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Synchronization of estrus in post-partum mares with progesterone and estradiol 17β
THERIOGENOLOGY SYNCHRONIZATION OF ESTRUS IN POST-PARTUM MARES WITH PROGESTERONE AND ESTRADIOL 17~ F. Bristol, K.A. Jacobs, and V. Pawlyshyn Department of Herd Medicine and Theriogenology Western College of Veterinary Medicine University of Saskatchewan Saskatoon, Sask. S7N OWO Received for publication: October 11, 1982
Accepted:
March 18, 1983
ABSTRACT Thirty-six mares which foaled over a lo-day period were given 1 to 10 daily intramuscular injections of a combination of 150 mg. progesterone and 10 mg. estradiol 178. The first injection was given within 18 hours after parturition. Because individual mares foaled on different dates during the 10 day period, commencement of treatment varied, but treatment for all mares ceased on the same day. Teasing and breeding began seven days after the final treatment. The mares were teased daily for 10 days and artifically inseminated every second day until ovulation occurred. The mean interval from the end of treatment to beginning of estrus was 9.4 days (range 7 to 14) and 33 of 26 mares (94.7%) ovulated 10 to 16 days after the final treatment. Both estrus and ovulation were effectively synchronized, resulting in a first estrus pregnancy rate of 80.6% (29 of 36). INTRODUCTION Estrus has been synchronized in mares using prostaglandin (PG)F CY and its analogues alone or in combination with'either human chorionl.Z gonadotrophin (hCG) or gonadotrophin releasing hormone (GnRH) (l-4). Prostaglandins have not been applicable for estrus synchronization in early post-partum mares because it has been shown that the interval from parturition to first ovulation varied from 5 to more than 25 days, however 93% had ovulated by day 15 post-partum therefore a large proportion of mares do not have a corpus luteum that is sensitive to PG until approximately 18 days after parturition (5). Various regimes of progesterone or progestogens, alone or in combination with PG or hCG or both have been used to control the estrous cycle in cycling mares (2,6,7). However, the time from the end of treat ment with progesterone to ovulation has been unpredictable and not uniformly distributed. A new synthetic progestogen (ally1 trenbolone), ACKNOWLEDGEMENT The study was supported by the Western College of Veterinary Medicine Equine Health Research Fund. We wish to thank Mr. H. Clement who provided the mares and facilities for this experiment.
JUNE
1983 VOL. 19 NO. 6
779
when administered orally or by impregnated vaginal sponges, has been shown to be effective in the control and synchronization of estrus in cycling mares (7,8,9). Loy and co-workers showed that a combination of 150 mg. progesterone and 10 mg. estradiol 178 administered daily after an injection of PGF M provided a highly effective method of controlling estrus and ovulation in cycling mares (10). The first post-partum estrus in mares has been delayed for varying lengths of time using daily injections of 100 to 200 mg. progesterone beginning on the 5th day post-partum; but not all ovulations were blocked, and the interval from the end of treatment to ovulation varied from 3 to 14 days (10,11,12). When progesterone treatment was begun on the first day post-partum, there were 2 groups of ovulatjons separated by approximately a week. It was suggested that this bimodal response occurred because the first follicles to develop after parturition ovulated after withdrawal of progesterone while others regressed and were replaced by new follicles which ovulated later (10). Recently Lay et al showed that 200 mg. progesterone and 10 mg. estradiol 178 administered daily for 5 days, co~encing the day of parturjtion, effectively delayed onset of the first post-partum estrus (13). The present experiment was designed to investigate the effectiveness of progesterone and estradiol 178 for synchronizing estrus and ovulation in early post-partum mares. MATERIALS AND METHODS Thirty-six crossbred draft-type mares weighing between 400 and 650 kg. were used in this experiment. The animals were in good bodily condition and maintained on a mixed grass-legume pasture. The mares foaled on various days during a lo-day period (May 18 to 27). They were given daily intramuscular injections of 150 mg. progesterone and 10 mg. estradiol 178 dissolved in cottonseed oil beginning within 18 hours of parturition. Because individual mares foaled on different days during the 10 day period, the number of treatments for each mare varied. The last treatment was given to all mares 7 days before teasing, and breeding began on June 3 (Table I). Table I.
Distribution and treatment frequency for post-partum mares receiving daily injections of 150 mg. progesterone and 10 mg. estradiol 178.
18 No. of mares commencing treatment
Date on which treatment began (May) 20 21 22 23 24 25 26
27
1780222464
No. of treatments per mare 10
780
19
9
8
7
6
5
4
3
2
1
JUNE 1983 VOL. 19 NO. 6
0
Mares ovulating Total No.
%
33.3
8.3
% 0
12
3
Mares in estrus Total No.
8
7
0
58.3
21
9
14 38.8
30.6
8.3
88.6
32
12
11
88.6
32
11
3
86.1
31
10
50.0
18
97.2
35
13
66.7
24
100
36
14
80.6
29
100
36
15
Synchronization of estrus and ovulation after treatment of postpartum mares with progesterone and estradiol 178. The numbers are those of all mares in estrus and ovulating by that day.
Days after last treatment
Table II.
91.7
33
100
36
16
THERIOGENOLOGY The mares were teased every day for 10 days and they were examined daily by rectal palpation during estrus to determine the time of ovulation. They were inseminated on alternate days during estrus beginning on the first or second day. Semen was collected from a Belgian stallion by artificial vagina and the gel free portion was divided equally amongst the mgres to be inseminated. Semen was kept in a warm water bath at $7 C prior to insemination. Each mare received a minimum of 140 x 10 progressively motile spermatozoa per insemination. Three days after the end of the experiment, the mares were assigned to pasture groups and turned into pastures with stallions to cover mares returning to estrus. They were examined for pregnancy by rectal palpation 38 days after the end of artificial insemination. For analysis of the results, the mares were divided into two equal groups of 18 mares, namely, those receiving treatment for 1 to 5 days post-partum and those receiving treatment for 6 to 10 days. The mean intervals from last treatment to onset of estrus and ovulation in these two groups were compared using Students 't' test and the differences in pregnancy rate were compared by Chi square analysis. RESULTS
AND DISCUSSION
The results of the experiment shown in Tables II and III indicate that the combination of progesterone and estradiol 176, when administered daily beginning within 18 hours of parturition, will delay the first post-partum estrus and result in synchronized estrus and ovulation in post-partum mares. Ovulations occurred from the 10th day after the end of the treatment period and continued until probably shortly after the 16th day post treatment.because three mares which had not ovulated had follicles greater than 45 mm (Table II). Table
III.
Effect of progesterone and estradiol and ovulation in post-partum mares. Onset of estrus days post treatment
170 on control
Ovulation days post treatment
of estrus
Number Pregnant
n
Mean
Range
n
Mean
Range
Mares treated 1 to 5 days post-partum
18
9.33
7-14
15*
13.67
11-16
14
Wares treated 6 to 10 days post-partum
18
9.44
8-13
18
12.56
lo-16
15
All mares
36
9.39
7-14
33
13.06
lo-16
29
*
Three mares
782
did not ovulate
during
the examination
period.
JUNE 1983 VOL. 19 NO. 6
THERIOGENOLOGY Mean interval from the end of treatment to the onset of estrus and ovulation in the two groups of mares were not different (P>O.O5), indicating synchronization of estrus and ovulation were similar regardless of the duration of treatment. The mean interval from the end of treatment to ovulation of 13.06 days was slightly longer than 10.9 days reported by Loy et al (10) and the range (IO-16 vs 9-13) was also greater. A slightly longer period of observation may have been necessary to detect all ovulations. The variable duration of treatment and slightly lower dose of progesterone may have caused this difference. During the last month of pregnancy, circulating progestogens increase markedly while estrogens decrease slightly. At or just prior to parturition, both hormones drop dramatically and, after parturition, the levels of estrogen and progesterone remain low until estrogen rises at the first estrus and progesterone rises shortly thereafter (14,15). A surge of follicle stimulating hormone (FSH) occurs during parturition and is thought to be due to removal of some inhibitory factor produced by the fetoplacental unit or hormonal changes associated with parturition (16,17). Following this surge, FSH levels remain low until the first estrus, while levels of luteinizing hormone (LH) gradually increase during the post-partum period and peak around the first ovulation (17). Loy et al (13) found that a combination of 200 mg. progesterone and 10 mg. estradiol 17s caused a more rapid decline of FSH within 24 hours of parturition when compared to untreated mares. LH remained low during treatment followed by a significant increase within 24 hours of the last progesterone-estradiol treatment similar to that seen within 24 hours of foaling in untreated mares (13). Apparently, the steroid combination delayed the hormonal changes and ovarian activity after parturition for the duration of treatment and resulted in a uniform distribution of ovulation. The fertility of mares after this method of estrus synchronization was satisfactory and the overall pregnancy rate at the delayed postpartum estrus of 80.6% (29 of 36) was comparable to that previously reoorted bv Lov et al (11) and Pooe et al (12). There was no statistica difference"betheen the‘pregnancy rates of those mares treated for 1 to 5 days and those treated 6 to 10 days (X P>O.Ol). Delaying the first post-partum estrus may result in a higher iertility than that achieved when mares are bred at the first estrus (13).
JUNE
1983 VOL. 19 NO. 6
783
THERIOGENOLOCY REFERENCES
1.
Palmer, E. and Jousset, B. prostaglandin analogue 269-274 (1975).
2.
Holtan, D.W., Douglas, R.H. and Ginther, O.J. Estrus, ovulation and conception following synchronization with progesterone, prostaglandin F cx and human chorionic gonadotrophin in pony mares. J. Anim. $ci . 44 : 431-437 (1977).
3.
Hyland, J.H. and Bristol, F. Synchronization of oestrus and timed J. Reprod. Fert., Suppl. 27 : 251-255 insemination of mares. (1979).
4.
Voss, J.L., Wallace, R.A., Squires, E.L., Pickett, B.W. and Shideler, R.K. Effects of synchronization and frequency of insemination on fertility. J. Reprod. Fert., Suppl. 27 : 257-261 (1979).
5.
Loy, R.G. Characteristics of postpartum reproduction in mares. Clin. No. Amer. Lrg. Anim. Pratt. 2 : 345-359 (1980).
6.
Loy, R.G. and Swan, S.M. phenomena in mares.
7.
Palmer, E. Different techniques for synchronization of ovulation mare. Proc. 8th Int. Congr. Anim. Reprod. and A.I., Kracow, 495-498 (1976).
8.
Palmer, E. Reproductive management of mares without J. Reprod. Fert., Suppl. 27 : 263-270 (1979).
9.
Squires. E.L.. Stevens. W.B.. McGlothlin. D.E. and Pickett. B.W. Effect of an-oral progestin-on the estrous cycle and fertility of mares. J. Anim. Sci. 49 : 729-735 (1979).
Synchronization of oestrus in mares with a and HCG. J. Reprod. Fert., Suppl. -23 :
Vet.
Effects of exogenous progestogens on reproductive J. Anim. Sci. 25_ : 821-826 (1966).
detection
in the pp.
of oestrus.
10.
LOY, R.G., Pemstein,
11.
LOY, R.G., Hughes, J.P., Richards,
12.
Pope, A.M. , Campbell, D.L. and Uavidson, J.P. Endometrial histology of post partum mares treated with progesterone and synthetic GnRH (AY-24,031). J. Reprod. Fert. Suppl., 27 : 587-591 (1979).
13.
Loy, R.G., Evans, M.J., Zent, W.R., Pemstein, R. and Taylor, T.B. Effects of injected ovarian steroids on reproductive patterns and performance in post-partum mares. Proc. Brit. Eq. Vet. Assoc. 20th Ann. Congr. p, 27 (1981).
784
R., O'Canna, D. and Douglas R.H. Control in cycling mares with ovarian steroids and prostaglandin. genology 15 : 191-200 (1981).
of ovulation Therio-
W.P.C. and Swan, S.M. Effects terone on reproductive function in mares after parturition. Reprod. Fert., Suppl. 23 : 291-295 (1975).
of progesJ.
JUNE 1983 VOL. 19 NO. 6
THERIOGENOLOGY 14.
Ganjam, V.K., knney, R.W. and Flickinger, G. Plasma progestagens in cyclic, pregnant and post-pat-turn mares. 3. Reprod. Fert., Suppl. 2: 441-447 (1975).
15.
Lovell, J.D., Stabenfeldt, G.H., Hughes, J.P. and Evans, J.W. Endocrine patterns of the mare at term. 3. Reprod. Fert., Suppl. -23 : 449456 (1975).
16.
Sharma, O.P. Release of oxytocin elicited by suckling stimulus in mares. J. Reprod. Fert. 37 : 421-424 (1974).
17.
Turner, D.D., Garcia, M.C., Miller, K.F., Holtan, U.W. and Ginther, O.J. FSH and LH concentrations in periparturient mares. J, Reprod. Fert., Suppl. 27 : 547-553 (1979).
JUNE 1983VOL. 19NO. 6
785
Accepted:
March 18, 1983
ABSTRACT Thirty-six mares which foaled over a lo-day period were given 1 to 10 daily intramuscular injections of a combination of 150 mg. progesterone and 10 mg. estradiol 178. The first injection was given within 18 hours after parturition. Because individual mares foaled on different dates during the 10 day period, commencement of treatment varied, but treatment for all mares ceased on the same day. Teasing and breeding began seven days after the final treatment. The mares were teased daily for 10 days and artifically inseminated every second day until ovulation occurred. The mean interval from the end of treatment to beginning of estrus was 9.4 days (range 7 to 14) and 33 of 26 mares (94.7%) ovulated 10 to 16 days after the final treatment. Both estrus and ovulation were effectively synchronized, resulting in a first estrus pregnancy rate of 80.6% (29 of 36). INTRODUCTION Estrus has been synchronized in mares using prostaglandin (PG)F CY and its analogues alone or in combination with'either human chorionl.Z gonadotrophin (hCG) or gonadotrophin releasing hormone (GnRH) (l-4). Prostaglandins have not been applicable for estrus synchronization in early post-partum mares because it has been shown that the interval from parturition to first ovulation varied from 5 to more than 25 days, however 93% had ovulated by day 15 post-partum therefore a large proportion of mares do not have a corpus luteum that is sensitive to PG until approximately 18 days after parturition (5). Various regimes of progesterone or progestogens, alone or in combination with PG or hCG or both have been used to control the estrous cycle in cycling mares (2,6,7). However, the time from the end of treat ment with progesterone to ovulation has been unpredictable and not uniformly distributed. A new synthetic progestogen (ally1 trenbolone), ACKNOWLEDGEMENT The study was supported by the Western College of Veterinary Medicine Equine Health Research Fund. We wish to thank Mr. H. Clement who provided the mares and facilities for this experiment.
JUNE
1983 VOL. 19 NO. 6
779
when administered orally or by impregnated vaginal sponges, has been shown to be effective in the control and synchronization of estrus in cycling mares (7,8,9). Loy and co-workers showed that a combination of 150 mg. progesterone and 10 mg. estradiol 178 administered daily after an injection of PGF M provided a highly effective method of controlling estrus and ovulation in cycling mares (10). The first post-partum estrus in mares has been delayed for varying lengths of time using daily injections of 100 to 200 mg. progesterone beginning on the 5th day post-partum; but not all ovulations were blocked, and the interval from the end of treatment to ovulation varied from 3 to 14 days (10,11,12). When progesterone treatment was begun on the first day post-partum, there were 2 groups of ovulatjons separated by approximately a week. It was suggested that this bimodal response occurred because the first follicles to develop after parturition ovulated after withdrawal of progesterone while others regressed and were replaced by new follicles which ovulated later (10). Recently Lay et al showed that 200 mg. progesterone and 10 mg. estradiol 178 administered daily for 5 days, co~encing the day of parturjtion, effectively delayed onset of the first post-partum estrus (13). The present experiment was designed to investigate the effectiveness of progesterone and estradiol 178 for synchronizing estrus and ovulation in early post-partum mares. MATERIALS AND METHODS Thirty-six crossbred draft-type mares weighing between 400 and 650 kg. were used in this experiment. The animals were in good bodily condition and maintained on a mixed grass-legume pasture. The mares foaled on various days during a lo-day period (May 18 to 27). They were given daily intramuscular injections of 150 mg. progesterone and 10 mg. estradiol 178 dissolved in cottonseed oil beginning within 18 hours of parturition. Because individual mares foaled on different days during the 10 day period, the number of treatments for each mare varied. The last treatment was given to all mares 7 days before teasing, and breeding began on June 3 (Table I). Table I.
Distribution and treatment frequency for post-partum mares receiving daily injections of 150 mg. progesterone and 10 mg. estradiol 178.
18 No. of mares commencing treatment
Date on which treatment began (May) 20 21 22 23 24 25 26
27
1780222464
No. of treatments per mare 10
780
19
9
8
7
6
5
4
3
2
1
JUNE 1983 VOL. 19 NO. 6
0
Mares ovulating Total No.
%
33.3
8.3
% 0
12
3
Mares in estrus Total No.
8
7
0
58.3
21
9
14 38.8
30.6
8.3
88.6
32
12
11
88.6
32
11
3
86.1
31
10
50.0
18
97.2
35
13
66.7
24
100
36
14
80.6
29
100
36
15
Synchronization of estrus and ovulation after treatment of postpartum mares with progesterone and estradiol 178. The numbers are those of all mares in estrus and ovulating by that day.
Days after last treatment
Table II.
91.7
33
100
36
16
THERIOGENOLOGY The mares were teased every day for 10 days and they were examined daily by rectal palpation during estrus to determine the time of ovulation. They were inseminated on alternate days during estrus beginning on the first or second day. Semen was collected from a Belgian stallion by artificial vagina and the gel free portion was divided equally amongst the mgres to be inseminated. Semen was kept in a warm water bath at $7 C prior to insemination. Each mare received a minimum of 140 x 10 progressively motile spermatozoa per insemination. Three days after the end of the experiment, the mares were assigned to pasture groups and turned into pastures with stallions to cover mares returning to estrus. They were examined for pregnancy by rectal palpation 38 days after the end of artificial insemination. For analysis of the results, the mares were divided into two equal groups of 18 mares, namely, those receiving treatment for 1 to 5 days post-partum and those receiving treatment for 6 to 10 days. The mean intervals from last treatment to onset of estrus and ovulation in these two groups were compared using Students 't' test and the differences in pregnancy rate were compared by Chi square analysis. RESULTS
AND DISCUSSION
The results of the experiment shown in Tables II and III indicate that the combination of progesterone and estradiol 176, when administered daily beginning within 18 hours of parturition, will delay the first post-partum estrus and result in synchronized estrus and ovulation in post-partum mares. Ovulations occurred from the 10th day after the end of the treatment period and continued until probably shortly after the 16th day post treatment.because three mares which had not ovulated had follicles greater than 45 mm (Table II). Table
III.
Effect of progesterone and estradiol and ovulation in post-partum mares. Onset of estrus days post treatment
170 on control
Ovulation days post treatment
of estrus
Number Pregnant
n
Mean
Range
n
Mean
Range
Mares treated 1 to 5 days post-partum
18
9.33
7-14
15*
13.67
11-16
14
Wares treated 6 to 10 days post-partum
18
9.44
8-13
18
12.56
lo-16
15
All mares
36
9.39
7-14
33
13.06
lo-16
29
*
Three mares
782
did not ovulate
during
the examination
period.
JUNE 1983 VOL. 19 NO. 6
THERIOGENOLOGY Mean interval from the end of treatment to the onset of estrus and ovulation in the two groups of mares were not different (P>O.O5), indicating synchronization of estrus and ovulation were similar regardless of the duration of treatment. The mean interval from the end of treatment to ovulation of 13.06 days was slightly longer than 10.9 days reported by Loy et al (10) and the range (IO-16 vs 9-13) was also greater. A slightly longer period of observation may have been necessary to detect all ovulations. The variable duration of treatment and slightly lower dose of progesterone may have caused this difference. During the last month of pregnancy, circulating progestogens increase markedly while estrogens decrease slightly. At or just prior to parturition, both hormones drop dramatically and, after parturition, the levels of estrogen and progesterone remain low until estrogen rises at the first estrus and progesterone rises shortly thereafter (14,15). A surge of follicle stimulating hormone (FSH) occurs during parturition and is thought to be due to removal of some inhibitory factor produced by the fetoplacental unit or hormonal changes associated with parturition (16,17). Following this surge, FSH levels remain low until the first estrus, while levels of luteinizing hormone (LH) gradually increase during the post-partum period and peak around the first ovulation (17). Loy et al (13) found that a combination of 200 mg. progesterone and 10 mg. estradiol 17s caused a more rapid decline of FSH within 24 hours of parturition when compared to untreated mares. LH remained low during treatment followed by a significant increase within 24 hours of the last progesterone-estradiol treatment similar to that seen within 24 hours of foaling in untreated mares (13). Apparently, the steroid combination delayed the hormonal changes and ovarian activity after parturition for the duration of treatment and resulted in a uniform distribution of ovulation. The fertility of mares after this method of estrus synchronization was satisfactory and the overall pregnancy rate at the delayed postpartum estrus of 80.6% (29 of 36) was comparable to that previously reoorted bv Lov et al (11) and Pooe et al (12). There was no statistica difference"betheen the‘pregnancy rates of those mares treated for 1 to 5 days and those treated 6 to 10 days (X P>O.Ol). Delaying the first post-partum estrus may result in a higher iertility than that achieved when mares are bred at the first estrus (13).
JUNE
1983 VOL. 19 NO. 6
783
THERIOGENOLOCY REFERENCES
1.
Palmer, E. and Jousset, B. prostaglandin analogue 269-274 (1975).
2.
Holtan, D.W., Douglas, R.H. and Ginther, O.J. Estrus, ovulation and conception following synchronization with progesterone, prostaglandin F cx and human chorionic gonadotrophin in pony mares. J. Anim. $ci . 44 : 431-437 (1977).
3.
Hyland, J.H. and Bristol, F. Synchronization of oestrus and timed J. Reprod. Fert., Suppl. 27 : 251-255 insemination of mares. (1979).
4.
Voss, J.L., Wallace, R.A., Squires, E.L., Pickett, B.W. and Shideler, R.K. Effects of synchronization and frequency of insemination on fertility. J. Reprod. Fert., Suppl. 27 : 257-261 (1979).
5.
Loy, R.G. Characteristics of postpartum reproduction in mares. Clin. No. Amer. Lrg. Anim. Pratt. 2 : 345-359 (1980).
6.
Loy, R.G. and Swan, S.M. phenomena in mares.
7.
Palmer, E. Different techniques for synchronization of ovulation mare. Proc. 8th Int. Congr. Anim. Reprod. and A.I., Kracow, 495-498 (1976).
8.
Palmer, E. Reproductive management of mares without J. Reprod. Fert., Suppl. 27 : 263-270 (1979).
9.
Squires. E.L.. Stevens. W.B.. McGlothlin. D.E. and Pickett. B.W. Effect of an-oral progestin-on the estrous cycle and fertility of mares. J. Anim. Sci. 49 : 729-735 (1979).
Synchronization of oestrus in mares with a and HCG. J. Reprod. Fert., Suppl. -23 :
Vet.
Effects of exogenous progestogens on reproductive J. Anim. Sci. 25_ : 821-826 (1966).
detection
in the pp.
of oestrus.
10.
LOY, R.G., Pemstein,
11.
LOY, R.G., Hughes, J.P., Richards,
12.
Pope, A.M. , Campbell, D.L. and Uavidson, J.P. Endometrial histology of post partum mares treated with progesterone and synthetic GnRH (AY-24,031). J. Reprod. Fert. Suppl., 27 : 587-591 (1979).
13.
Loy, R.G., Evans, M.J., Zent, W.R., Pemstein, R. and Taylor, T.B. Effects of injected ovarian steroids on reproductive patterns and performance in post-partum mares. Proc. Brit. Eq. Vet. Assoc. 20th Ann. Congr. p, 27 (1981).
784
R., O'Canna, D. and Douglas R.H. Control in cycling mares with ovarian steroids and prostaglandin. genology 15 : 191-200 (1981).
of ovulation Therio-
W.P.C. and Swan, S.M. Effects terone on reproductive function in mares after parturition. Reprod. Fert., Suppl. 23 : 291-295 (1975).
of progesJ.
JUNE 1983 VOL. 19 NO. 6
THERIOGENOLOGY 14.
Ganjam, V.K., knney, R.W. and Flickinger, G. Plasma progestagens in cyclic, pregnant and post-pat-turn mares. 3. Reprod. Fert., Suppl. 2: 441-447 (1975).
15.
Lovell, J.D., Stabenfeldt, G.H., Hughes, J.P. and Evans, J.W. Endocrine patterns of the mare at term. 3. Reprod. Fert., Suppl. -23 : 449456 (1975).
16.
Sharma, O.P. Release of oxytocin elicited by suckling stimulus in mares. J. Reprod. Fert. 37 : 421-424 (1974).
17.
Turner, D.D., Garcia, M.C., Miller, K.F., Holtan, U.W. and Ginther, O.J. FSH and LH concentrations in periparturient mares. J, Reprod. Fert., Suppl. 27 : 547-553 (1979).
JUNE 1983VOL. 19NO. 6
785