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The effect of limiting the number of days of contact with boars, season and herd of origin on the attainment of puberty in gilts
Animal Reproduction Science, 18 (1989) 293-301
293
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
The Effect of L i m i t i n g the N u m b e r of D a y s of Contact w i t h Boars, Season and Herd of Origin on the Attainment of P u b e r t y in Gilts A.M. PATERSON 1, P.E. HUGHES 2'4and G.P. PEARCE 3
1Department o/Agriculture, South Perth, W.A. 6151 (Australia) 2Muresh Institute o/Agriculture, Northam, W.A. 6401 (Australia) 3Animal Science Group, School o/Agriculture, University o/Western Australia, Nedlands, W.A. 6009 (Australia) 4present address: School of Agriculture and Forestry, University of Melbourne, Parkville, Vic. 3052, Australia. (Accepted 5 September 1988)
ABSTRACT Paterson, A.M., Hughes, P.E. and Pearce, G.P., 1989. The effect of limiting the number of days of contact with boars, season and herd of origin on the attainment of puberty in gilts. Anita. Reprod. Sci., 18: 293-301. The effect of limiting the number of days of boar exposure on the attainment of puberty in gilts was examined in two herds (Medina and Muresk) and in two seasons (winter and summer). The treatments used were full boar contact for 20-30 rain per day from 165 days of age for (a) I day, (b) 10 consecutive days or (c) daily until puberty or slaughter and (d) an isolated control. Those not detected in oestrus were slaughtered 80 days (Medina) or 135 days (Muresk) after first boar exposure. In the winter neither treatment nor herd had any significant effect on the proportion of gilts which reached puberty (0.93 in both herds). In the summer a lower proportion of control gilts reached puberty at Medina than in the winter (0.33 vs. 1.0, P<0.01) but the proportion of each group exposed to boars which reached puberty was not affected by season. At Muresk the proportion of gilts reaching puberty in the summer was lower than in the winter for control (0 vs. 0.86, P < 0.01 ), 1-day (0 vs. 1.0, P < 0.01 ) and 10-day (0.38 vs. 1.0, P < 0.05) gilts but the proportion of daily gilts was 0.86 in both seasons. In both herds 10-day and daily gilts reached puberty earlier than control gilts (P<0.01). At each treatment level the Medina gilts reached puberty significantly earlier ( P < 0.001 ) than the Muresk gilts. When boar exposure was limited to I day or 10 consecutive days a degree of stimulation took place but daily exposure continuing until puberty was necessary for the maximum stimulation of puberty.
INTRODUCTION I t is n o w well e s t a b l i s h e d t h a t e x p o s u r e t o m a t u r e b o a r s c a n i n d u c e p r e c o c i o u s p u b e r t y in gilts ( B r o o k s a n d Cole, 1970; M a v r o g e n i s a n d R o b i s o n , 1976;
0378-4320/89/$03.50
© 1989 Elsevier Science Publishers B.V.
294 Thompson and Savage, 1978) and many of the factors which affect the response have been identified (see Hughes, 1982, for review). The boar effect is believed to be mediated by a synergism between pheromones released by the boar and stress induced by physical contact with the boar (Kirkwood et al., 1981; Pearce and Hughes, 1987a,b) but exactly when the signal (s) which initiate final development towards puberty are perceived by the gilt is unknown. Indeed it is not known whether stimulation by the boar is necessary on a daily basis or whether a limited period of stimulation can initiate the sequence of events leading to puberty. This knowledge would be useful at the practical level because it may simplify the application of the boar effect. At the physiological level it may identify the period after boar exposure when the changes preceding puberty take place and therefore allow detailed study of these changes. Many authors have reported that season of birth affects age at puberty but the data are equivocal. Some studies indicate that autumn-born gilts reach puberty earlier than spring-born gilts (Gossett and Sorensen, 1959; Mavrogenis and Robison, 1976), some indicate the opposite (Sorensen et al., 1961; Scanlon and Krishnamurthy, 1974) and some have found no difference at all (Wise and Robertson, 1953; Christenson and Ford, 1979). Such inconsistency suggests that stimulatory or inhibitory influences other than season may have confounded the results. In this study we investigated whether limiting contact with boars to one or 10 days at the beginning of the period of exposure would be as effective at inducing puberty as contact on a continuing daily basis. Identical protocols of boar contact were applied in the winter and the summer in two herds which differed markedly in housing and genotype to examine the effect of season and herd of origin on the attainment of puberty of the gilts under the differing boar exposure regimes. MATERIALSAND METHODS
Animals and housing The herds at Muresk Institute of Agriculture and the Western Australian Department of Agriculture's Intensive Industries Research Centre, Medina were used in this experiment. In each herd one replicate was performed in each of two seasons with boar contact commencing in either the winter (July/August) or the summer (December/January). At Muresk a total of 58 gilts (28 in winter, 30 in summer) were used. They were the progeny of sows from a Large White/Landrace synthetic line which had been selected for prolificacy (Tomes and Newman, 1982 ) mated to Large White boars. The gilts were housed in groups of 6-9 in pens comprising an enclosed area of 3.1 X 2.8 m with access to an outdoor area 3.1 × 4.7 m. These pens were at least 50 m from any buildings housing mature boars.
295 At Medina a total of 65 Large White/Landrace crossbred gilts (29 in winter, 36 in summer) were housed in groups of 6-9 in pens 3.2X2.4 m (including 1.2 X 2.4 m slatted area) in a fully enclosed grower shed with a capacity of 720 grower pigs. The shed contained no mature boars or male growers over 180 days of age and no males over 100 days of age were housed within 25 m of the gilts.
Treatments
In each herd the gilts in each replicate were allocated according to litter of origin and body weight so that litter mates were spread across the four treatment groups which were: (1) Exposure to a boar on 1 day. (2) Exposure to a boar on 10 consecutive days. (3) Exposure to a boar daily until puberty or termination. (4) Isolated controls receiving no boar exposure. Boar exposure commenced at an overall mean age of 164.6 _+0.44 days (range 160-169 days, n = 123 ). It consisted of walking each pen of gilts which required exposure to a set of yards and allowing them full physical contact with a mature working boar over 18 months old for 30 min. At Muresk the yards were located over 40 m from and out of sight of the gilts' home pens while at Medina the yards were adjacent to and outside the grower shed, about 30 m from the gilts' home pens. In each herd the same boar was used for the summer and winter replicate. When more than 1 group required exposure on a single day (day 110) the order in which the groups were presented to the boar were rotated each day. The gilts were checked daily for signs of oestrus. In groups whose protocol on that day involved boar exposure oestrus detection was performed using the back-pressure test (Signoret, 1972) while the gilts had fenceline contact with the boar in the yards immediately before exposure. In groups which did not receive boar exposure the gilts were checked in the home pens using the backpressure test in the absence of a boar in conjunction with visual criteria such as development of the vulva and behavioural changes such as mounting of pen mates. Gilts were slaughtered within 15 days of being detected in oestrus and their reproductive tracts recovered to confirm their reproductive status. Two gilts classified as in oestrus in the absence of a boar were prepubertal at slaughter and no data from these gilts are included here. Gilts not detected in oestrus by 80 days (Medina) or 135 days (Muresk) after first boar exposure were slaughtered to confirm they were prepubertal at this stage.
296
Statistical analysis All statistical analyses were performed on a PRIME computer using the GENSTAT statistical programme (Rothamstead Experimental Station, U.K.). Generalized linear regression models (McCullagh and Nelder, 1983) were fitted to the data for initial weight, initial age and days from first boar exposure to puberty in preference to analysis of variance procedures because the numbers of animals were not proportionally balanced for the factorial treatments which were level of boar exposure ( 4 ) x h e r d (2))
297 110
100
90
8O ~3
70 .¢,., J~
,-,
60
m
50
e-
"
40
30
20
10
0
1
10
Daily
Treatment
Fig.1. Effect of herd on mean interval to puberty attainment. Dark shading = Muresk; light shadi n g = Medina; a, b, c and w, x, y, z: values with different letters are significantly different ( P < 0.01 ). Significant difference between herd (P<0.001).
DISCUSSION
The inverse relationship between the number of days of boar exposure and the interval to puberty indicates that although limited boar exposure did affect the attainment of puberty the repeated stimulus provided by daily boar contact was necessary for maximum stimulation of puberty. Even though the age at puberty varied widely between the herds one day of boar exposure advanced puberty by about 10 days, ten days of exposure by about 22 days and daily exposure by about 37 days compared with isolated controls in both herds. The Medina herd was the more responsive and gilts exposed to boars for 10
298 MEDINA .p
I00 90 80 70 u
60 50 40
30 20 o
10 0 1
10
Daily
1
10
Daily
MURESK lO0 ,.Q o.,
90 8o
.,4 ,=::: u nJ
7o
Ii
5o
4~ .,.-i
60
4o 30 20
o
lO o o
Treatment
Fig. 2. Effect of season on the percentage of gilts attaining puberty before "cut-off". Dark shading= winter; light shading= summer; a vs. b: within herd values with different letters are significantly different (P<0.05). days at Medina started reaching puberty before boar exposure was complete. In contrast the first of the Muresk gilts exposed for 10 days did not reach puberty until day 47 and the median for the group was 73 days. Gilts exposed to boars for one day also exhibited a long delay between the completion of exposure and the onset of puberty. It appears t h a t both 1 and 10 days of exposure to the boar initiated changes which finally resulted in the gilts attaining puberty earlier t h a n isolated controls but what these changes were and why their influence was delayed for so long is not known.
299 The proportion of gilts reaching puberty when isolated from boars in both herds or in response to limited boar exposure at Muresk was depressed in the summer and the mean interval from first boar exposure to puberty tended to be longer in summer for gilts receiving daily boar contact in both herds. Seasonal effects on reproductive activity in many species are mediated by changes in photoperiod a n d / o r ambient temperature (Tavant et al., 1966; Lincoln and Short, 1980; Arendt, 1986). Studies with controlled photoperiod indicate that long daylengths do not delay puberty in gilts (Hacker et al., 1979; Diekman and Hoagland, 1983 ) but high ambient temperatures may (Pearce and Hughes, 1985; Flowers et al., 1986). In the present study limited boar exposure was less effective in the summer and the seasonal effect on natural attainment of puberty was more pronounced at Muresk where the housing provided less control over the environment and the summer temperatures were higher which suggests that high ambient temperature may be the major environmental factor responsible for the seasonal effects observed in this study. Wide between-herd variation in age at puberty was observed in both seasons. Apart from differences in the climatic and physical environment the genotype of the gilts was different in each herd. In the Muresk herd selection of breeding stock was based solely on litter size but at Medina selection was based on growth rate and carcase traits. As a result the Medina pigs grew much faster and this was reflected by the large differences in body weight at the start of boar exposure. However the between-herd variation in response to treatment can not be explained by body weight differences alone. Regression analysis showed no relationship between initial body weight and the interval from exposure to puberty. At Medina the gilts were heavier in the summer than in the winter but at Muresk the gilts were lighter in the summer. Despite this reversal of the body weight differences the seasonal effect on attainment of puberty followed the same pattern in both herds. In other puberty studies at Medina (Paterson and Lindsay, 1980) the gilts were much lighter, weighing about the same as our Muresk gilts but their response to daily boar exposure and the onset of puberty in isolated control gilts was almost identical to that observed at Medina in the present study. This suggests that genotype rather than initial body weight was the major determinant of the between-herd variation in the onset of puberty. In conclusion the results of this experiment indicate that although limited boar exposure can stimulate puberty relative to isolated controls continued daily contact with boars until puberty is necessary for maximum stimulation of puberty in gilts. ACKNOWLEDGEMENTS This work was supported by a grant from the Pig Research Council. We thank Mr. M. D'Antuono for his assistance with the statistical analysis of the
300
data and Dr. P.H. Hemsworth and Prof. D.R. Lindsay for comments on the manuscript.
REFERENCES Arendt, J., 1986. Role of the pineal gland and melatonin in seasonal reproductive function in mammals. Oxf. Rev. Reprod. Biol., 8: 266-320. 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. Fert., 23: 435-440. Christenson, R.K. and Ford, J.J., 1979. Puberty and estrus in confinement-reared gilts. J. Anim. Sci., 49: 743-751. Diekman, M.A. and Hoagland, T.A., 1983. Influence of supplemental lighting during periods of increasing or decreasing daylength on the onset of puberty in gilts. J. Anita. Sci., 57: 12351242. Flowers, B.F., Cantley, T.C., Martin, M.J. and Day, B.N., 1986. Effects of elevated ambient temperature on age of puberty in gilts. J. Anita. Sci., 63 (Suppl. 1 ): p. 126 (abstract). Gossett, J.W. and Sorensen, A.M., 1959. The effect of two levels of energy and seasons on reproductive phenomena in gilts. J. Anita. Sci., 18: 40-47. Hacker, R.R., King, G.J., Ntunde, B.N. and Narendran, R., 1979. Plasma estrogen, progesterone and other reproductive responses of gilts to photoperiod. J. Reprod. Fert., 57: 447-451. Hughes, P.E., 1982. Factors affecting the natural attainment of puberty in the gilt. In: D.J.A. Cole and G.R. Foxcroft (Editors), Control of Pig Reproduction. Butt~rworths, London, pp. 117138. Kirkwood, R.N., Forbes, J.M. and Hughes, P.E., 1981. Influence of boar contact on attainment of puberty in gilts after removal of the olfactory bulbs. J. Reprod. Fert., 61: 193-196. Lincoln, G.A. and Short, R.V., 1980. Seasonal breeding: Nature's contraceptive. Rec. Prog. Horm. Res., 36: 1-52. Mavrogenis, A.P. and Robison, O.W., 1976. Factors affecting puberty in swine. J. Anita. Sci., 42: 1251-1255. I McCullagh, P. and Nelder, J.A., 1983. Generalized Linear Models. Chapman and Hall, London, pp. 1-261. Paterson, A.M. and Lindsay, D.R., 1980. Induction of puberty in gilts. 1. The effects of rearing conditions on reproductive performance and response to mature boars after early puberty. Anita. Prod., 31: 291-297. Pearce, G.P. and Hughes, P.E., 1985. The influence of daily movement of gilts and the environment in which boar exposure occurs on the efficacy of boar-induced precocious puberty in the gilt. Anita. Prod., 40: 161-167. Pearce, G.P. and Hughes, P.E., 1987a. The influence of boar-component stimuli on puberty attainment in the gilt. Anim. Prod., 44: 293-302. Pearce, G.P. and Hughes, P.E., 1987b. The influence of male contact on plasma cortisol concentrations in the prepubertal gilt. J. Reprod. Fert., 80: 417-424. Scanlon, P.F. and Krishnamurthy, S., 1974. Puberty attainment in slaughter weight gilts in relation to month examined. J. Anita. Sci., 39:160 (abstract). Signoret, J.P., 1972. The mating behaviour of the sow. In: D.J.A. Cole (Editor), Pig Production. Butterworths, London, pp. 295-313. Sorensen, A.M., Thomas, W.B. and Gossett, J.W., 1961. A further study on the influence of level of energy intake and season on reproductive performance of gilts. J. Anim. Sei., 20: 347-359. Tavant, R., Pelletier, J. and Signoret, J.P., 1966. Regulation of breeding season and oestrus cycles by light and external stimuli in some mammals. J. Anim. Sci., 25:119-139.
301 Thompson, L.H. and Savage, J.S., 1978. Age at puberty and ovulation rate in confinement as influenced by exposure to a boar. J. Anita. Sci., 47: 1141-1144. Tomes, G.J. and Newman, R.B., 1982. Initial responses to selection for litter size in pigs. Proc. Aust. Soc. Anita. Prod., 14: 569-571. Wise, F.S. and Robertson, G.L., 1953. Some effects of sexual age on reproductive performance in gilts. J. Anita. Sci., 12:957 (abstract).
293
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
The Effect of L i m i t i n g the N u m b e r of D a y s of Contact w i t h Boars, Season and Herd of Origin on the Attainment of P u b e r t y in Gilts A.M. PATERSON 1, P.E. HUGHES 2'4and G.P. PEARCE 3
1Department o/Agriculture, South Perth, W.A. 6151 (Australia) 2Muresh Institute o/Agriculture, Northam, W.A. 6401 (Australia) 3Animal Science Group, School o/Agriculture, University o/Western Australia, Nedlands, W.A. 6009 (Australia) 4present address: School of Agriculture and Forestry, University of Melbourne, Parkville, Vic. 3052, Australia. (Accepted 5 September 1988)
ABSTRACT Paterson, A.M., Hughes, P.E. and Pearce, G.P., 1989. The effect of limiting the number of days of contact with boars, season and herd of origin on the attainment of puberty in gilts. Anita. Reprod. Sci., 18: 293-301. The effect of limiting the number of days of boar exposure on the attainment of puberty in gilts was examined in two herds (Medina and Muresk) and in two seasons (winter and summer). The treatments used were full boar contact for 20-30 rain per day from 165 days of age for (a) I day, (b) 10 consecutive days or (c) daily until puberty or slaughter and (d) an isolated control. Those not detected in oestrus were slaughtered 80 days (Medina) or 135 days (Muresk) after first boar exposure. In the winter neither treatment nor herd had any significant effect on the proportion of gilts which reached puberty (0.93 in both herds). In the summer a lower proportion of control gilts reached puberty at Medina than in the winter (0.33 vs. 1.0, P<0.01) but the proportion of each group exposed to boars which reached puberty was not affected by season. At Muresk the proportion of gilts reaching puberty in the summer was lower than in the winter for control (0 vs. 0.86, P < 0.01 ), 1-day (0 vs. 1.0, P < 0.01 ) and 10-day (0.38 vs. 1.0, P < 0.05) gilts but the proportion of daily gilts was 0.86 in both seasons. In both herds 10-day and daily gilts reached puberty earlier than control gilts (P<0.01). At each treatment level the Medina gilts reached puberty significantly earlier ( P < 0.001 ) than the Muresk gilts. When boar exposure was limited to I day or 10 consecutive days a degree of stimulation took place but daily exposure continuing until puberty was necessary for the maximum stimulation of puberty.
INTRODUCTION I t is n o w well e s t a b l i s h e d t h a t e x p o s u r e t o m a t u r e b o a r s c a n i n d u c e p r e c o c i o u s p u b e r t y in gilts ( B r o o k s a n d Cole, 1970; M a v r o g e n i s a n d R o b i s o n , 1976;
0378-4320/89/$03.50
© 1989 Elsevier Science Publishers B.V.
294 Thompson and Savage, 1978) and many of the factors which affect the response have been identified (see Hughes, 1982, for review). The boar effect is believed to be mediated by a synergism between pheromones released by the boar and stress induced by physical contact with the boar (Kirkwood et al., 1981; Pearce and Hughes, 1987a,b) but exactly when the signal (s) which initiate final development towards puberty are perceived by the gilt is unknown. Indeed it is not known whether stimulation by the boar is necessary on a daily basis or whether a limited period of stimulation can initiate the sequence of events leading to puberty. This knowledge would be useful at the practical level because it may simplify the application of the boar effect. At the physiological level it may identify the period after boar exposure when the changes preceding puberty take place and therefore allow detailed study of these changes. Many authors have reported that season of birth affects age at puberty but the data are equivocal. Some studies indicate that autumn-born gilts reach puberty earlier than spring-born gilts (Gossett and Sorensen, 1959; Mavrogenis and Robison, 1976), some indicate the opposite (Sorensen et al., 1961; Scanlon and Krishnamurthy, 1974) and some have found no difference at all (Wise and Robertson, 1953; Christenson and Ford, 1979). Such inconsistency suggests that stimulatory or inhibitory influences other than season may have confounded the results. In this study we investigated whether limiting contact with boars to one or 10 days at the beginning of the period of exposure would be as effective at inducing puberty as contact on a continuing daily basis. Identical protocols of boar contact were applied in the winter and the summer in two herds which differed markedly in housing and genotype to examine the effect of season and herd of origin on the attainment of puberty of the gilts under the differing boar exposure regimes. MATERIALSAND METHODS
Animals and housing The herds at Muresk Institute of Agriculture and the Western Australian Department of Agriculture's Intensive Industries Research Centre, Medina were used in this experiment. In each herd one replicate was performed in each of two seasons with boar contact commencing in either the winter (July/August) or the summer (December/January). At Muresk a total of 58 gilts (28 in winter, 30 in summer) were used. They were the progeny of sows from a Large White/Landrace synthetic line which had been selected for prolificacy (Tomes and Newman, 1982 ) mated to Large White boars. The gilts were housed in groups of 6-9 in pens comprising an enclosed area of 3.1 X 2.8 m with access to an outdoor area 3.1 × 4.7 m. These pens were at least 50 m from any buildings housing mature boars.
295 At Medina a total of 65 Large White/Landrace crossbred gilts (29 in winter, 36 in summer) were housed in groups of 6-9 in pens 3.2X2.4 m (including 1.2 X 2.4 m slatted area) in a fully enclosed grower shed with a capacity of 720 grower pigs. The shed contained no mature boars or male growers over 180 days of age and no males over 100 days of age were housed within 25 m of the gilts.
Treatments
In each herd the gilts in each replicate were allocated according to litter of origin and body weight so that litter mates were spread across the four treatment groups which were: (1) Exposure to a boar on 1 day. (2) Exposure to a boar on 10 consecutive days. (3) Exposure to a boar daily until puberty or termination. (4) Isolated controls receiving no boar exposure. Boar exposure commenced at an overall mean age of 164.6 _+0.44 days (range 160-169 days, n = 123 ). It consisted of walking each pen of gilts which required exposure to a set of yards and allowing them full physical contact with a mature working boar over 18 months old for 30 min. At Muresk the yards were located over 40 m from and out of sight of the gilts' home pens while at Medina the yards were adjacent to and outside the grower shed, about 30 m from the gilts' home pens. In each herd the same boar was used for the summer and winter replicate. When more than 1 group required exposure on a single day (day 110) the order in which the groups were presented to the boar were rotated each day. The gilts were checked daily for signs of oestrus. In groups whose protocol on that day involved boar exposure oestrus detection was performed using the back-pressure test (Signoret, 1972) while the gilts had fenceline contact with the boar in the yards immediately before exposure. In groups which did not receive boar exposure the gilts were checked in the home pens using the backpressure test in the absence of a boar in conjunction with visual criteria such as development of the vulva and behavioural changes such as mounting of pen mates. Gilts were slaughtered within 15 days of being detected in oestrus and their reproductive tracts recovered to confirm their reproductive status. Two gilts classified as in oestrus in the absence of a boar were prepubertal at slaughter and no data from these gilts are included here. Gilts not detected in oestrus by 80 days (Medina) or 135 days (Muresk) after first boar exposure were slaughtered to confirm they were prepubertal at this stage.
296
Statistical analysis All statistical analyses were performed on a PRIME computer using the GENSTAT statistical programme (Rothamstead Experimental Station, U.K.). Generalized linear regression models (McCullagh and Nelder, 1983) were fitted to the data for initial weight, initial age and days from first boar exposure to puberty in preference to analysis of variance procedures because the numbers of animals were not proportionally balanced for the factorial treatments which were level of boar exposure ( 4 ) x h e r d (2))
297 110
100
90
8O ~3
70 .¢,., J~
,-,
60
m
50
e-
"
40
30
20
10
0
1
10
Daily
Treatment
Fig.1. Effect of herd on mean interval to puberty attainment. Dark shading = Muresk; light shadi n g = Medina; a, b, c and w, x, y, z: values with different letters are significantly different ( P < 0.01 ). Significant difference between herd (P<0.001).
DISCUSSION
The inverse relationship between the number of days of boar exposure and the interval to puberty indicates that although limited boar exposure did affect the attainment of puberty the repeated stimulus provided by daily boar contact was necessary for maximum stimulation of puberty. Even though the age at puberty varied widely between the herds one day of boar exposure advanced puberty by about 10 days, ten days of exposure by about 22 days and daily exposure by about 37 days compared with isolated controls in both herds. The Medina herd was the more responsive and gilts exposed to boars for 10
298 MEDINA .p
I00 90 80 70 u
60 50 40
30 20 o
10 0 1
10
Daily
1
10
Daily
MURESK lO0 ,.Q o.,
90 8o
.,4 ,=::: u nJ
7o
Ii
5o
4~ .,.-i
60
4o 30 20
o
lO o o
Treatment
Fig. 2. Effect of season on the percentage of gilts attaining puberty before "cut-off". Dark shading= winter; light shading= summer; a vs. b: within herd values with different letters are significantly different (P<0.05). days at Medina started reaching puberty before boar exposure was complete. In contrast the first of the Muresk gilts exposed for 10 days did not reach puberty until day 47 and the median for the group was 73 days. Gilts exposed to boars for one day also exhibited a long delay between the completion of exposure and the onset of puberty. It appears t h a t both 1 and 10 days of exposure to the boar initiated changes which finally resulted in the gilts attaining puberty earlier t h a n isolated controls but what these changes were and why their influence was delayed for so long is not known.
299 The proportion of gilts reaching puberty when isolated from boars in both herds or in response to limited boar exposure at Muresk was depressed in the summer and the mean interval from first boar exposure to puberty tended to be longer in summer for gilts receiving daily boar contact in both herds. Seasonal effects on reproductive activity in many species are mediated by changes in photoperiod a n d / o r ambient temperature (Tavant et al., 1966; Lincoln and Short, 1980; Arendt, 1986). Studies with controlled photoperiod indicate that long daylengths do not delay puberty in gilts (Hacker et al., 1979; Diekman and Hoagland, 1983 ) but high ambient temperatures may (Pearce and Hughes, 1985; Flowers et al., 1986). In the present study limited boar exposure was less effective in the summer and the seasonal effect on natural attainment of puberty was more pronounced at Muresk where the housing provided less control over the environment and the summer temperatures were higher which suggests that high ambient temperature may be the major environmental factor responsible for the seasonal effects observed in this study. Wide between-herd variation in age at puberty was observed in both seasons. Apart from differences in the climatic and physical environment the genotype of the gilts was different in each herd. In the Muresk herd selection of breeding stock was based solely on litter size but at Medina selection was based on growth rate and carcase traits. As a result the Medina pigs grew much faster and this was reflected by the large differences in body weight at the start of boar exposure. However the between-herd variation in response to treatment can not be explained by body weight differences alone. Regression analysis showed no relationship between initial body weight and the interval from exposure to puberty. At Medina the gilts were heavier in the summer than in the winter but at Muresk the gilts were lighter in the summer. Despite this reversal of the body weight differences the seasonal effect on attainment of puberty followed the same pattern in both herds. In other puberty studies at Medina (Paterson and Lindsay, 1980) the gilts were much lighter, weighing about the same as our Muresk gilts but their response to daily boar exposure and the onset of puberty in isolated control gilts was almost identical to that observed at Medina in the present study. This suggests that genotype rather than initial body weight was the major determinant of the between-herd variation in the onset of puberty. In conclusion the results of this experiment indicate that although limited boar exposure can stimulate puberty relative to isolated controls continued daily contact with boars until puberty is necessary for maximum stimulation of puberty in gilts. ACKNOWLEDGEMENTS This work was supported by a grant from the Pig Research Council. We thank Mr. M. D'Antuono for his assistance with the statistical analysis of the
300
data and Dr. P.H. Hemsworth and Prof. D.R. Lindsay for comments on the manuscript.
REFERENCES Arendt, J., 1986. Role of the pineal gland and melatonin in seasonal reproductive function in mammals. Oxf. Rev. Reprod. Biol., 8: 266-320. 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. Fert., 23: 435-440. Christenson, R.K. and Ford, J.J., 1979. Puberty and estrus in confinement-reared gilts. J. Anim. Sci., 49: 743-751. Diekman, M.A. and Hoagland, T.A., 1983. Influence of supplemental lighting during periods of increasing or decreasing daylength on the onset of puberty in gilts. J. Anita. Sci., 57: 12351242. Flowers, B.F., Cantley, T.C., Martin, M.J. and Day, B.N., 1986. Effects of elevated ambient temperature on age of puberty in gilts. J. Anita. Sci., 63 (Suppl. 1 ): p. 126 (abstract). Gossett, J.W. and Sorensen, A.M., 1959. The effect of two levels of energy and seasons on reproductive phenomena in gilts. J. Anita. Sci., 18: 40-47. Hacker, R.R., King, G.J., Ntunde, B.N. and Narendran, R., 1979. Plasma estrogen, progesterone and other reproductive responses of gilts to photoperiod. J. Reprod. Fert., 57: 447-451. Hughes, P.E., 1982. Factors affecting the natural attainment of puberty in the gilt. In: D.J.A. Cole and G.R. Foxcroft (Editors), Control of Pig Reproduction. Butt~rworths, London, pp. 117138. Kirkwood, R.N., Forbes, J.M. and Hughes, P.E., 1981. Influence of boar contact on attainment of puberty in gilts after removal of the olfactory bulbs. J. Reprod. Fert., 61: 193-196. Lincoln, G.A. and Short, R.V., 1980. Seasonal breeding: Nature's contraceptive. Rec. Prog. Horm. Res., 36: 1-52. Mavrogenis, A.P. and Robison, O.W., 1976. Factors affecting puberty in swine. J. Anita. Sci., 42: 1251-1255. I McCullagh, P. and Nelder, J.A., 1983. Generalized Linear Models. Chapman and Hall, London, pp. 1-261. Paterson, A.M. and Lindsay, D.R., 1980. Induction of puberty in gilts. 1. The effects of rearing conditions on reproductive performance and response to mature boars after early puberty. Anita. Prod., 31: 291-297. Pearce, G.P. and Hughes, P.E., 1985. The influence of daily movement of gilts and the environment in which boar exposure occurs on the efficacy of boar-induced precocious puberty in the gilt. Anita. Prod., 40: 161-167. Pearce, G.P. and Hughes, P.E., 1987a. The influence of boar-component stimuli on puberty attainment in the gilt. Anim. Prod., 44: 293-302. Pearce, G.P. and Hughes, P.E., 1987b. The influence of male contact on plasma cortisol concentrations in the prepubertal gilt. J. Reprod. Fert., 80: 417-424. Scanlon, P.F. and Krishnamurthy, S., 1974. Puberty attainment in slaughter weight gilts in relation to month examined. J. Anita. Sci., 39:160 (abstract). Signoret, J.P., 1972. The mating behaviour of the sow. In: D.J.A. Cole (Editor), Pig Production. Butterworths, London, pp. 295-313. Sorensen, A.M., Thomas, W.B. and Gossett, J.W., 1961. A further study on the influence of level of energy intake and season on reproductive performance of gilts. J. Anim. Sei., 20: 347-359. Tavant, R., Pelletier, J. and Signoret, J.P., 1966. Regulation of breeding season and oestrus cycles by light and external stimuli in some mammals. J. Anim. Sci., 25:119-139.
301 Thompson, L.H. and Savage, J.S., 1978. Age at puberty and ovulation rate in confinement as influenced by exposure to a boar. J. Anita. Sci., 47: 1141-1144. Tomes, G.J. and Newman, R.B., 1982. Initial responses to selection for litter size in pigs. Proc. Aust. Soc. Anita. Prod., 14: 569-571. Wise, F.S. and Robertson, G.L., 1953. Some effects of sexual age on reproductive performance in gilts. J. Anita. Sci., 12:957 (abstract).