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Estrus synchronisation and fertility in gilts using a synthetic progestagen (Allyl Trenbolone) and inseminated with fresh stored or frozen semen
THERIOGENOLOCY ESTRUS SYNCHRONISATION AND FERTILITY IN GILTS USING A SYNTHETlCPROGESTAGEN(AL~YLTRENBOLONE~ANOlNSEMlNATEO WITH FRESH STORED OR .FROZEN SEMEN P. J. 0' Reilly, Rosemary MC Cormack, K: 0' Mahony, C. Murphy, Veterinary Research Laboratory, Abbotstown, Castleknock, Co. Dublin, Ireland. Received for Publication: February 15, 1979 Abstract An orally active synthetic progestagen was administered at two dosage levels to synchronise estrus in gilts. Fertility following insemination with either fresh stored or frozen semen was determined by examining surgically recovered ova for cleavage, and numbers of spermatozoa attached to the zona pellucida, or enumeration of embryos.,ingilts slaughtered 30 days post insemination. There was no significant difference (P70.05) between treated and control groups in the duration of estrus or in fertility as determined by cleavage of ova. A significant :ly (P,O.OOl) shorter interval to estrtisand better synchronisation was obtained with both treatment groups than with the control group. The mean interval from the end of treatment to the onset of estrus for the untreated controls and the treated groups feceiving 12.5 and 15 mg compound per day was 11.25 f 10.4 SD; 5.6 - 0.52 SD and 7.3 ? 5.3 SD. Fresh semen yielded significantly (kO.01) more cleaved ova than frozen semen. Introduction A number of approaches to the control and synchronisation of estrus in gilts over the past 25 years have included the administration of progesterone, (1) synthetic progestagens, (2, 3, 4, 5, 6, 7, 8) and a non-steroidal pituitary inhibitor. (9). Progesterone and progestagens have generally proved unsuitable either because the method of administration necessitated daily injections or the dosage level required to prevent the formation of cystic ovaries resulted in unsatisfactory synchronisation or fertility. Acknowledgements The authors are indebted to the Director of the Veterinary Research Laboratory for permission to publish this manuscript and to Dr. Steve Webel of Abbott Laboratories for making the compound available and for his critical comnents on the manuscript. Thanks are also due to Miss Maura MC Grane for typing and to Frank Smith for seeing to the needs of the experimental animals.
SEPTEMBER 1979 VOL. 12 NO. 3
131
THERIOGENOLOGY
The non-steroidal pituitary inhibitor Methallibure was satisfactory as a synchronising agent but was commercially withdrawn because of its teratogenic effect when fed to pregnant animals(l0). This experiment was designed to test a new progestagen compound A-35957 (17 alpha-allylestratriene 4-9-11 Beta-ol-3-onej (Regumate supplied by Dr. S. K. Vehel, Abbott Laboratories, North Chiago, Illinois). This is one of two compounds which have recently produced satisfactory results (5, 6, 7, 8). The object was to examine its efficiency as a synchronising agent and to determine the level of fertility obtained with fresh or frozen semen in treated and control gilts. Materials and Methods Forty-eight gilts were randomly allocated to three groups matched on a weight basis. Group one received no compound and served as a control. Groups two and three were fed so as to receive an average daily dose of 12.5 mgm. and 15 mgm. of progestagen in 2.24 Kg of feed. The gilts within a group were collectively fed and the numbers of animals in each group replicate were eight, four and four respectively. The compound was fed to each treatment group for 19 to 21 days. The last day of treatment was taken as day 0. Estrus was observed twice daily using a teaser boar and the time of onset was taken as being the time of observation plus half the interval from the previous observation. Likewise,the end of estrus was taken as being the last observed time of standing estrus plus half the interval to the next observation. Fresh and frozen semen were prepared as previously described (11, 12). Gilts were inseminated 6-18 hours after first beina observed in estrus and thereafter every morning and evening uniil the end of estrus. The fresh semen used for each gilt was that collected on the day of first insemination. The frozen semen used was a composite of a number of ejaculates frozen at different times from boars currently at stud in Gilts at any given estrous period were inseminated with semen AI. from one boar. A laparotomy was performed on half of each group two days after the end of estrus and ova were recovered as described (13). Ova were exanined for cleavage, and sperm attached to the zona pellucida were enumerated. The remaining animals were slaughtered 30 days after insemination if they failed to return to estrus. Gilts returning to estrus were inseminated with the opposite type of semen to that previously used. The numbers of embryos and corpora lutea were determined in slaughtered gilts. Results The gilts were randomised on a weight basis;thus there was no significant difference between the weights of the gilts in the three groups. The mean weight of the gilts at the commencement of treatment was 110.842 21.6 Kg with a range of 74.2 to 140.6 Kg.
132
SEPTEMBER
1979 VOL. 12 NO. 3
THERIOCENOLOGY
The mean interval from the last feed of compound to the onset of estrus, and the'number showing behavioural estrus within 26 days, are shown in table I. TABLE
I
Interval, in days, from last feed of compound to the onset of estrus Treatment No. Animals No. animals in estrus Within 26 days Mean interval to estrus + SD Range
Control
12.5 mg /day
16 12
16 11
11.66+ 7.81a) O-25
15 mg /day :!
5.84.2(b) 5-6
7.3c 5,3(b) 4-26
a.b. Columns with different super script significantly different PLO.OO1. There was no significant difference between the control and either of the treated groups in the duration of estrus which had a mean of 55.32 13.8 hours and a range of 24-89 hours. The fertility, as determined by the cleavage of ova and the presence of spermatozoa in the zona pellucida, is shown in Table II. TABLE
II
Fertility as determined by ova cleavage and sperm no. in the zona pelucida of gilts inseminated with fresh or frozen semen Treatment
Control
Type of Semen
Fresh
No. Animals No. Ovulations No. Recovered No. Ova Cleaved with sperm in zona N;dn,f Sperm in
SEPTEMBER
Frozen
3
3
:;
12.5 mg/day Fresh
15 mglday
Frozen Fresh
Frozen
::
3 39 33
3 36 28
t:
Z
::
3 1
29 27
17 18
30 34
G!
189
2
1995
61
2669
85
1979 VOL. 12 NO. 3
4
4
133
THERIOGENOLOGY
One animal in the 12.5 mgmlday frozen semen category was excluded from the data because it had a uterus unicornuus, from which four uncleaved ova were recovered. There was no significant difference (P>O.O5) in the proportion of ova cleaved in the treated and control groups but significantly more ova (P-0.01) were cleaved with fresh than with frozen semen. The numbers of gilts pregnant at slaughter 30 days after insemination and the total number of embryos are shown in Table III. Fertility of gilts inseminated at their first return to estrus whether ova had previously been flushed from their reproductive tracts or not is shown in Table IV. TABLE
III
Fertility as determined by pregnancy rate and litter size at slaughter 30 days post insemination Control
Treatment Type of Semen No. Animals No. Embryos Pregnant
Fresh
12.5 mg /day
Frozen
Fresh
Frozen
15 mg /day Fresh
Frozen
3
:
3
3
2
3
1:
0
23 2
7 1
11
:
TABLE
IV
Fertility of animals which returned to estrus and were slaughtered 30 days after a repeat insemination with fresh or frozen semen Operated
Previous Surgery Type of Semen No. Animals No. Pregnant No. Embryos
Fresh
Un-operated Frozen
Fresh
7
6
5
3;
1:
122
Frozen 25 14
Estrus was not observed in four gilts in group 1, five in group 2 and one in group 3. These gilts were in the lower weight ranges at the commencement of the experiment but at the time of slaughter averaged 140 Kg. They had small inactive ovaries when examined at slaughter.
134
SEPTEMBER 1979 VOL. 12 NO. 3
THERIOGENOLOGY One cyst 15mm. in diameter was observed at laparotomy on the ovary one of the gilts receiving 15 mgm of compound per day. Yormal ovulations had also occurred.
of
Discussion The mean duration of estrus observed in this experiment of 55.32 13.8 hours agrees well with that reported (14) of 53.1: 0.36, although the variation observed here seems to be greater and is probably due to different methods of observation. The mean interval between the end of treatment and estrus was shorter (PC 0.01) and the degree of synchronisation for those observed in estrus was better for the treated than the controls. A spread of 2 days (days 5 and 6) in onset of estrus was observed among the gilts on 12.5 Thirteen of the fifteen animals on 15 mg per day came mg per day. into estrus within a four day (days 4-7) period, while only four of the controls came into estrus within any four day period. The mean interval from the end of treatment to the onset of estrus for those i ts coming into estrus within a 10 day period was 6.5f 0.52 and %A- 2 1.4 days respectively for the two treated groups. These are about one day shorter than those previously reported (15) of 6.92 1.2 and 7.0? 1.0 days. The difference may be related to the method of calculation of interval, time of feeding or estrus observation. The failure of some gilts to come into estrus may be related to their low body weight at the commencement of treatment or to the sequence in environmental change. Most of the problems arose in one particular consignment of gilts which when brought to the research centre was maintained there for three weeks prior to treatment because of low body Thus the effects of stress of movement, change of environment weight. and exposure to boars may have worn off (16, 17). The feeding of this compound would seem to have little or no effect in stimulating the onset of puberty in prepubertal gilts. There was no significant difference in the treated and control groups: 115) which showed no difference in and'control gilts, although in the sperm per ova were greater for the
in the proportion of ova cleaved this is in line with other results fertilitv or litter size for treated present experiment the numbers of treated groups of animals.
Fresh semen yielded significantly more cleaved ova (Pr0.01) than frozen semen, which appears to be due largely to the rather poor results obtained using frozen semen from three of the four boars used. All of the boars used had normal or above normal fertility with farrowing rates of 70 per cent when used in a commercial fresh semen AI delivery service. Thus the rather low cleavage rates and the numbers of spermatozoa attached to the zona pellucida obtained amongst the controls with both fresh and frozen semen with cleavage rates of 33 and 18.7 per cent respectively is surprising. However significantly improved litter sizes in gilts fed on this compound have been reported (15). Other workers (17, 18, 19) reported fertilization rates of 50.8 per cent, 21.7 per cent and 83.5 per cent in gilts inseminated with fresh diluted semen
SEPTEMBER
1979 VOL. 12 NO. 3
135
THERIOGENOLOGY Fertilization rates of 87 per cent using frozen thawed semen have been reported (12) but authors did not reFer to any difference noted between boars in fertility with frozen semen. The numbers of embryos recorded in slaughtered gilts are very much in line with the fertility as estimated by the examination of ova, although the figures are too small to arrive at a statistical conclusion. In conclusion Ally1 Trenbolone is an effective synchronising agent for gilts which does not depress fertility. However since the objective is to inseminate gilts without the necessity of estrus detection or to treat gilts which either have not reached puberty or whose breeding history is unknown, additional treatments using gonadotrophins or gonadotrophin releasing hormones are likely to be necessary. References 1.
Ulberg, L.C., Grummer, R.H., Casida, L.E. (1951). The Effects of Progesterone upon Ovarian Function in Gilts. J. Anim. Sci. lo, 665.
2.
Polge, C. (1965). Fertility in Gilts following Dziuk, P.J., Induced Ovulation. Vet. Rec. -77, 236.
3.
Ray, D.E., Seerley, R.W. (1966). Estrus and ovarian morphology in gilts following treatment with orally effective steroids. Nature, -211, 1102.
4.
Pond, W.G., Hansel, W., Dunn, J.A., Bratton, R.W., Foote, R.H. (1965). Estrus Cycle Synchronisation and Fertility of Gilts Fed Progestational and Estrogenic Compounds. J. Anim. Sci. _' 24 536.
5.
Webel, S.K. (1976). Estrus Control in Swine with a Progestagen. 3. Anim. Sci. 42, 1358. ABSTR.
6.
Mayer, P., Schutze, E. (1977). A new progestin (S a 45.249) for cycle control in pigs. Communication 1: Hormonal activities and dosage. Theriogenology 8 357.
7.
Schutze, E. Mayer, P. (1977). A new progestin (S a 45.249) for Duration of treatment cycle control in pigs. Communication II: and effectiveness as an estrous cycle sychronizer. Theriogenology -8 367.
8.
Zerobin , K., Schutze, E., Mayer, P. (1977). A new progestin (S a 45.249) for cycle control in pigs. III; confirmation of effectiveness in field trials Theriogenology -8 378.
9.
Polge, C., Day, B.N., Groves, T.W. (1968). Synchronisation of Ovulation and Artificial Insemination in Pigs. Vet. Rec. 83, 136.
10.
Wrathall, A.E., Herbert, Nancy., Hoskin, B.O. (1972). Vente, J.P., Quantative Anatomical Study of Methallibure Induced Malformations in Piglets. Res. Vet. Sci. -13, 169.
136
SEPTEMBER
1979 VOL. 12 NO. 3
THERIOGENOLOGY
11.
Bennett, G.H., 0' Hagan, C. (1964). of Artificial Insemination of Pigs. Reprod, and AI. Trento -vi, 481.
Factors Influencing the Success Proc. Vth Int. Congr. Anim.
12.
Freezing Pursel, V.G., Johnson, L.A. (1975). Fertilizing Capacity with Concentrated Semen Procedure. J. Anim. Sci. 40, No. 1, 99.
13.
Dziuk, P. (1971). Obtaining Eggs and Embryos from Methods in Mammalian Embryology. ed. J.C. Daniel, Freeman & Co. San Francisco.
14.
Signoret, J.P. (1967). Duree du cycle Estrien et de l"Estrus chez la Truis Action du Benzoate d'Estradio1 chez la Femelle Ovariectonises. Ann. Bio. Anim. Bioch. Biophys. L, (4), 407.
15.
Webel,
16
Mavrogenis, A.P., Puberty in Swine.
17.
Brooks, P.H., Cole, D.J.A. (1970). The Effect of Presence of a Boar on the Attainment of Puberty in Gilts. 3. Reprod. Fertil. -23, 435.
18.
Baker, R.D., Dziuk, P.J., Norton, H.W. (1968). The Effect of Volume of Semen Number of Sperm and Drugs on the Transport of Sperm in Artificially Inseminated Gilts. J. Anim. Sci. -27, 88.
19.
The Effect of Semen Volume Hancock, J.L., Hovell, G.T. (1961). and Numbers of Spermatozoa on the Fertility of Intra-uterine Inseminated Gilts. Anim. Prod. 3, 153.
of Boar Spermatozoa and a New Thawing
Sheep and Pigs. 76. W.H.
S.K. (1978). Ovulation Control in the Pig. In Proc. Control of Ovulation, Nottingham, 421. Ed. D.B. Crighton, Symp. N.B. Haynes, G.R. Foxcroft, G.E. Lamming.
SEPTEMBER
Robinson, J. Anim.
1979 VOL. 12 NO. 3
O.W. (1976). Factors Sci. -42, 1251.
Affecting
137
SEPTEMBER 1979 VOL. 12 NO. 3
131
THERIOGENOLOGY
The non-steroidal pituitary inhibitor Methallibure was satisfactory as a synchronising agent but was commercially withdrawn because of its teratogenic effect when fed to pregnant animals(l0). This experiment was designed to test a new progestagen compound A-35957 (17 alpha-allylestratriene 4-9-11 Beta-ol-3-onej (Regumate supplied by Dr. S. K. Vehel, Abbott Laboratories, North Chiago, Illinois). This is one of two compounds which have recently produced satisfactory results (5, 6, 7, 8). The object was to examine its efficiency as a synchronising agent and to determine the level of fertility obtained with fresh or frozen semen in treated and control gilts. Materials and Methods Forty-eight gilts were randomly allocated to three groups matched on a weight basis. Group one received no compound and served as a control. Groups two and three were fed so as to receive an average daily dose of 12.5 mgm. and 15 mgm. of progestagen in 2.24 Kg of feed. The gilts within a group were collectively fed and the numbers of animals in each group replicate were eight, four and four respectively. The compound was fed to each treatment group for 19 to 21 days. The last day of treatment was taken as day 0. Estrus was observed twice daily using a teaser boar and the time of onset was taken as being the time of observation plus half the interval from the previous observation. Likewise,the end of estrus was taken as being the last observed time of standing estrus plus half the interval to the next observation. Fresh and frozen semen were prepared as previously described (11, 12). Gilts were inseminated 6-18 hours after first beina observed in estrus and thereafter every morning and evening uniil the end of estrus. The fresh semen used for each gilt was that collected on the day of first insemination. The frozen semen used was a composite of a number of ejaculates frozen at different times from boars currently at stud in Gilts at any given estrous period were inseminated with semen AI. from one boar. A laparotomy was performed on half of each group two days after the end of estrus and ova were recovered as described (13). Ova were exanined for cleavage, and sperm attached to the zona pellucida were enumerated. The remaining animals were slaughtered 30 days after insemination if they failed to return to estrus. Gilts returning to estrus were inseminated with the opposite type of semen to that previously used. The numbers of embryos and corpora lutea were determined in slaughtered gilts. Results The gilts were randomised on a weight basis;thus there was no significant difference between the weights of the gilts in the three groups. The mean weight of the gilts at the commencement of treatment was 110.842 21.6 Kg with a range of 74.2 to 140.6 Kg.
132
SEPTEMBER
1979 VOL. 12 NO. 3
THERIOCENOLOGY
The mean interval from the last feed of compound to the onset of estrus, and the'number showing behavioural estrus within 26 days, are shown in table I. TABLE
I
Interval, in days, from last feed of compound to the onset of estrus Treatment No. Animals No. animals in estrus Within 26 days Mean interval to estrus + SD Range
Control
12.5 mg /day
16 12
16 11
11.66+ 7.81a) O-25
15 mg /day :!
5.84.2(b) 5-6
7.3c 5,3(b) 4-26
a.b. Columns with different super script significantly different PLO.OO1. There was no significant difference between the control and either of the treated groups in the duration of estrus which had a mean of 55.32 13.8 hours and a range of 24-89 hours. The fertility, as determined by the cleavage of ova and the presence of spermatozoa in the zona pellucida, is shown in Table II. TABLE
II
Fertility as determined by ova cleavage and sperm no. in the zona pelucida of gilts inseminated with fresh or frozen semen Treatment
Control
Type of Semen
Fresh
No. Animals No. Ovulations No. Recovered No. Ova Cleaved with sperm in zona N;dn,f Sperm in
SEPTEMBER
Frozen
3
3
:;
12.5 mg/day Fresh
15 mglday
Frozen Fresh
Frozen
::
3 39 33
3 36 28
t:
Z
::
3 1
29 27
17 18
30 34
G!
189
2
1995
61
2669
85
1979 VOL. 12 NO. 3
4
4
133
THERIOGENOLOGY
One animal in the 12.5 mgmlday frozen semen category was excluded from the data because it had a uterus unicornuus, from which four uncleaved ova were recovered. There was no significant difference (P>O.O5) in the proportion of ova cleaved in the treated and control groups but significantly more ova (P-0.01) were cleaved with fresh than with frozen semen. The numbers of gilts pregnant at slaughter 30 days after insemination and the total number of embryos are shown in Table III. Fertility of gilts inseminated at their first return to estrus whether ova had previously been flushed from their reproductive tracts or not is shown in Table IV. TABLE
III
Fertility as determined by pregnancy rate and litter size at slaughter 30 days post insemination Control
Treatment Type of Semen No. Animals No. Embryos Pregnant
Fresh
12.5 mg /day
Frozen
Fresh
Frozen
15 mg /day Fresh
Frozen
3
:
3
3
2
3
1:
0
23 2
7 1
11
:
TABLE
IV
Fertility of animals which returned to estrus and were slaughtered 30 days after a repeat insemination with fresh or frozen semen Operated
Previous Surgery Type of Semen No. Animals No. Pregnant No. Embryos
Fresh
Un-operated Frozen
Fresh
7
6
5
3;
1:
122
Frozen 25 14
Estrus was not observed in four gilts in group 1, five in group 2 and one in group 3. These gilts were in the lower weight ranges at the commencement of the experiment but at the time of slaughter averaged 140 Kg. They had small inactive ovaries when examined at slaughter.
134
SEPTEMBER 1979 VOL. 12 NO. 3
THERIOGENOLOGY One cyst 15mm. in diameter was observed at laparotomy on the ovary one of the gilts receiving 15 mgm of compound per day. Yormal ovulations had also occurred.
of
Discussion The mean duration of estrus observed in this experiment of 55.32 13.8 hours agrees well with that reported (14) of 53.1: 0.36, although the variation observed here seems to be greater and is probably due to different methods of observation. The mean interval between the end of treatment and estrus was shorter (PC 0.01) and the degree of synchronisation for those observed in estrus was better for the treated than the controls. A spread of 2 days (days 5 and 6) in onset of estrus was observed among the gilts on 12.5 Thirteen of the fifteen animals on 15 mg per day came mg per day. into estrus within a four day (days 4-7) period, while only four of the controls came into estrus within any four day period. The mean interval from the end of treatment to the onset of estrus for those i ts coming into estrus within a 10 day period was 6.5f 0.52 and %A- 2 1.4 days respectively for the two treated groups. These are about one day shorter than those previously reported (15) of 6.92 1.2 and 7.0? 1.0 days. The difference may be related to the method of calculation of interval, time of feeding or estrus observation. The failure of some gilts to come into estrus may be related to their low body weight at the commencement of treatment or to the sequence in environmental change. Most of the problems arose in one particular consignment of gilts which when brought to the research centre was maintained there for three weeks prior to treatment because of low body Thus the effects of stress of movement, change of environment weight. and exposure to boars may have worn off (16, 17). The feeding of this compound would seem to have little or no effect in stimulating the onset of puberty in prepubertal gilts. There was no significant difference in the treated and control groups: 115) which showed no difference in and'control gilts, although in the sperm per ova were greater for the
in the proportion of ova cleaved this is in line with other results fertilitv or litter size for treated present experiment the numbers of treated groups of animals.
Fresh semen yielded significantly more cleaved ova (Pr0.01) than frozen semen, which appears to be due largely to the rather poor results obtained using frozen semen from three of the four boars used. All of the boars used had normal or above normal fertility with farrowing rates of 70 per cent when used in a commercial fresh semen AI delivery service. Thus the rather low cleavage rates and the numbers of spermatozoa attached to the zona pellucida obtained amongst the controls with both fresh and frozen semen with cleavage rates of 33 and 18.7 per cent respectively is surprising. However significantly improved litter sizes in gilts fed on this compound have been reported (15). Other workers (17, 18, 19) reported fertilization rates of 50.8 per cent, 21.7 per cent and 83.5 per cent in gilts inseminated with fresh diluted semen
SEPTEMBER
1979 VOL. 12 NO. 3
135
THERIOGENOLOGY Fertilization rates of 87 per cent using frozen thawed semen have been reported (12) but authors did not reFer to any difference noted between boars in fertility with frozen semen. The numbers of embryos recorded in slaughtered gilts are very much in line with the fertility as estimated by the examination of ova, although the figures are too small to arrive at a statistical conclusion. In conclusion Ally1 Trenbolone is an effective synchronising agent for gilts which does not depress fertility. However since the objective is to inseminate gilts without the necessity of estrus detection or to treat gilts which either have not reached puberty or whose breeding history is unknown, additional treatments using gonadotrophins or gonadotrophin releasing hormones are likely to be necessary. References 1.
Ulberg, L.C., Grummer, R.H., Casida, L.E. (1951). The Effects of Progesterone upon Ovarian Function in Gilts. J. Anim. Sci. lo, 665.
2.
Polge, C. (1965). Fertility in Gilts following Dziuk, P.J., Induced Ovulation. Vet. Rec. -77, 236.
3.
Ray, D.E., Seerley, R.W. (1966). Estrus and ovarian morphology in gilts following treatment with orally effective steroids. Nature, -211, 1102.
4.
Pond, W.G., Hansel, W., Dunn, J.A., Bratton, R.W., Foote, R.H. (1965). Estrus Cycle Synchronisation and Fertility of Gilts Fed Progestational and Estrogenic Compounds. J. Anim. Sci. _' 24 536.
5.
Webel, S.K. (1976). Estrus Control in Swine with a Progestagen. 3. Anim. Sci. 42, 1358. ABSTR.
6.
Mayer, P., Schutze, E. (1977). A new progestin (S a 45.249) for cycle control in pigs. Communication 1: Hormonal activities and dosage. Theriogenology 8 357.
7.
Schutze, E. Mayer, P. (1977). A new progestin (S a 45.249) for Duration of treatment cycle control in pigs. Communication II: and effectiveness as an estrous cycle sychronizer. Theriogenology -8 367.
8.
Zerobin , K., Schutze, E., Mayer, P. (1977). A new progestin (S a 45.249) for cycle control in pigs. III; confirmation of effectiveness in field trials Theriogenology -8 378.
9.
Polge, C., Day, B.N., Groves, T.W. (1968). Synchronisation of Ovulation and Artificial Insemination in Pigs. Vet. Rec. 83, 136.
10.
Wrathall, A.E., Herbert, Nancy., Hoskin, B.O. (1972). Vente, J.P., Quantative Anatomical Study of Methallibure Induced Malformations in Piglets. Res. Vet. Sci. -13, 169.
136
SEPTEMBER
1979 VOL. 12 NO. 3
THERIOGENOLOGY
11.
Bennett, G.H., 0' Hagan, C. (1964). of Artificial Insemination of Pigs. Reprod, and AI. Trento -vi, 481.
Factors Influencing the Success Proc. Vth Int. Congr. Anim.
12.
Freezing Pursel, V.G., Johnson, L.A. (1975). Fertilizing Capacity with Concentrated Semen Procedure. J. Anim. Sci. 40, No. 1, 99.
13.
Dziuk, P. (1971). Obtaining Eggs and Embryos from Methods in Mammalian Embryology. ed. J.C. Daniel, Freeman & Co. San Francisco.
14.
Signoret, J.P. (1967). Duree du cycle Estrien et de l"Estrus chez la Truis Action du Benzoate d'Estradio1 chez la Femelle Ovariectonises. Ann. Bio. Anim. Bioch. Biophys. L, (4), 407.
15.
Webel,
16
Mavrogenis, A.P., Puberty in Swine.
17.
Brooks, P.H., Cole, D.J.A. (1970). The Effect of Presence of a Boar on the Attainment of Puberty in Gilts. 3. Reprod. Fertil. -23, 435.
18.
Baker, R.D., Dziuk, P.J., Norton, H.W. (1968). The Effect of Volume of Semen Number of Sperm and Drugs on the Transport of Sperm in Artificially Inseminated Gilts. J. Anim. Sci. -27, 88.
19.
The Effect of Semen Volume Hancock, J.L., Hovell, G.T. (1961). and Numbers of Spermatozoa on the Fertility of Intra-uterine Inseminated Gilts. Anim. Prod. 3, 153.
of Boar Spermatozoa and a New Thawing
Sheep and Pigs. 76. W.H.
S.K. (1978). Ovulation Control in the Pig. In Proc. Control of Ovulation, Nottingham, 421. Ed. D.B. Crighton, Symp. N.B. Haynes, G.R. Foxcroft, G.E. Lamming.
SEPTEMBER
Robinson, J. Anim.
1979 VOL. 12 NO. 3
O.W. (1976). Factors Sci. -42, 1251.
Affecting
137