Effects of Equine Chorionic Gonadotropin on Reproductive Performance in Anestrous Mink1 William B. Wehrenbergz, Kenneth J. Kurt, and Reinhold J. Hutz Departments of Health Sciences and Biological Sciences, University of Wisconsin, Milwaukee 5321 1 and J K J Mink Faxm, Union Grove, WI 53216
IU of hCG on March 18. Females were paired with males beginning on March 22. None of the salinetreated mink mated. In contrast, reproductive performance of the anestrous mink was signiflcantly improved by treatment with eCG. This included proportion of mink breeding (47to 100%), proportion giving birth (33 to 80%1, and average litter size (2.6 to 4.0 kits per whelping female). Reproductive efficiency improved with increasing doses of eCG and was not further improved by addition of hCG. These results demonstrate that eCG has a potential application for treating anestrous mink during the breeding season.
Key Words: Mink, HCG, Anestrus, Reproduction, Pregnancy
J. Anim. Sci. 1992. 70:499-502
Introduction Mink are seasonally breeding animals that demonstrate estrus during the month of March in the northern hemisphere (Joergensen, 1985). These reproductive events are regulated by photoperiod (Turek and Campbell, 1979). In general, females demonstrate two to four periods of estrus at 7-d intervals. Follicular development is in synchrony with these periods of estrus (Murphy, 1987; Murphy et al., 1987; Sundquist et al., 1988). Ovulation is induced by copulation (Hannson, 1947;Enders, 1952).Almost always, implantation is delayed and the female enters another period of
'Research supported by JKJ Fur Farm, Union Grove, WI. We thank L. Stagg for technical assistance and D. Hackenberg for typing the manuscript. * o whom correspondence should be addressed: Dept. of Health Sci., Univ. of Wisconsin, P.O.Box 413, Milwaukee, 53201. Received May 28, 1991. Accepted August 29, 1991.
499
estrus with its accompanying follicular development. If the animal is bred again during this estrus, the litter comprises offspring from both matings. It has been estimated that 2 to 5% of the female breeding herd remains anestrous during the breeding season. Thus, based on USDA statistics (Anonymous, 19901,the number of anestrous mink during the breeding season approaches 25,000 in the United States alone. The economic impact of these infertile mink is obvious. Administration of gonadotropins to enhance reproductive performance is a common practice in numerous species. Its application in the management of anestrous mink is limited by a lack of research (Hammond, 1952; Adams, 1976; Hattenhauer, 1984; Murphy, 1987; Murphy et al., 1987). The present study was undertaken to evaluate the effects of exogenous equine chorionic gonadotropin (eCG) and hCG treatment on the reproductive performance of idiopathic, anestrous mink.
Downloaded from https://academic.oup.com/jas/article-abstract/70/2/499/4631970 by 04860000 user on 14 January 2019
ABSTRACT: The incidence of anestrous mink during the normal breeding season has been reported to be as high as 5%. We sought to induce estrus in these mink by using various doses of equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCGI. Seventyfive female mink maintained under standard ranch conditions failed to demonstrate estrus during the annual breeding season in March. These anestrous mink were randomly assigned to treatment groups. Treatments were given on March 16 and again on March 18. On these 2 d, mink were treated with equal doses of saline, 25,50,or 100 IU of eCG, or 50 IU of eCG on March 16 and 50 of IU of eCG + 100
WEHRENBERG ET AL.
500
Table 1. Breeding performance in mink treated with equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCG)”
Treatment 2 x
Saline
2 x 25 2 x 50 2
breedina
15
IU of eCG IU of eCG
IU of eCG +
+
@
15
gd
15
15”
15
lld
IU of hCG aData are expressed as means i SEM.
50 IU of eCG
oc
100
Breeding Datternb 0 Single
First day No. giving of breedina birth 0
OC
6.8 f .3
5d
8.7 f .2
9”
0 Double
Single Double single Double Single Double Single 8 Double 4
3 5 3 6 9 3
6.8 f .2 7.5 f .2
12”
9d”
bSingle refers to females only bred once,double refers to females bred once then rebred 8 to 24 h later. c*d*eDifTerent superscripts within a column denote significant differences (P < .OS), ANOVA, Studentized range statistic.
Experimental Procedure Animals. Seventy-five female ranch mink of the dark variety were maintained under standard ranch conditions that included natural photoperiod (Joergensen, 1985; Anonymous, 1991). All animals were 1 to 2 yr of age. Animals were fed six times per week with a wet mink feed consisting of 39% CP and 19% fat. During the breeding season, feeding was increased to seven times per week. Water was available at all times. The care and use of the animals was approved by the University of Wisconsin-Milwaukee Animal Care and Use Cornmittee. Breeding Practices. Breeding was initiated in these animals on March 3. Breeding procedures at the J K J Fur Farm consisted of moving females to the male’s cage. The females were watched closely to determine their receptivity for breeding. If the male was successful in mounting the female and intromission occurred ( c o n f i i e d visually by the posture of the animals), the animals were left together for 15 to 20 min. This was defined as “bred.”If the female was nonreceptive, the animals were separated within 5 min. This was defined as an “attempt to breed.” All males used in the study were of confirmed fertility. The day of the first breeding was termed d 0 of gestation. After attempting to breed the animals used in this study five to seven times a t approximately 3-d intervals, it was determined by the ranch management that the likelihood that these animals would breed was very low. Thus, on March 16 animals were randomly assigned to one of five treatment groups. Four groups, saline and 25,50, or 100 IU of eCG aot #27047, Intervet America, Millsboro, DE), were treated with equal doses on March 16 and March 18th. The fifth group received 50 IU of eCG on March 16 and 50 IU of eCG + 100 IU of hCG on
March 18. Injections were administered i.m. in a rear leg at a volume of .25 to .50 mL. Breeding of the animals was reinitiated on March 22. Attempts to breed mink were delayed until 6 d after the initial injection to allow follicular maturation to occur. After termination of the breeding season, all females were maintained in their home cages. During the whelping season, animals were observed daily for parturition. Both date of parturition and litter size were noted. Statistical Analysis. Significant treatment effects were detected by ANOVA for average day of breeding and the number of kits born and raised. Treatment effects on breeding performance were detected by Fisher exact-probability test.
Results Number of attempts to breed each female before hormone treatment averaged 5.6 f .1 to 6.1 f .1 among treatment groups. In no instance were any of the animals successfully bred. This failure to breed served as the criterion for the selection of animals in this study. Number of attempts to breed each mink after treatment with eCG or eCG + hCG averaged 1.6 f .I to 2.5 f .1 among treatment groups. Saline-treated females were clearly nonreceptive to the male mink and therefore attempts to breed these females ceased after two or three unsuccessful attempts. Breeding performance in response to gonadotropin treatment is reflected by the number of mink breeding and giving birth (Table 11. None of 15 anestrous mink bred after saline treatment, compared with 50 to 100% of the animals treated with eCG or eCG + hCG. Between 33 and 80% of the females assigned to the gonadotropin treatment groups gave birth. Most animals bred be-
Downloaded from https://academic.oup.com/jas/article-abstract/70/2/499/4631970 by 04860000 user on 14 January 2019
50
No.
mink 15
IU of eCG
x 100
No. of
ECG/HCG IN ANESTROUS MINK
501
Table 2. Reproductive performance in mink treated with equine chorionic gonadotropin [eCG) and human chorionic gonadotropin (hCG)" ~~
~
All
Treatment
female
Ob
Ob .87 1.33 2.67 2.40
f f f f
Whelping female
.41b .46bc .71' .65'
1.81 2.50 2.67 3.18
f f f f
*Data are expressed as means f SEM. b,cDifferent superscripts within a column denote significant differences
tween d 6 and 7 posttreatment (Table 1). In most instances (23 of 411, the bred mink were receptive to a second mating 8 to 24 h later. Table 2 presents the effects of gonadotropin treatment on the number of kits born and raised. In no instance did saline-treated mink bear offspring. In contrast, the number of kits born per bred and whelping female was significantly enhanced by treatment with gonadotropin (P < .05, Table 21. When averaged over all females, the number of kits born was reduced compared with the number of kits born per bred or whelping females because not all mink bred or bore offspring after eCG and eCG + hCG treatment. The number of kits alive at 24 h reflected neonatal mortality of less than .8 kits per litter (Table 2).
Discussion These results demonstrate clearly that exogenous gonadotropin treatment can improve reproductive performance in anestrous mink. Litter size in whelping females (2.6 to 4.0) treated with two doses of eCG or eCG + hCG were within the expected range [Murphy, 1987; Anonymous, 1991). The efficiency of eCG or eCG + hCG treatment was evident in that gonadotropin-treated mink successfully bore offspring whereas the salinetreated animals were barren. These results are consistent with our earlier observation that a single dose of 37 to 50 IU of eCG significantly enhanced breeding performance and litter size in female mink rendered anestrous by photoperiod (Wehrenberg, 1989). Similarly, Murphy et al. (1987) observed that a single injection of 100 IU of eCG to 226 anestrous mink resulted in 80% breeding and approximately 50% whelping. Litter size was approximately two kits per litter, slightly lower than litter size in our experiment. The incidence of neonatal mortality in the present study was similar to neonatal kit loss for normal mink (Murphy, 1987).
.74' .63' .71' .7OC
[P
2.60 2.86 3.33 4.00
Ob f .8lC f .59'
f .mC f 37'
Kits alive at 24 h postpartum per whelping female Ob 2.20 2.00 2.67 3.67
f 1.02c f 58'
f f
.76'
.7lC
< .051,ANOVA, Studentized range statistic.
We previously investigated the effects of a single injection of eCG (25 to 50 IU1 on reproductive performance in anestrous mink [Wehrenberg, et al., 1989). That study and other preliminary studies in which two doses of eCG at 2-d intervals were administered led to the conclusion by mink ranch management that the two-dose regimen of eCG treatment led to greater ease in breeding (unpublished observations). Based on this information, we chose to increase the number of injections of eCG from a single injection to two injections 2 d apart. Although it is not strictly appropriate to make comparisons between the present study and our former study (Wehrenberg, et al., 19891 because of the different study sites and ranch management practices, the number of "attempts to breed after a single injection of 50 IU of eCG was 3.3 f .4 and resulted in 36% of the females giving birth. In contrast, the number of "attempts to breed" after 2 x 50 IU of eCG was 2.1 k .2 and resulted in 47% of the females giving birth. The present study expands our knowledge by Eurther characterizing the dose-response relationship between exogenous gonadotropins and reproductive performance. The fact that reproductive performance among the mink treated with the 2 x 50 IU dose, the 2 x 100 IU dose, and 50 IU of eCG + 50 IU of eCG + 100 IU of hCG was similar and was better than the 2 x 25 IU dose leads us to suggest that the 2 x 50 IU dose is the lowest dose for optimal enhancement of reproductive performance. In addition, use of the 2 x 50 IU dose may be prudent because we have observed increased follicular atresia with higher doses Nehrenberg, et al., 19891. However, further investigations may be necessary because approximately 50% of the anestrous mink still did not yield offspring. Such studies should center on anatomical and physiological dysfunction of the reproductive tract itself because dysfunction of the pituitary and higher brain centers is bypassed by gonadotropin treatment. However, central nervous system involve-
Downloaded from https://academic.oup.com/jas/article-abstract/70/2/499/4631970 by 04860000 user on 14 January 2019
2 x Saline 2 x 25 IU of eCG 2 x 50 IU of eCG 2 x 100 IU of eCG 50 IU of eCG + 50 IU of eCG + 100 IU of hCG
Kits born per Bred female
502
WEHRENBERG ET
Implications On the average, nonbreeding mink represent 2 to 5% of the female breeding herd. The present study demonstrates that administration of equine chorionic gonadotropin leads to successful pregnancy in approximately 50% of the animals. The ability of the mink rancher to salvage these
animals by artificial gonadotropin treatment has direct economic benefits. These include decreased work in breeding the mink, decreased percentage of infertile mink, and increased number of offspring.
Literature Cited Adams, C. E. 1976. Current research problems in the reproduction of mink. Proc. First Int. Congr. Anim. Prod. Anonymous. 1990. Mink. Agriculture Statistics Board, National Agricultural Statistics Service, USDA, Washington, DC. Anonymous. 1991.Blue Book of Fur Farming. Communications Marketing, Inc., Eden Prairie, MN. Enders, R. K. 1952. Reproduction in the Mink (Mustela visonl. Proc. Am. Philos. SOC.96:691. Hammond. J. 1952. Gonadotropin-induced ovulation in the mink. J. Mam~~iol. 33:218. Hannson, A. 1947. The physiology of reproduction in mink (Mustela vison) with special reference to delayed implant& tion. Acta Zool. Stockh. 28:l. Hattenhauer, H., H. Pingel, and K. Eke. 1984. Use of PMSG and hCG in mink breeding. Monatsh. Veterinaermed. 39:520. Hutz, R. J., L. C. Sayles, and W. B. Wehrenberg. 1990. Effect of gonadotropin regimen on induction of follicular growth in mink. Biol. Reprod. 42 (Suppl. 1):47 (Abstr.1. Joergensen, G. 1985. Mink Production. Scient*, Hilleroed, Denmark. Murphy, B. D. 1987. Reproductive physiology of female mustelids. In: US.Seal, E. T. Thorne, M. A. Bogan, and S. H. Anderson [Ed.) Reproductive Physiology of the BlackFooted Ferret. pp 107-123. Yale Univ. Press, New Haven, CT. Murphy, B. D., D. B. Hinter, D. K. Onderka, and J. Hazelwood. 1987.Use of equine chorionic gonadotropin in female mink. Theriogenology. 28:667. Sundquist, C., L. C. Ellis, and A. Bartke. 1988. Reproductive endocrinology in the mink (Mustela vison). Endocr. Rev. 9: 247.
Turek, F. W., and C. S.Campbell. 1979.Photoperiodic regulation of neuroendocrine-gonadal activity. Biol. Reprod. 20:32. Wehrenberg, W. B., S.M. Bejvan, J. Kurhajec, and R. J. Hutz. 1989. The role of pregnant mare serum gonadotropin (PMSG) in the practical management of non-breeding female mink. Domest. Anim. Endocrinol. 6:371.
Downloaded from https://academic.oup.com/jas/article-abstract/70/2/499/4631970 by 04860000 user on 14 January 2019
ment cannot be ruled out completely because reproductive failure might reflect behavioral anestrus. We sought to enhance ovulation and not just follicular development in those mink treated with eCG and hCG. This was based on our histological observations that two injections of 25 IU of eCG 48 h apart or identical treatment followed by a third injection of 37 IU of eCG on d 6 failed to result in ovulation (Hutz et al., 1990). Our concern for this issue was based on the hypothesis that if anestrus is due to hypothalamic or pituitary dysfunction, then regardless of successful exogenous gonadotropin-induced follicular development, reproductive failure would persist because ovulation could not occur. Our present data suggest that hCG supplementation is not effective in improving reproductive performance. Effective application of the information obtained from this study on a commercial mink ranch will in part reflect normal breeding practices on a ranch (Le., the identification and selection of anestrous females). As evidenced by the results obtained in saline-treated mink, anestrous females were correctly identified in this study. Results from applying these procedures on ranches where close scrutiny of breeding mink is not practiced may yield different results. It is not known whether eCG treatment has any detrimental effect on pregnant mink.
AL.
IU of hCG on March 18. Females were paired with males beginning on March 22. None of the salinetreated mink mated. In contrast, reproductive performance of the anestrous mink was signiflcantly improved by treatment with eCG. This included proportion of mink breeding (47to 100%), proportion giving birth (33 to 80%1, and average litter size (2.6 to 4.0 kits per whelping female). Reproductive efficiency improved with increasing doses of eCG and was not further improved by addition of hCG. These results demonstrate that eCG has a potential application for treating anestrous mink during the breeding season.
Key Words: Mink, HCG, Anestrus, Reproduction, Pregnancy
J. Anim. Sci. 1992. 70:499-502
Introduction Mink are seasonally breeding animals that demonstrate estrus during the month of March in the northern hemisphere (Joergensen, 1985). These reproductive events are regulated by photoperiod (Turek and Campbell, 1979). In general, females demonstrate two to four periods of estrus at 7-d intervals. Follicular development is in synchrony with these periods of estrus (Murphy, 1987; Murphy et al., 1987; Sundquist et al., 1988). Ovulation is induced by copulation (Hannson, 1947;Enders, 1952).Almost always, implantation is delayed and the female enters another period of
'Research supported by JKJ Fur Farm, Union Grove, WI. We thank L. Stagg for technical assistance and D. Hackenberg for typing the manuscript. * o whom correspondence should be addressed: Dept. of Health Sci., Univ. of Wisconsin, P.O.Box 413, Milwaukee, 53201. Received May 28, 1991. Accepted August 29, 1991.
499
estrus with its accompanying follicular development. If the animal is bred again during this estrus, the litter comprises offspring from both matings. It has been estimated that 2 to 5% of the female breeding herd remains anestrous during the breeding season. Thus, based on USDA statistics (Anonymous, 19901,the number of anestrous mink during the breeding season approaches 25,000 in the United States alone. The economic impact of these infertile mink is obvious. Administration of gonadotropins to enhance reproductive performance is a common practice in numerous species. Its application in the management of anestrous mink is limited by a lack of research (Hammond, 1952; Adams, 1976; Hattenhauer, 1984; Murphy, 1987; Murphy et al., 1987). The present study was undertaken to evaluate the effects of exogenous equine chorionic gonadotropin (eCG) and hCG treatment on the reproductive performance of idiopathic, anestrous mink.
Downloaded from https://academic.oup.com/jas/article-abstract/70/2/499/4631970 by 04860000 user on 14 January 2019
ABSTRACT: The incidence of anestrous mink during the normal breeding season has been reported to be as high as 5%. We sought to induce estrus in these mink by using various doses of equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCGI. Seventyfive female mink maintained under standard ranch conditions failed to demonstrate estrus during the annual breeding season in March. These anestrous mink were randomly assigned to treatment groups. Treatments were given on March 16 and again on March 18. On these 2 d, mink were treated with equal doses of saline, 25,50,or 100 IU of eCG, or 50 IU of eCG on March 16 and 50 of IU of eCG + 100
WEHRENBERG ET AL.
500
Table 1. Breeding performance in mink treated with equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCG)”
Treatment 2 x
Saline
2 x 25 2 x 50 2
breedina
15
IU of eCG IU of eCG
IU of eCG +
+
@
15
gd
15
15”
15
lld
IU of hCG aData are expressed as means i SEM.
50 IU of eCG
oc
100
Breeding Datternb 0 Single
First day No. giving of breedina birth 0
OC
6.8 f .3
5d
8.7 f .2
9”
0 Double
Single Double single Double Single Double Single 8 Double 4
3 5 3 6 9 3
6.8 f .2 7.5 f .2
12”
9d”
bSingle refers to females only bred once,double refers to females bred once then rebred 8 to 24 h later. c*d*eDifTerent superscripts within a column denote significant differences (P < .OS), ANOVA, Studentized range statistic.
Experimental Procedure Animals. Seventy-five female ranch mink of the dark variety were maintained under standard ranch conditions that included natural photoperiod (Joergensen, 1985; Anonymous, 1991). All animals were 1 to 2 yr of age. Animals were fed six times per week with a wet mink feed consisting of 39% CP and 19% fat. During the breeding season, feeding was increased to seven times per week. Water was available at all times. The care and use of the animals was approved by the University of Wisconsin-Milwaukee Animal Care and Use Cornmittee. Breeding Practices. Breeding was initiated in these animals on March 3. Breeding procedures at the J K J Fur Farm consisted of moving females to the male’s cage. The females were watched closely to determine their receptivity for breeding. If the male was successful in mounting the female and intromission occurred ( c o n f i i e d visually by the posture of the animals), the animals were left together for 15 to 20 min. This was defined as “bred.”If the female was nonreceptive, the animals were separated within 5 min. This was defined as an “attempt to breed.” All males used in the study were of confirmed fertility. The day of the first breeding was termed d 0 of gestation. After attempting to breed the animals used in this study five to seven times a t approximately 3-d intervals, it was determined by the ranch management that the likelihood that these animals would breed was very low. Thus, on March 16 animals were randomly assigned to one of five treatment groups. Four groups, saline and 25,50, or 100 IU of eCG aot #27047, Intervet America, Millsboro, DE), were treated with equal doses on March 16 and March 18th. The fifth group received 50 IU of eCG on March 16 and 50 IU of eCG + 100 IU of hCG on
March 18. Injections were administered i.m. in a rear leg at a volume of .25 to .50 mL. Breeding of the animals was reinitiated on March 22. Attempts to breed mink were delayed until 6 d after the initial injection to allow follicular maturation to occur. After termination of the breeding season, all females were maintained in their home cages. During the whelping season, animals were observed daily for parturition. Both date of parturition and litter size were noted. Statistical Analysis. Significant treatment effects were detected by ANOVA for average day of breeding and the number of kits born and raised. Treatment effects on breeding performance were detected by Fisher exact-probability test.
Results Number of attempts to breed each female before hormone treatment averaged 5.6 f .1 to 6.1 f .1 among treatment groups. In no instance were any of the animals successfully bred. This failure to breed served as the criterion for the selection of animals in this study. Number of attempts to breed each mink after treatment with eCG or eCG + hCG averaged 1.6 f .I to 2.5 f .1 among treatment groups. Saline-treated females were clearly nonreceptive to the male mink and therefore attempts to breed these females ceased after two or three unsuccessful attempts. Breeding performance in response to gonadotropin treatment is reflected by the number of mink breeding and giving birth (Table 11. None of 15 anestrous mink bred after saline treatment, compared with 50 to 100% of the animals treated with eCG or eCG + hCG. Between 33 and 80% of the females assigned to the gonadotropin treatment groups gave birth. Most animals bred be-
Downloaded from https://academic.oup.com/jas/article-abstract/70/2/499/4631970 by 04860000 user on 14 January 2019
50
No.
mink 15
IU of eCG
x 100
No. of
ECG/HCG IN ANESTROUS MINK
501
Table 2. Reproductive performance in mink treated with equine chorionic gonadotropin [eCG) and human chorionic gonadotropin (hCG)" ~~
~
All
Treatment
female
Ob
Ob .87 1.33 2.67 2.40
f f f f
Whelping female
.41b .46bc .71' .65'
1.81 2.50 2.67 3.18
f f f f
*Data are expressed as means f SEM. b,cDifferent superscripts within a column denote significant differences
tween d 6 and 7 posttreatment (Table 1). In most instances (23 of 411, the bred mink were receptive to a second mating 8 to 24 h later. Table 2 presents the effects of gonadotropin treatment on the number of kits born and raised. In no instance did saline-treated mink bear offspring. In contrast, the number of kits born per bred and whelping female was significantly enhanced by treatment with gonadotropin (P < .05, Table 21. When averaged over all females, the number of kits born was reduced compared with the number of kits born per bred or whelping females because not all mink bred or bore offspring after eCG and eCG + hCG treatment. The number of kits alive at 24 h reflected neonatal mortality of less than .8 kits per litter (Table 2).
Discussion These results demonstrate clearly that exogenous gonadotropin treatment can improve reproductive performance in anestrous mink. Litter size in whelping females (2.6 to 4.0) treated with two doses of eCG or eCG + hCG were within the expected range [Murphy, 1987; Anonymous, 1991). The efficiency of eCG or eCG + hCG treatment was evident in that gonadotropin-treated mink successfully bore offspring whereas the salinetreated animals were barren. These results are consistent with our earlier observation that a single dose of 37 to 50 IU of eCG significantly enhanced breeding performance and litter size in female mink rendered anestrous by photoperiod (Wehrenberg, 1989). Similarly, Murphy et al. (1987) observed that a single injection of 100 IU of eCG to 226 anestrous mink resulted in 80% breeding and approximately 50% whelping. Litter size was approximately two kits per litter, slightly lower than litter size in our experiment. The incidence of neonatal mortality in the present study was similar to neonatal kit loss for normal mink (Murphy, 1987).
.74' .63' .71' .7OC
[P
2.60 2.86 3.33 4.00
Ob f .8lC f .59'
f .mC f 37'
Kits alive at 24 h postpartum per whelping female Ob 2.20 2.00 2.67 3.67
f 1.02c f 58'
f f
.76'
.7lC
< .051,ANOVA, Studentized range statistic.
We previously investigated the effects of a single injection of eCG (25 to 50 IU1 on reproductive performance in anestrous mink [Wehrenberg, et al., 1989). That study and other preliminary studies in which two doses of eCG at 2-d intervals were administered led to the conclusion by mink ranch management that the two-dose regimen of eCG treatment led to greater ease in breeding (unpublished observations). Based on this information, we chose to increase the number of injections of eCG from a single injection to two injections 2 d apart. Although it is not strictly appropriate to make comparisons between the present study and our former study (Wehrenberg, et al., 19891 because of the different study sites and ranch management practices, the number of "attempts to breed after a single injection of 50 IU of eCG was 3.3 f .4 and resulted in 36% of the females giving birth. In contrast, the number of "attempts to breed" after 2 x 50 IU of eCG was 2.1 k .2 and resulted in 47% of the females giving birth. The present study expands our knowledge by Eurther characterizing the dose-response relationship between exogenous gonadotropins and reproductive performance. The fact that reproductive performance among the mink treated with the 2 x 50 IU dose, the 2 x 100 IU dose, and 50 IU of eCG + 50 IU of eCG + 100 IU of hCG was similar and was better than the 2 x 25 IU dose leads us to suggest that the 2 x 50 IU dose is the lowest dose for optimal enhancement of reproductive performance. In addition, use of the 2 x 50 IU dose may be prudent because we have observed increased follicular atresia with higher doses Nehrenberg, et al., 19891. However, further investigations may be necessary because approximately 50% of the anestrous mink still did not yield offspring. Such studies should center on anatomical and physiological dysfunction of the reproductive tract itself because dysfunction of the pituitary and higher brain centers is bypassed by gonadotropin treatment. However, central nervous system involve-
Downloaded from https://academic.oup.com/jas/article-abstract/70/2/499/4631970 by 04860000 user on 14 January 2019
2 x Saline 2 x 25 IU of eCG 2 x 50 IU of eCG 2 x 100 IU of eCG 50 IU of eCG + 50 IU of eCG + 100 IU of hCG
Kits born per Bred female
502
WEHRENBERG ET
Implications On the average, nonbreeding mink represent 2 to 5% of the female breeding herd. The present study demonstrates that administration of equine chorionic gonadotropin leads to successful pregnancy in approximately 50% of the animals. The ability of the mink rancher to salvage these
animals by artificial gonadotropin treatment has direct economic benefits. These include decreased work in breeding the mink, decreased percentage of infertile mink, and increased number of offspring.
Literature Cited Adams, C. E. 1976. Current research problems in the reproduction of mink. Proc. First Int. Congr. Anim. Prod. Anonymous. 1990. Mink. Agriculture Statistics Board, National Agricultural Statistics Service, USDA, Washington, DC. Anonymous. 1991.Blue Book of Fur Farming. Communications Marketing, Inc., Eden Prairie, MN. Enders, R. K. 1952. Reproduction in the Mink (Mustela visonl. Proc. Am. Philos. SOC.96:691. Hammond. J. 1952. Gonadotropin-induced ovulation in the mink. J. Mam~~iol. 33:218. Hannson, A. 1947. The physiology of reproduction in mink (Mustela vison) with special reference to delayed implant& tion. Acta Zool. Stockh. 28:l. Hattenhauer, H., H. Pingel, and K. Eke. 1984. Use of PMSG and hCG in mink breeding. Monatsh. Veterinaermed. 39:520. Hutz, R. J., L. C. Sayles, and W. B. Wehrenberg. 1990. Effect of gonadotropin regimen on induction of follicular growth in mink. Biol. Reprod. 42 (Suppl. 1):47 (Abstr.1. Joergensen, G. 1985. Mink Production. Scient*, Hilleroed, Denmark. Murphy, B. D. 1987. Reproductive physiology of female mustelids. In: US.Seal, E. T. Thorne, M. A. Bogan, and S. H. Anderson [Ed.) Reproductive Physiology of the BlackFooted Ferret. pp 107-123. Yale Univ. Press, New Haven, CT. Murphy, B. D., D. B. Hinter, D. K. Onderka, and J. Hazelwood. 1987.Use of equine chorionic gonadotropin in female mink. Theriogenology. 28:667. Sundquist, C., L. C. Ellis, and A. Bartke. 1988. Reproductive endocrinology in the mink (Mustela vison). Endocr. Rev. 9: 247.
Turek, F. W., and C. S.Campbell. 1979.Photoperiodic regulation of neuroendocrine-gonadal activity. Biol. Reprod. 20:32. Wehrenberg, W. B., S.M. Bejvan, J. Kurhajec, and R. J. Hutz. 1989. The role of pregnant mare serum gonadotropin (PMSG) in the practical management of non-breeding female mink. Domest. Anim. Endocrinol. 6:371.
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ment cannot be ruled out completely because reproductive failure might reflect behavioral anestrus. We sought to enhance ovulation and not just follicular development in those mink treated with eCG and hCG. This was based on our histological observations that two injections of 25 IU of eCG 48 h apart or identical treatment followed by a third injection of 37 IU of eCG on d 6 failed to result in ovulation (Hutz et al., 1990). Our concern for this issue was based on the hypothesis that if anestrus is due to hypothalamic or pituitary dysfunction, then regardless of successful exogenous gonadotropin-induced follicular development, reproductive failure would persist because ovulation could not occur. Our present data suggest that hCG supplementation is not effective in improving reproductive performance. Effective application of the information obtained from this study on a commercial mink ranch will in part reflect normal breeding practices on a ranch (Le., the identification and selection of anestrous females). As evidenced by the results obtained in saline-treated mink, anestrous females were correctly identified in this study. Results from applying these procedures on ranches where close scrutiny of breeding mink is not practiced may yield different results. It is not known whether eCG treatment has any detrimental effect on pregnant mink.
AL.
