect of the ovarian morphology. Representative tissue pieces from the uterine mucosa were examined histologically. Statistics
The material was slightly unbalanced, and so the effects of t r e a t m e n t group and experimental group were estimated by the m e t h o d of least squares analysis, using the Statistical Analysis System (Barr et al., 1979). The following model was used. Y~ik = u + ai + b i + (ab)~" + eii k
316
where Y~ik = the i j k t h observation u = general mean a i = effect of ith treatment group (i = 1, 2, 3) b j = effect o f j t h experimental group {j = 1, 2, 3, 4, 5, 6) (ab)o" = effect of interaction between ith treatment group and jth experimental group. eijk = residual random term.
RESULTS
All gilts except one showed external heat symptoms -- reddening and swelling of the vulva -- at regular intervals during the experimental period. The length of the first oestrous cycle of the irregular gilt was 28 days. At the expected third oestrus this gilt showed neither external heat symptoms nor standing reflex. Thirty-four days after the second oestrus laparoscopic examination revealed old corpora lutea and small follicles. Fifty-two days after the second oestrus the gilt showed pronounced oestrous symptoms and ovulated. Despite reddening and swelling of the vulva, a few gilts did not stand for the boar at each oestrous period (Table II). As can be seen from this table, failure of standing reflex occurred independently of oestrous number. There was, however, a tendency to a higher incidence among the boar-exposed gilts. Judging from the appearance of the ovaries and from the progesterone levels, all gilts, including those that showed reddening and swelling of the vulva but refused to stand for the boar, ovulated. Only gilts showing standing oestrus at puberty are included in the statistical calculations. According to the finding at laparoscopic examination (presence of older corpora lutea) and plasma progesterone levels, one gilt belonging to group C (experimental group VI) had ovulated before the experimental period. This gilt therefore, together with her two litter-mates, was excluded from the presentation. TABLE II Occurrence of "silent h e a t " in the gilts during the experimental period Treatment group
A B C Total
Number of gilts not standing for a boar 1st heat
2nd heat
3rd heat
2/20 2*/20 0/20 4/60
1/20 2*/20 1/20 4/60
2**/20 5*/20 0/20 7/60
* One gilt did not stand for the boar on any occasion. **One gilt had inactive ovaries at the expected time of her third oestrus.
317 TABLE III Least squares m e a n s
of age (days) and body weight (kg) of the giltsat puberty
At puberty
Treatment groups
Age Mean S.E. Weight Mean S.E. Number of gilts
Group A
Group B
Group C
163 x 2.1 82 x 2.9 17
183 Y 2.1 98 y 2.9 17
198 z 2.0 108 z 2.8 19
XYZ: Means with these superscripts are highly significant (P < 0.001). xyz: Means with these superscripts are significant (P < 0.05).
TABLE IV Le~tsquares means ofage(days) at puberty o f t h e g i l t s i n the experimentalgroupsI--VI Experimental ~oup I II III IV V VI
Treatment ~ o u p Group A
Group B
Group C
173 180 164 163 150 148
180 182 193 176 182 186
205 183 228 218 186 168
L e a s t squares m e a n s o f age a n d b o d y w e i g h t at p u b e r t y o f t h e gilts b e l o n g i n g t o t r e a t m e n t g r o u p s A, B a n d C are p r e s e n t e d in T a b l e III. Gilts b e l o n g i n g t o g r o u p A s h o w e d t h e i r first oestrus o n average 20 d a y s earlier t h a n gilts in g r o u p B a n d 35 d a y s earlier t h a n gilts in g r o u p C. T h e s e d i f f e r e n c e s are highly significant (P < 0.001). H o w e v e r , t h e s t i m u l a t o r y e f f e c t o f t h e b o a r o n t h e age at p u b e r t y varied b e t w e e n t h e six e x p e r i m e n t a l g r o u p s ( T a b l e IV), a n d t h e i n t e r a c t i o n b e t w e e n t r e a t m e n t a n d e x p e r i m e n t a l g r o u p s was significant (P < 0.01). In all e x p e r i m e n t a l g r o u p s gilts b e l o n g i n g to g r o u p A r e a c h e d p u b e r t y earlier t h a n gilts b e l o n g i n g t o g r o u p s B or C. T h e lengths o f t h e sexual cycles w e r e n o t i n f l u e n c e d b y t h e d i f f e r e n t t r e a t m e n t s (P > 0.05). No pathological abnormalities were found on post-mortem examination of oviducts, u t e r u s , cervix a n d vagina. Histological e x a m i n a t i o n o f t h e u t e r i n e wall revealed c h r o n i c e n d o m e t r i t i s in o n e gilt. T h e m a j o r i t y o f t h e c o r p o r a l u t e a w e r e solid, b u t in s o m e gilts t h e r e was a c o m b i n a t i o n o f solid a n d cystic c o r p o r a lutea, a n d o c c a s i o n a l l y o n e o r t w o l u t e i n i z e d cysts. N o follicle cysts w e r e f o u n d at p o s t - m o r t e m e x a m i n a t i o n . N o d i f f e r e n c e in n u m b e r o f c o r p o r a l u t e a or n u m b e r s o f cystic c o r p o r a l u t e a a n d l u t e i n i z e d c y s t s was f o u n d b e t w e e n t h e t r e a t m e n t groups.
318 DISCUSSION
The gilts belonging to group B in the present study reached puberty at a mean age of 183 days. The boar stimulation to which these gilts were exposed corresponds well with the management in gilt-producing herds in Sweden. Swedish crossbred gilts, kept at the clinic under similar environmental conditions for puberty studies, were sexually mature at approximately the same age (Andersson and Einarsson, 1980; Karlbom et al., 1982). The boar-exposed gilts reached puberty earlier than unexposed gilts. This was demonstrated earlier in several studies (e.g. Brooks and Cole, 1970; Thompson and Savage, 1978). In contrast to the opinion advanced by Brooks (1978) that gilts must be reared in isolation from boars in order to be able to respond to the "male effect", the present results clearly demonstrate a positive effect on puberty in gilts exposed to boars from 3 months of age. Daily introduction of the boars to the gilts from an age of approximately 142 days stimulated the gilts to attain puberty still earlier. Mavrogenis and Robison (1976) observed that a higher incidence of "silent heats '~ occurred before the first detected oestrus among gilts exposed to boars than among unexposed gilts. Alliston et al. (1974) found active ovaries at slaughter in a number of boar-exposed gilts, not observed in heat for 46 days. In the present study some boar-exposed gilts with reddening and swelling of the vulva and subsequent formation of corpora lutea refused to stand for the boar. This occurred regardless of oestrous number and was therefore not related to puberty. The reason for failure of the standing reaction in gilts is not known. ACKNOWLEDGEMENTS
This study was supported by grants from the Swedish Council for Forestry and Agricultural Research. The author wishes to acknowledge the valuable help with the statistical analyses given by Leif GSransson and the technical assistance given by Mari-Anne Carlsson, Eva Hedberg, Britt J 5nsson, Kerstin Lindblad and Mari WaUbring.
REFERENCES Alliston, C.W., Haglof, S.A., Wilson, S.P., Aberle, E.D. and Judge, M.D., 1974. Presence of boars and sexual maturity in gilts. J. Anim. Sci., 3 9 : 9 6 7 (Abstr.). Andersson, A.-M. and Einarsson, S., 1980. Studies on the oestrus and ovarian activity during five successive oestrous cycles in gilts. Acta Vet. Scand., 21: 677---688. Bane, A., Einarsson, S. and Larsson, K., 1976. Effect of season on age at puberty in crossbred gilts in Sweden. Proc. VIIIth Int. Congr. Anim. Reprod. and A.I., Cracow, pp. 117--119. Barr, A.J., Goodnight, J.H., Sall, J.P., Blair, W.H. and Chilka, D.M., 1979. SAS User's Guide. North Carolina State University, Raleigh, NC.
319 Bosu, W.T., Edqvist, L.-E., Lindberg, P., Martinsson, K. and Johansson, E.D.B., 1976. The effect of various dosages of lynesterol on the plasma level of oestrogen and progesterone during the menstrual cycle in the rhesus monkey. Contraception, 13: 677--684. Bourn, P., Carlson, R., Lantz, B. and Zimmerman, D.R., 1974. Age at puberty in gilts as influenced by age at boar exposure and transport. J. Anim. Sci., 3 9 : 9 8 7 (Abstr.). Brooks, P.H., 1978. Early sexual maturity and mating of gilts. A.D.A.S.Q. Rev., 30: 139--152. Brooks, P.H. and Cole, D.J.A., 1970. The effect of the presence of a boar on the attainment of puberty in gilts. J. Reprod. Fertil., 23: 435--440. Eriksson, S., Sanne, S. and Thomke, S., 1972. Fodermedlen (Feedstuff). LT:s fSrlag. Karlbom, I., Einarsson, S. and Edqvist. L.-E., 1982. Attainment of puberty in female pigs: clinical appearance and patterns of progesterone, oestradiol-17~ and LH. Anita. Reprod. Sci., 4: 301--312. Mavrogenis, A.P. and Robison, O.W., 1976. Factors affecting puberty in swine. J. Anita. Sci., 42: 1251--1255. Thompson, L.H. and Savage, J.S., 1978. Age at puberty and ovulation rate in gilts in confinement as influenced by exposure to a boar. J. Anim. Sei., 47: 1141--1144. Wiggins, E.L., Casida, L.E. and Grummer, R.H., 1950. The effect of season of birth on sexual development in gilts. J. Anita. Sci., 9: 277--280. Wildt, D.E., Fujimoto, S., Spencer, J.L. and Dukelow, W.R., 1973. Direct ovarian observation in the pig by means of laparoscopy. J. Reprod. Fertil., 35: 541--543. Zimmerman, D.R., Carlson, R. and Lantz, B., 1974. The influence of exposure to the boar and movement on pubertal development in the gilt. J. Anita. Sci., 3 9 : 2 3 0 (Abstr.).
313
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ATTAINMENT OF PUBERTY IN FEMALE PIGS: INFLUENCE OF BOAR STIMULATION
INGRID KARLBOM
Department o f Obstetrics and Gynaecology and Department o f Clinical Chemistry, Swedish University o f Agricultural Sciences, S-750 07 Uppsala (Sweden) (Accepted 26 October 1981)
ABSTRACT Karlbom, I., 1982. Attainment of puberty in female pigs: influence of boar stimulation. Anita. Reprod. Sci., 4: 313--319. The object of the present study was to investigate whether the presence of a boar is of importance for the age at puberty and the expression of external heat symptoms in female pigs. Altogether 60 crossbred gilts were purchased at an age of 12 weeks and grouped, three or four per pen, in three separate barns (treatment groups A, B and C). Treatment group A: Boars were kept in adjacent pens to the gilts during the whole period. One vasectomized boar was penned with the gilts daily for 20 rain during the experimental period, which started at a mean age of 142 days. Treatment group B: Boars were kept in adjacent pens to the gilts during the whole period. Treatment group C: No boars were kept in the barn at any time. The study was performed six times. Daily heat control was carried out during the experimental period, which was terminated when the gilts had passed at least two oestrous cycles. Blood samples for progesterone analysis were taken once a week. Laparoscopy was performed in all gilts after their first observed heat. The gilts were slaughtered after their third or fourth heat. The effects of treatment group were estimated by the method of least squares analysis. All gilts except one showed external heat symptoms at regular intervals during the experimental period. A few gilts did not stand for the boar at each oestrous period, regardless of oestrous number. Gilts belonging to group A showed their first oestrus on average 20 days earlier than gilts in group B and 35 days earlier than gilts in group C. These differences were highly significant (P < 0.0Ol). The stimulatory effect of the boar on the age at puberty varied between the six experimental groups and the interaction between treatment and experimental groups was significant (P < 0.01).
INTRODUCTION
Recently studies on p u b e r t y in gilts have shown that gilts of Swedish crossbreeds are sexually mature at an age of approximately 185 days and a b o d y weight of approximately 100 kg (Andersson and Einarsson, 1980; Karlbom et al., 1982). Several factors influence the age at puberty. Gilts born in spring attain p u b e r t y later than those born in autumn (e.g. Wiggins
0378-4320/82/0000--0000/$02.75
© 1982 Elsevier Scientific Publishing Company
314
et al., 1950; Bane et al., 1976). The group size of gilts in confinement is also considered a factor of importance (Mavrogenis and Robison, 1976). In several studies carried o u t during the last decade it has been shown that gilts exposed to a boar attained p u b e r t y at an earlier age than unexposed gilts (e.g. Brooks and Cole, 1970; Thompson and Savage, 1978). The efficacy of the boar stimulatory effect is to some extent related to the age of the gilts at exposure (Brooks and Cole, 1970; Bourn et al., 1974; Zimmerman et al., 1974). According to Mavrogenis and Robison (1976) and Alliston et al. (1974) there is a higher incidence of silent heats among gilts exposed to boars than among unexposed gilts. The control of the sexual functions of gilts in the above-mentioned studies was effected by checking for heat with or without the aid of a boar. The ovarian activity was n o t followed directly by laparoscopic examination or indirectly by measurement of the blood plasma progesterone levels. The results from the aforementioned studies must therefore be judged with caution. The object of the present study was to investigate whether the presence of a boar is of importance for the age at p u b e r t y and the expression of external heat s y m p t o m s in female pigs. M A T E R I A L AND METHODS
The present study, carried o u t during 1977--1979, involved a total of 60 crossbred gilts (Swedish Landrace X Swedish Yorkshire). The gilts were purchased from different herds at an age of approximately 12 weeks. On arrival they were grouped three or four per pen in three separate barns (treatment groups A, B and C). Three gilts selected from each of 14 litters and six gilts from each of three other litters were allocated to the three treatment groups. The gilts were fed according to Swedish breeding-stock standards (Eriksson et al., 1972}. The allowance was based on mean b o d y weight per animal in each pen. The gilts were weighed once a week until their first heat, and thereafter every 14th day. Treatment group A. Boars were kept in adjacent pens to the gilts during the whole period. One vasectomized boar was penned with the gilts daily for 20 min during the experimental period until all pen-mates had shown their first heat. Treatment group B. Boars were kept in adjacent pens to the gilts during the whole period, b u t were never penned with the gilts. Treatment group C. No boars were kept in the barn at any time during the entire period. This study was performed six times (see Table I). Daily heat control was carried out during the whole experimental period. It comprised inspection of the vulva (swelling and reddening) and test of standing reflex. In treatment groups A and B the standing reflex was tested in the presence of a boar. The gilts belonging to treatment group C were checked without the assistance of a boar. In the case of a weak response to the "riding test" the gilt was taken
315 TABLE I Birth date, age and body weight of the gilts at the start of the experiment in the different experimental groups Experiment
Number of gilts
Birth date
I
12
August 1977
II
12
September 1977
III
9
April 1978
IV
9
April 1978
V
9
August 1979
VI
9
August 1979
Total
60
Start of experiment Mean age (days)
Mean weight (kg)
146 (143--148) 134 (132--140) 141 (137--146) 141 (137--146) 148 (141--155) 139 (136--141) 142
61 (49--75) 68 (57--78) 65 (53--89) 65 (59--79) 66 (61--74) 67 (62--73) 65
o u t o f the barn and checked in the presence of a boar. The experimental period ter min ate d when the gilts had passed at least t w o oestrous cycles. Blood samples were taken by p u n c t u r e o f an ear vein once a week during the whole experimental period and collected in heparinized tubes. After centrifugation, the plasma was r e m oved and stored in plastic tubes at --20°C until assayed. Levels o f plasma progesterone were determined using a radioimmunoassay system (Bosu et al., 1976). Laparoscopy, according to Wildt et al. (1973), was p e r f o r m e d in all gilts after their first observed heat. The ovaries were inspected for ovulations and f o r m a t i o n of corpora lutea. The occurrence o f older corpora lutea was also carefully recorded. The gilts were slaughtered after their third or fourth heat. The genital organs were removed and examined macroscopically, especially in respect of the ovarian morphology. Representative tissue pieces from the uterine mucosa were examined histologically. Statistics
The material was slightly unbalanced, and so the effects of t r e a t m e n t group and experimental group were estimated by the m e t h o d of least squares analysis, using the Statistical Analysis System (Barr et al., 1979). The following model was used. Y~ik = u + ai + b i + (ab)~" + eii k
316
where Y~ik = the i j k t h observation u = general mean a i = effect of ith treatment group (i = 1, 2, 3) b j = effect o f j t h experimental group {j = 1, 2, 3, 4, 5, 6) (ab)o" = effect of interaction between ith treatment group and jth experimental group. eijk = residual random term.
RESULTS
All gilts except one showed external heat symptoms -- reddening and swelling of the vulva -- at regular intervals during the experimental period. The length of the first oestrous cycle of the irregular gilt was 28 days. At the expected third oestrus this gilt showed neither external heat symptoms nor standing reflex. Thirty-four days after the second oestrus laparoscopic examination revealed old corpora lutea and small follicles. Fifty-two days after the second oestrus the gilt showed pronounced oestrous symptoms and ovulated. Despite reddening and swelling of the vulva, a few gilts did not stand for the boar at each oestrous period (Table II). As can be seen from this table, failure of standing reflex occurred independently of oestrous number. There was, however, a tendency to a higher incidence among the boar-exposed gilts. Judging from the appearance of the ovaries and from the progesterone levels, all gilts, including those that showed reddening and swelling of the vulva but refused to stand for the boar, ovulated. Only gilts showing standing oestrus at puberty are included in the statistical calculations. According to the finding at laparoscopic examination (presence of older corpora lutea) and plasma progesterone levels, one gilt belonging to group C (experimental group VI) had ovulated before the experimental period. This gilt therefore, together with her two litter-mates, was excluded from the presentation. TABLE II Occurrence of "silent h e a t " in the gilts during the experimental period Treatment group
A B C Total
Number of gilts not standing for a boar 1st heat
2nd heat
3rd heat
2/20 2*/20 0/20 4/60
1/20 2*/20 1/20 4/60
2**/20 5*/20 0/20 7/60
* One gilt did not stand for the boar on any occasion. **One gilt had inactive ovaries at the expected time of her third oestrus.
317 TABLE III Least squares m e a n s
of age (days) and body weight (kg) of the giltsat puberty
At puberty
Treatment groups
Age Mean S.E. Weight Mean S.E. Number of gilts
Group A
Group B
Group C
163 x 2.1 82 x 2.9 17
183 Y 2.1 98 y 2.9 17
198 z 2.0 108 z 2.8 19
XYZ: Means with these superscripts are highly significant (P < 0.001). xyz: Means with these superscripts are significant (P < 0.05).
TABLE IV Le~tsquares means ofage(days) at puberty o f t h e g i l t s i n the experimentalgroupsI--VI Experimental ~oup I II III IV V VI
Treatment ~ o u p Group A
Group B
Group C
173 180 164 163 150 148
180 182 193 176 182 186
205 183 228 218 186 168
L e a s t squares m e a n s o f age a n d b o d y w e i g h t at p u b e r t y o f t h e gilts b e l o n g i n g t o t r e a t m e n t g r o u p s A, B a n d C are p r e s e n t e d in T a b l e III. Gilts b e l o n g i n g t o g r o u p A s h o w e d t h e i r first oestrus o n average 20 d a y s earlier t h a n gilts in g r o u p B a n d 35 d a y s earlier t h a n gilts in g r o u p C. T h e s e d i f f e r e n c e s are highly significant (P < 0.001). H o w e v e r , t h e s t i m u l a t o r y e f f e c t o f t h e b o a r o n t h e age at p u b e r t y varied b e t w e e n t h e six e x p e r i m e n t a l g r o u p s ( T a b l e IV), a n d t h e i n t e r a c t i o n b e t w e e n t r e a t m e n t a n d e x p e r i m e n t a l g r o u p s was significant (P < 0.01). In all e x p e r i m e n t a l g r o u p s gilts b e l o n g i n g to g r o u p A r e a c h e d p u b e r t y earlier t h a n gilts b e l o n g i n g t o g r o u p s B or C. T h e lengths o f t h e sexual cycles w e r e n o t i n f l u e n c e d b y t h e d i f f e r e n t t r e a t m e n t s (P > 0.05). No pathological abnormalities were found on post-mortem examination of oviducts, u t e r u s , cervix a n d vagina. Histological e x a m i n a t i o n o f t h e u t e r i n e wall revealed c h r o n i c e n d o m e t r i t i s in o n e gilt. T h e m a j o r i t y o f t h e c o r p o r a l u t e a w e r e solid, b u t in s o m e gilts t h e r e was a c o m b i n a t i o n o f solid a n d cystic c o r p o r a lutea, a n d o c c a s i o n a l l y o n e o r t w o l u t e i n i z e d cysts. N o follicle cysts w e r e f o u n d at p o s t - m o r t e m e x a m i n a t i o n . N o d i f f e r e n c e in n u m b e r o f c o r p o r a l u t e a or n u m b e r s o f cystic c o r p o r a l u t e a a n d l u t e i n i z e d c y s t s was f o u n d b e t w e e n t h e t r e a t m e n t groups.
318 DISCUSSION
The gilts belonging to group B in the present study reached puberty at a mean age of 183 days. The boar stimulation to which these gilts were exposed corresponds well with the management in gilt-producing herds in Sweden. Swedish crossbred gilts, kept at the clinic under similar environmental conditions for puberty studies, were sexually mature at approximately the same age (Andersson and Einarsson, 1980; Karlbom et al., 1982). The boar-exposed gilts reached puberty earlier than unexposed gilts. This was demonstrated earlier in several studies (e.g. Brooks and Cole, 1970; Thompson and Savage, 1978). In contrast to the opinion advanced by Brooks (1978) that gilts must be reared in isolation from boars in order to be able to respond to the "male effect", the present results clearly demonstrate a positive effect on puberty in gilts exposed to boars from 3 months of age. Daily introduction of the boars to the gilts from an age of approximately 142 days stimulated the gilts to attain puberty still earlier. Mavrogenis and Robison (1976) observed that a higher incidence of "silent heats '~ occurred before the first detected oestrus among gilts exposed to boars than among unexposed gilts. Alliston et al. (1974) found active ovaries at slaughter in a number of boar-exposed gilts, not observed in heat for 46 days. In the present study some boar-exposed gilts with reddening and swelling of the vulva and subsequent formation of corpora lutea refused to stand for the boar. This occurred regardless of oestrous number and was therefore not related to puberty. The reason for failure of the standing reaction in gilts is not known. ACKNOWLEDGEMENTS
This study was supported by grants from the Swedish Council for Forestry and Agricultural Research. The author wishes to acknowledge the valuable help with the statistical analyses given by Leif GSransson and the technical assistance given by Mari-Anne Carlsson, Eva Hedberg, Britt J 5nsson, Kerstin Lindblad and Mari WaUbring.
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