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The effect of early contact with mature boars on reproductive efficiency in the gilt
Animal Reproduction Science, 6 (1983) 51--57
51
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
THE EFFECT OF EARLY CONTACT WITH MATURE BOARS ON REPRODUCTIVE EFFICIENCY IN THE GILT
G.M. CRONIN
Department of Agriculture, Animal Research Institute, Werribee, Vict. 3030 (Australia) Present address: Ethology Section, Department of Animal Husbandry, Agricultural University, Wageningen (The Netherlands) (Accepted 14 February 1983)
ABSTRACT Cronin, G.M., 1983. The effect o f early contact with mature boars on reproductive efficiency in the gilt. Anita. Reprod. Sci., 6: 51--57. The effect of first contact of gilts with a mature boar at 23 or 28 weeks of age on their subsequent reproductive efficiency was studied over a 12-month period at a large intensive piggery in southern Australia. Following this contact, the gilts entered the mating shed at 29 weeks of age and were checked daily for oestrus, as assessed by the back-pressure test in the presence of the boar. Gilts that showed moderate or high responses were taken to a boar for mating. Sexual receptivity was then assessed by the time taken to " s t a n d " after the first m o u n t by the boar. Gilts that remained unmated at 35 weeks of age were culled, and their ovaries were examined. Of the 2660 gilts in the study, 2349 were mated and they had a farrowing rate of 88.2% with a mean litter size of 9.5 piglets, of which 0.7 piglets (7.4%) were born dead. The reproductive efficiency of the gilts following earlier contact with the boar was consistently higher than that of gilts exposed later. The mating rate of the week 23 gilts was greater than that of the week 28 gilts (70.1 vs 66.0%, P < 0.01), more appeared to show a high level of sexual receptivity (97.0 and 94.6%, N.S.) and fewer failed to mate when put to a boar (6.1 vs 9.5%, P < 0.01). The percentage of prepubertal gilts at 35 weeks of age was also lower (1.46 vs 3.03%, P < 0.01). The improved reproductive performance was estimated to be equivalent to 0.24 extra piglets born per gilt.
INTRODUCTION
The importance of contact with mature boars during rearing on reducing age at puberty of gilts is well established (Brooks and Cole, 1970; Hughes and Cole, 1976; Kirkwood and Hughes, 1979). Further, gilts that mated at the second oestrus after an induced puberty had similar lifetime production to gilts mated after a natural puberty (Brooks and Smith, 1980). Nevertheless, little is known of the effects of induced puberty on other reproductive characters and overall reproductive efficiency. Hemsworth et al. (1982) have shown that lack of mature boar contact during the rearing period can
0378-4320/83/$03.00
© 1983 Elsevier Science Publishers B.V.
52
lower oestrous expression and sexual receptivity. The aim of this study was to measure the reproductive efficiency of gilts exposed to mature boars from either 23 or 28 weeks of age in a commercial herd. Criteria measured were incidence of oestrus (sexual receptivity: Roberts, 1971), intensity of oestrus, acceptance of the boar and proportion of gilts that reached puberty by 35 weeks of age. The study was performed over a 12-month period at a large intensive piggery in central Victoria. MATERIALS AND METHODS
Animals Similar numbers of gilts experienced each treatment (week 23: 1474, week 28: 1438). There were three breeds of gilts (Large White, Landrace and a synthetic breed (M) selected from crossbreds of the former breeds on the basis of large litter size). Also, there were crossbred gilts of these three breeds. At 21 weeks of age, gilts were selected for breeding on the basis of growth rate and back-fat thickness.
Feeding The gilts were fed ad libitum on a grower ration to 21 weeks of age, when feed was restricted to 2.5 kg/gilt/day until week 27. During weeks 27 and 28, feed was again available ad libitum.
Early boar contact Alternate weekly groups of gilts, which ranged in size from 45 to 73 gilts, were allotted to the two treatments. The two mature boars (A and B) used as contact boars were 3 years old at the c o m m e n c e m e n t of the study. Treatment 1: Gilts week 23. From 23 to 27 weeks of age, the gilts were housed in shed A and contact boar A was introduced into their pen for 15 min daily. At 27 weeks, the gilts were transferred to shed B, and each day during the following week contact boar B was introduced for 15 min. Treatment 2: Gilts week 28. From 23 to 27 weeks of age, the gilts were housed in a shed identical to shed A, but w i t h o u t boars present. At 27 weeks of age the gilts were transferred to shed B, and during the following week contact boar B was introduced into the gilts' pen for 15 min dally.
Oestrous detection At 29 weeks of age, the gilts were transferred to an adjoining boar shed for mating over the subsequent 6-week period to young breeding boars. Groups of five gilts were housed in 1.8 X 3.0 m pens, 1.0 m from the boar pens, and were fed 3.0 kg per gilt per day.
53 One of three trained herdsmen tested each gilt daily for oestrus, using the back-pressure test in the presence of a boar (Signoret and Du Mesnil du Buisson, 1961). Intensity of response was classified as low, moderate or high depending upon whether gilts stood for < 10, 15--30, or > 30 s. Those which showed a moderate or high response were taken individually to a boar in a mating pen. Any period of 3 or more consecutive days, in which a positive back-pressure test (low, moderate or high) was recorded, was termed a positive response period.
Sexual receptivity On entry to the mating pen, sexual receptivity was classified according to the response to the boar. Receptivity was classified as high or low upon whether gilts stood immediately to the first mounting attempt or resisted before allowing copulation. Gilts that showed the appropriate back-pressure test response the following day were mated again to the same boar. At the end of each week, mated gilts were transferred to the dry sow shed and checked daily for returns to oestrus. Data from the return mating were not included in the analysis. Gilts unmated by 35 weeks were slaughtered at a local abattoir, and their ovaries examined for evidence of puberty. Farrowing data from the piggery's records were analysed for differences between treatments in oestrous behaviour, mating rate, fertility and fecundity. RESULTS
Mating performance at the first positive response period The mating rate of gilts was significantly greater for the week 23 than for the week 28 gilts (70.1 and 66.0%, P < 0.01). Further, a lower proportion of week 23 than week 28 gilts failed to mate when taken to the boar (6.1 and 9.5%, P < 0.01), b u t this was not indicated by the respective percentage of low responses to the back-pressure test (25.3 and 27.3%, N.S.).
Mating performance over all positive response periods A greater percentage of week 23 than week 28 gilts expressed a high level of sexual receptivity at the first mating in gilts which received a double mating (i.e. two matings 24 h apart) (89.7 and 86.8%, respectively; P < 0.01). There was a significantly higher incidence of prepubertal gilts at 35 weeks of age in the week 28 gilts than in the week 23 animals (3.03 and 1.46%, P < 0.01), b u t no difference in the incidence of post-pubertal, unmated gilts (11.9 and 11.5%).
88.4
--
11.5
100.0
88.5 87.7 100.0
70.1 x 18.0 0.4
9.5
--
9.4 a 9.9 b 10.6 b
Mean litter size (piglets)
747
--
587 156 4
Piglets per 100 gilts
100.0
11.9
65.9 y 21.3 0.8
Gilts (%)
87.9
88.9 84.9 88.9
Farrowing rate (%)
Week 28 (n = 1 3 1 0 )
a b V a l u e s w i t h i n c o l u m n s w i t h d i f f e r e n t l e t t e r s d i f f e r significantly, P < 0.05. x,YValues b e t w e e n c o l u m n s w i t h d i f f e r e n t l e t t e r s d i f f e r significantly, P < 0.05.
Total
First Second Third N o t m a t e d by 35 weeks
Gilts (%)
(PRP w h e n m a t e d )
Farrowing rate (%)
Week 23 (n = 1350)
Treatment
R e p r o d u c t i v e p e r f o r m a n c e o f t h e p o s t - p u b e r t a l gilts (PRP: Positive r e s p o n s e p e r i o d )
TABLE I
9.5
9.4 a 9.8 b 10.1 b
Mean litter size (piglets)
735
551 177 7
Piglets per 100 gilts
55 TABLE II Sources of wastage beyond the first PRP among the gilts, based on data for 100 gilts in each treatment (PRP: Positive Response Period) Treatment
Week 23 (n = 1370)
(Source of wastage beyond first PRP)
Gilts (%)
Prepubertal gilts Post-pubertal, unmated gilts Gilts mated at the second PRP Gilts mated at the third PRP Total
1.4 a
Days unmated
(Week 28 (n = 1351) Pig days
42
61.3
11.3
42
475.0
17.7 x
21
0.4
42
Gilts (%) 3.0 b
Days unmated
Pig days
42
127.2
11.5
42
485.1
372.5
20.6 y
21
433.6
18.5
0.8
42
34.0
927.3
Difference/gilt (pig days)
1080.0 +
1.5
abValues between columns with different letters differ significantly, P < 0.01. x'YValues between columns with different letters differ significantly, P < 0.05.
Farrowing performance The mean farrowing rate o f the 2349 mated gilts was 88.16%, with 8.86 live and 0.67 dead piglets. There were no significant differences between the treatments (~able I). However, week 23 gilts produced 845 piglets per 100 matings compared with 835 for the week 28 gilts. For gilts in both treatments, the mean litter size increased significantly from mating at the first positive response period to matings at the second or third (Table I). It can be calculated (Table II) that the week 23 treatment reduces the average number of unproductive days by 1.5 per gilt. DISCUSSION
Earlier contact with mature boars produced a n u m b e r of small but significant improvements in reproductive efficiency, equivalent to 1.5 less unproductive days per gilt. Of major importance was the improvement in sexual receptivity of the week 23 gilts, which resulted in a higher mating rate for that treatment. The improved sexual receptivity and mating rate in the present experiment agrees with the finding of Hemsworth et al. (1982), who found that gilts reared in isolation from boars until 180 days of age had a reduced mating rate due to reduction in both oestrous expression and sexual receptivity. While the week 23 gilts showed better oestrous expression, the improvement was not significantly different from that of the week 28 gilts.
56
There was also a small reduction in the number of prepubertal gilts at 35 weeks of age following earlier boar contact. This finding is consistent with those of Brooks and Cole (1970), who reported that the age of p u b e r t y was reduced in gilts introduced to boars at 23 rather than 27 weeks of age. The results of the study by Cronin et al. (1982) at the same piggery, b u t with boar introduction occurring only at 28 weeks, indicate that in the present study the first positive response period of the week 28 gilts may have been their pubertal oestrus. In contrast, on the basis of the results of Brooks and Cole (1970) the week 23 gilts mated after their pubertal oestrus. However, there is no evidence of treatment increasing litter size. It is of interest that in the present study there was a higher incidence of post-pubertal, unmated gilts at 35 weeks than in the study by Cronin et al. (1983) at the same piggery. As shown in this latter study, the higher stocking density that occurred while the present study was conducted may account for the higher incidence of the postpubertal, unmated gilts. In conclusion, the exposure to mature boars at 23 rather than 28 weeks of age increased the reproductive efficiency of the gilts in the present study (Table II). For every 100 gilts that entered the boar shed, 737 and 713 piglets were produced for the week 23 and 28 treatments, respectively. This represents an increase of 0.24 piglets per gilt for the week 23 treatment. ACKNOWLEDGEMENTS
The author gratefully acknowledges the advice and assistance of C.G. Winfield, Dr. P.H. Hemsworth, A. Makin, C. Hansen and A. Hand and the financial support of the Australian Pig Industry Research Committee. I would also like to thank Mr. G.T. Hope, General Manager of Mayfair Farms for allowing me to undertake this study at their Bendigo Piggery. I am also indebted to Less Riddell and Mario Belardinelli for their assistance in collecting the data.
REFERENCES Brooks, P.H. and Cole, D.J.A., 1970. The effects of the presence o f a boar on the attainment of puberty in gilts. J. Reprod. Fertil., 23: 435--440. Brooks, P.H. and Smith, D.A., 1980. The effects of mating age on the reproductive performance, food utilization and liveweight change of the female pig. Livestock Prod. Sci., 7 : 67--78. Cronin, G.M., Hemsworth, P.H. and Winfield, C.G., 1982. Oestrous behaviour in relation to fertility and fecundity of gilts. Anita. Reprod. Sci., 5: 117--125. Cronin, G.M., Hemsworth, P.H., Winfield, C.G., Muller, B. and Chamley, W.A., 1983. The incidence of, and factors associated with, failure to mate by 245 days of age in the gilt. Anim. Reprod. Sci., 5: 199--205. Hemsworth, P.H., Cronin, G.M. and Hansen, C., 1982. The i n f l u e n c e of social restriction during rearing on the sexual behaviour of the gilt. Anita. Prod., 35: 35--40. Hughes, P.E. and Cole, D.J.A., 1976. Reproduction in the gilt. 2. The i n f l u e n c e of gilt age at boar introduction on the a t t a i n m e n t of puberty. Anim. Prod., 23: 89---94.
57 Kirkwood, R.N. and Hughes, P.E., 1979. The influence of age at first boar contact on puberty attainment in the gilt. Anita. Prod., 29: 231--238. Roberts, S.J., 1971. Veterinary Obstetrics and General Disease. Ithaca, NY. Signoret, J.P. and Du Mesnil du Buisson, F., 1961. Study of the behaviour of the sow during oestrus. Proc. IVth Int. Congr. on Animal Beproduction, Vol. 11. The Hague, pp. 171--175.
51
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
THE EFFECT OF EARLY CONTACT WITH MATURE BOARS ON REPRODUCTIVE EFFICIENCY IN THE GILT
G.M. CRONIN
Department of Agriculture, Animal Research Institute, Werribee, Vict. 3030 (Australia) Present address: Ethology Section, Department of Animal Husbandry, Agricultural University, Wageningen (The Netherlands) (Accepted 14 February 1983)
ABSTRACT Cronin, G.M., 1983. The effect o f early contact with mature boars on reproductive efficiency in the gilt. Anita. Reprod. Sci., 6: 51--57. The effect of first contact of gilts with a mature boar at 23 or 28 weeks of age on their subsequent reproductive efficiency was studied over a 12-month period at a large intensive piggery in southern Australia. Following this contact, the gilts entered the mating shed at 29 weeks of age and were checked daily for oestrus, as assessed by the back-pressure test in the presence of the boar. Gilts that showed moderate or high responses were taken to a boar for mating. Sexual receptivity was then assessed by the time taken to " s t a n d " after the first m o u n t by the boar. Gilts that remained unmated at 35 weeks of age were culled, and their ovaries were examined. Of the 2660 gilts in the study, 2349 were mated and they had a farrowing rate of 88.2% with a mean litter size of 9.5 piglets, of which 0.7 piglets (7.4%) were born dead. The reproductive efficiency of the gilts following earlier contact with the boar was consistently higher than that of gilts exposed later. The mating rate of the week 23 gilts was greater than that of the week 28 gilts (70.1 vs 66.0%, P < 0.01), more appeared to show a high level of sexual receptivity (97.0 and 94.6%, N.S.) and fewer failed to mate when put to a boar (6.1 vs 9.5%, P < 0.01). The percentage of prepubertal gilts at 35 weeks of age was also lower (1.46 vs 3.03%, P < 0.01). The improved reproductive performance was estimated to be equivalent to 0.24 extra piglets born per gilt.
INTRODUCTION
The importance of contact with mature boars during rearing on reducing age at puberty of gilts is well established (Brooks and Cole, 1970; Hughes and Cole, 1976; Kirkwood and Hughes, 1979). Further, gilts that mated at the second oestrus after an induced puberty had similar lifetime production to gilts mated after a natural puberty (Brooks and Smith, 1980). Nevertheless, little is known of the effects of induced puberty on other reproductive characters and overall reproductive efficiency. Hemsworth et al. (1982) have shown that lack of mature boar contact during the rearing period can
0378-4320/83/$03.00
© 1983 Elsevier Science Publishers B.V.
52
lower oestrous expression and sexual receptivity. The aim of this study was to measure the reproductive efficiency of gilts exposed to mature boars from either 23 or 28 weeks of age in a commercial herd. Criteria measured were incidence of oestrus (sexual receptivity: Roberts, 1971), intensity of oestrus, acceptance of the boar and proportion of gilts that reached puberty by 35 weeks of age. The study was performed over a 12-month period at a large intensive piggery in central Victoria. MATERIALS AND METHODS
Animals Similar numbers of gilts experienced each treatment (week 23: 1474, week 28: 1438). There were three breeds of gilts (Large White, Landrace and a synthetic breed (M) selected from crossbreds of the former breeds on the basis of large litter size). Also, there were crossbred gilts of these three breeds. At 21 weeks of age, gilts were selected for breeding on the basis of growth rate and back-fat thickness.
Feeding The gilts were fed ad libitum on a grower ration to 21 weeks of age, when feed was restricted to 2.5 kg/gilt/day until week 27. During weeks 27 and 28, feed was again available ad libitum.
Early boar contact Alternate weekly groups of gilts, which ranged in size from 45 to 73 gilts, were allotted to the two treatments. The two mature boars (A and B) used as contact boars were 3 years old at the c o m m e n c e m e n t of the study. Treatment 1: Gilts week 23. From 23 to 27 weeks of age, the gilts were housed in shed A and contact boar A was introduced into their pen for 15 min daily. At 27 weeks, the gilts were transferred to shed B, and each day during the following week contact boar B was introduced for 15 min. Treatment 2: Gilts week 28. From 23 to 27 weeks of age, the gilts were housed in a shed identical to shed A, but w i t h o u t boars present. At 27 weeks of age the gilts were transferred to shed B, and during the following week contact boar B was introduced into the gilts' pen for 15 min dally.
Oestrous detection At 29 weeks of age, the gilts were transferred to an adjoining boar shed for mating over the subsequent 6-week period to young breeding boars. Groups of five gilts were housed in 1.8 X 3.0 m pens, 1.0 m from the boar pens, and were fed 3.0 kg per gilt per day.
53 One of three trained herdsmen tested each gilt daily for oestrus, using the back-pressure test in the presence of a boar (Signoret and Du Mesnil du Buisson, 1961). Intensity of response was classified as low, moderate or high depending upon whether gilts stood for < 10, 15--30, or > 30 s. Those which showed a moderate or high response were taken individually to a boar in a mating pen. Any period of 3 or more consecutive days, in which a positive back-pressure test (low, moderate or high) was recorded, was termed a positive response period.
Sexual receptivity On entry to the mating pen, sexual receptivity was classified according to the response to the boar. Receptivity was classified as high or low upon whether gilts stood immediately to the first mounting attempt or resisted before allowing copulation. Gilts that showed the appropriate back-pressure test response the following day were mated again to the same boar. At the end of each week, mated gilts were transferred to the dry sow shed and checked daily for returns to oestrus. Data from the return mating were not included in the analysis. Gilts unmated by 35 weeks were slaughtered at a local abattoir, and their ovaries examined for evidence of puberty. Farrowing data from the piggery's records were analysed for differences between treatments in oestrous behaviour, mating rate, fertility and fecundity. RESULTS
Mating performance at the first positive response period The mating rate of gilts was significantly greater for the week 23 than for the week 28 gilts (70.1 and 66.0%, P < 0.01). Further, a lower proportion of week 23 than week 28 gilts failed to mate when taken to the boar (6.1 and 9.5%, P < 0.01), b u t this was not indicated by the respective percentage of low responses to the back-pressure test (25.3 and 27.3%, N.S.).
Mating performance over all positive response periods A greater percentage of week 23 than week 28 gilts expressed a high level of sexual receptivity at the first mating in gilts which received a double mating (i.e. two matings 24 h apart) (89.7 and 86.8%, respectively; P < 0.01). There was a significantly higher incidence of prepubertal gilts at 35 weeks of age in the week 28 gilts than in the week 23 animals (3.03 and 1.46%, P < 0.01), b u t no difference in the incidence of post-pubertal, unmated gilts (11.9 and 11.5%).
88.4
--
11.5
100.0
88.5 87.7 100.0
70.1 x 18.0 0.4
9.5
--
9.4 a 9.9 b 10.6 b
Mean litter size (piglets)
747
--
587 156 4
Piglets per 100 gilts
100.0
11.9
65.9 y 21.3 0.8
Gilts (%)
87.9
88.9 84.9 88.9
Farrowing rate (%)
Week 28 (n = 1 3 1 0 )
a b V a l u e s w i t h i n c o l u m n s w i t h d i f f e r e n t l e t t e r s d i f f e r significantly, P < 0.05. x,YValues b e t w e e n c o l u m n s w i t h d i f f e r e n t l e t t e r s d i f f e r significantly, P < 0.05.
Total
First Second Third N o t m a t e d by 35 weeks
Gilts (%)
(PRP w h e n m a t e d )
Farrowing rate (%)
Week 23 (n = 1350)
Treatment
R e p r o d u c t i v e p e r f o r m a n c e o f t h e p o s t - p u b e r t a l gilts (PRP: Positive r e s p o n s e p e r i o d )
TABLE I
9.5
9.4 a 9.8 b 10.1 b
Mean litter size (piglets)
735
551 177 7
Piglets per 100 gilts
55 TABLE II Sources of wastage beyond the first PRP among the gilts, based on data for 100 gilts in each treatment (PRP: Positive Response Period) Treatment
Week 23 (n = 1370)
(Source of wastage beyond first PRP)
Gilts (%)
Prepubertal gilts Post-pubertal, unmated gilts Gilts mated at the second PRP Gilts mated at the third PRP Total
1.4 a
Days unmated
(Week 28 (n = 1351) Pig days
42
61.3
11.3
42
475.0
17.7 x
21
0.4
42
Gilts (%) 3.0 b
Days unmated
Pig days
42
127.2
11.5
42
485.1
372.5
20.6 y
21
433.6
18.5
0.8
42
34.0
927.3
Difference/gilt (pig days)
1080.0 +
1.5
abValues between columns with different letters differ significantly, P < 0.01. x'YValues between columns with different letters differ significantly, P < 0.05.
Farrowing performance The mean farrowing rate o f the 2349 mated gilts was 88.16%, with 8.86 live and 0.67 dead piglets. There were no significant differences between the treatments (~able I). However, week 23 gilts produced 845 piglets per 100 matings compared with 835 for the week 28 gilts. For gilts in both treatments, the mean litter size increased significantly from mating at the first positive response period to matings at the second or third (Table I). It can be calculated (Table II) that the week 23 treatment reduces the average number of unproductive days by 1.5 per gilt. DISCUSSION
Earlier contact with mature boars produced a n u m b e r of small but significant improvements in reproductive efficiency, equivalent to 1.5 less unproductive days per gilt. Of major importance was the improvement in sexual receptivity of the week 23 gilts, which resulted in a higher mating rate for that treatment. The improved sexual receptivity and mating rate in the present experiment agrees with the finding of Hemsworth et al. (1982), who found that gilts reared in isolation from boars until 180 days of age had a reduced mating rate due to reduction in both oestrous expression and sexual receptivity. While the week 23 gilts showed better oestrous expression, the improvement was not significantly different from that of the week 28 gilts.
56
There was also a small reduction in the number of prepubertal gilts at 35 weeks of age following earlier boar contact. This finding is consistent with those of Brooks and Cole (1970), who reported that the age of p u b e r t y was reduced in gilts introduced to boars at 23 rather than 27 weeks of age. The results of the study by Cronin et al. (1982) at the same piggery, b u t with boar introduction occurring only at 28 weeks, indicate that in the present study the first positive response period of the week 28 gilts may have been their pubertal oestrus. In contrast, on the basis of the results of Brooks and Cole (1970) the week 23 gilts mated after their pubertal oestrus. However, there is no evidence of treatment increasing litter size. It is of interest that in the present study there was a higher incidence of post-pubertal, unmated gilts at 35 weeks than in the study by Cronin et al. (1983) at the same piggery. As shown in this latter study, the higher stocking density that occurred while the present study was conducted may account for the higher incidence of the postpubertal, unmated gilts. In conclusion, the exposure to mature boars at 23 rather than 28 weeks of age increased the reproductive efficiency of the gilts in the present study (Table II). For every 100 gilts that entered the boar shed, 737 and 713 piglets were produced for the week 23 and 28 treatments, respectively. This represents an increase of 0.24 piglets per gilt for the week 23 treatment. ACKNOWLEDGEMENTS
The author gratefully acknowledges the advice and assistance of C.G. Winfield, Dr. P.H. Hemsworth, A. Makin, C. Hansen and A. Hand and the financial support of the Australian Pig Industry Research Committee. I would also like to thank Mr. G.T. Hope, General Manager of Mayfair Farms for allowing me to undertake this study at their Bendigo Piggery. I am also indebted to Less Riddell and Mario Belardinelli for their assistance in collecting the data.
REFERENCES Brooks, P.H. and Cole, D.J.A., 1970. The effects of the presence o f a boar on the attainment of puberty in gilts. J. Reprod. Fertil., 23: 435--440. Brooks, P.H. and Smith, D.A., 1980. The effects of mating age on the reproductive performance, food utilization and liveweight change of the female pig. Livestock Prod. Sci., 7 : 67--78. Cronin, G.M., Hemsworth, P.H. and Winfield, C.G., 1982. Oestrous behaviour in relation to fertility and fecundity of gilts. Anita. Reprod. Sci., 5: 117--125. Cronin, G.M., Hemsworth, P.H., Winfield, C.G., Muller, B. and Chamley, W.A., 1983. The incidence of, and factors associated with, failure to mate by 245 days of age in the gilt. Anim. Reprod. Sci., 5: 199--205. Hemsworth, P.H., Cronin, G.M. and Hansen, C., 1982. The i n f l u e n c e of social restriction during rearing on the sexual behaviour of the gilt. Anita. Prod., 35: 35--40. Hughes, P.E. and Cole, D.J.A., 1976. Reproduction in the gilt. 2. The i n f l u e n c e of gilt age at boar introduction on the a t t a i n m e n t of puberty. Anim. Prod., 23: 89---94.
57 Kirkwood, R.N. and Hughes, P.E., 1979. The influence of age at first boar contact on puberty attainment in the gilt. Anita. Prod., 29: 231--238. Roberts, S.J., 1971. Veterinary Obstetrics and General Disease. Ithaca, NY. Signoret, J.P. and Du Mesnil du Buisson, F., 1961. Study of the behaviour of the sow during oestrus. Proc. IVth Int. Congr. on Animal Beproduction, Vol. 11. The Hague, pp. 171--175.