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The effect of cloprostenol and cloprostenol + HCG on corpora lutea and serum progesterone in Brahman cows
THERIOGENOLOGY
THE EFFECT OF CLOPROSTENOL AND CLOPROSTENOL + HCG‘bN CORPORA LUTEA AND SERUM PROGESTERONE IN BRAHMAN COWS1,2,3 D. R. Hardin and R. D. Randel Texas A@1 University Agricultural Research and Extension Center, Overton 75684 Received for publication: Accepted: ABSTRACT
January 12, 1982 Plarch 25, 1982
Two independent trials were conducted to evaluate 1) the effect of cloprostenol (CLP; ICI 80996) on subsequent corpus luteum size and progesterone content and 2) the effect of CLP and CLP followed by HCG (1500 IU) at estrus on daily serum progesterone levels in Brahman cows. In Trial 1, cows were assigned as untreated controls (n=8) or to receive 500 ng CLP intramuscularly on day 8-12 postestrus (n=9). Corpora lutea were removed surgically and weighed on day 13 after the spontaneous or CLP induced estrus. CL progesterone (P4) was also monitored. In Trial 2, cows were assigned as untreated controls (n=15), to receive 500 vg CLP on day 9, 10 or 11 postestrus (n=lO), or to receive CLP as above, plus 1500 IU HCG 12 hr after the CLP-induced estrus (n=lOI. Daily blood samples were collected from all cows from day 2 postestrus through the second estrus, thus encompassing the period of CL development and regression. The data generated in Trial 1 indicated that CLP depressed CL weight (2.7 vs 4.7 mg; Pc.05) and CL P4 content (220.88 vs 367.43 vg; Pc.05) as compared to untreated controls. Serum P4 during the time period corresponding with CL development was lower (Pc.05) in CLPtreated cows in Trial 2. The most distinct reduction occurred from day 7 through 10. Postestrus treatment with 1500 IU HCG appeared to increase CL steroidogenic capacity, however, a significant (Pc.05) difference was not detected between CLP + HCG and CLP or control groups.
lJourna1 paper TA 17196 , Texas Agr. Exp. Sta. 2This study was a contribution to the Western Regional Research Project w-112, "Reproductive Performance in Domestic Ruminants". 3The authors acknowledge the generous gift of Cloprostenol and additional support by ICI, Americas, Inc., Wilmington, DE.
JUNE 1982 VOL. 17 NO. 6
669
TMERIOGENOLOGY
INTRODUCTION The potential benefits offered by control of the estrous cycle with PGF20 are numerous. The advantages of controlled breeding are the subject of several review articles (1, 2, 3, 4, 5). However, the economic feasibility of a controlled breeding program is initially dependent upon the response of the animals to PGF2a and secondly on the conception rate achieved at the synchronized breeding. The efficacy of PGF2a (6, 7) and CLP (8, 9) has been established. In fact, the hormonal changes which occur following treatment with PGFZo (10) or CLP (11) have been considered to be normal based on investigations in Bos taurus females. The response of the Brahman-crossbred (Bos indicus xos taurus) females to synchronization attempts with ??% hasn p= (12, 13) and below the results of contemporary groups of straightbred Bos taurus females (9). The endocrine signaling process for behavioral estrus and LH release differ between the Brahman (Bos indicus) and Hereford (Bos taurus females (14, 15, 16). The possibilityexists that the differencethat have been demonstrated (vide supra) may be responsible for asynchronies in hormone patterns similarto those reported by Hansel and Beal (17) which were indicated as major causes of synchrony failures. The intensive processing of Brahman females immediately prior to estrus has deleterious effects on the behavioral (13) and endocrine (Randel, unpublished data) changes associated with the periestrous period. The physiological cause-effect relationship has been presumed to be stress/adrenal oriented, but remains poorly documented. Until the mechanism for the phenomenon has been established, data collected by intensive sampling of the proestrous ______ Bos indicus female should be scrutinized closely. Conversely, the stage of the estrous cycle encompassing luteal development may be monitored by frequent blood sampling without altering the reproductive cycle of the Brahman female (Randel, unpublished data). Therefore, efforts to evaluate the factors responsible for the low response of the Bos indicus type female to estrous synchronization attempts wi-chprostaglandins have been confined to the effects on the subsequent cycle. Human Chorionic Gonadotropin (HCG) has been demonstrated to exert a luteotrophic effect in the bovine by increasing CL size and total CL P4 (18). The improvement in ovarian CL development and steroidogenic capacity has been presumed to be responsible for the improved pregnancy rates following HCG treatment at breeding (19,2(l). The possibility of using the luteotrophic activity of HCG to improve the fertility response of the Bos indicus female to synchronization with prostaglandins seemed realistic. ~ Hence, in addition to evaluating the effects of CLP on CL weight, CL P4 content and daily serum P4, the following studies were designed to determine if HCG could affect the luteal P4 production during the CLP-induced cycle.
670
JUNE
1982 VOL. 17 NO. 6
THERIOGENOLOGY
EXPERIMEWAL PROCEDURE Multiparous, nonlactating Brahman cows served as experimental animals. Estrus was confirmed at least once immediately prior to assignment of treatment groups. Estrous detection was intensified to at least 4 x daily during critical periods and continued at a minimum of 2 x daily throughout the trials. Estrous detection was conducted by visual observation aided by surgically altered bulls equipped with chin-ball marking devices. Estrus was confirmed by observation of behavioral estrus followed by the presence of CL detected by palpation E rectum. In Trial 1, cows were assigned to control (n=8) or CLP (n=9) treatments. Corpora lutea (CL) were removed on day 13 after either the second spontaneous estrus or the CLP induced estrus. CL were removed by exposure of the CL bearing ovary through a paralumbar incision. CL weights were recorded immediately after removal. CL P4 content was determined according to a procedure by Armstrong -et al. (21). In Trial 2, animals were assigned to either control (n=l5), CLP (n=lO), or CLP + HCG (n=lO) treatment groups. CLP cows received 500 ug CLP on either day 9, 10 or 11 postestrus. CLP + HCG cows received 500 ug as above followed by 1500 IU HCG 12 hr after detection of the CLP-induced estrus. Blood samples were obtained from day 2 through either the subsequent estrus or day 45 whichever occurred earlier. Blood samples were collected via tail vessel puncture, processed to yield serum and stored at -20 C. Serum P4 content was determined by the procedure of Erb -et al. (22). CL weights and total CL P4 data were analyzed by analysis of variance (23). Serum P4 data were pooled over days where no difference in P4 level was detected and pooled means analyzed by analysis of variance. Pooling was done to increase degrees of freedom for each treatment subgroup and provide a more reliable test.
RESULTS AND DISCUSSION The data collected in Trial 1 indicated that a single luteolytic dose of CLP administered at midcycle reduced (Pc.05) weight and total P4 content of the subsequently developed CL (Table I). The apparent effect on the steroidogenic capacity of the developing CL was supported by the daily serum P4 levels observed in Trial 2 (Figure l), since CL size and peripheral P4 have been shown (24) to be highly correlated et al. (25) observed a similar suppression of milk (r = .89). Jackson -P4 following CLP in one of five herds in England. The impaired development of the subsequent CL and serum P4 production were reminiscent of the lower P4 levels of infertile or nonpregnant females reported by Henricks -et al. (26) and Erb et al. (27). These data collectively would suggest that a direct suppreGi= effect of CLP on the ovary may beresponsible for the lower fertility of Brahman type females reported (9, 12).
JUNE 1982 VOL. 17 NO. 6
671
THERIOGENOLOGY The need for development of normal luteal function at the time of second PGF2ffinjection (day 8-10) is a basic premise for the theoretical response in a 10 to 12 day double injection regime. The reduced serum P4 by day 8 through 10 postestrus, as observed in Trial 2, provides sufficient justification to suspect that impaired CL formation may be one causative factor in the poor estrous response of the Brahman-type female to PGF2e synchronization (13). The postestrus treatment with 1500 IU HCG increased (Pq.05) serum P4 levels slightly above P4 levels in CLP cows. The timing and/or dose of HCG following the CLP-induced estrus may have impaired improvement of CL development to control levels of P4 production. Additional research directed toward elucidation of the causative factors of the depressed CL and serum P4 parameters seems warranted.
TABLE I. EFFECT OF CLOPROSTENOL (CLP) ON CORPUS LUTEUM WEIGHT AND PROGESTERONE CONTENT IN BRAHMAN COWS. Corpus luteum Treatment
Weight
Progesterone
CLP
2.70 + 0.42 ga
220.88 + 38.85 ngc
Control
4.71 2 0.21 gb
367.43 + 38.86 ngd
"b'c'dMeans
+ SE with different superscripts differ (Pc.05).
SERUM PROGESTERONE IN BRAHMAN COWS ANOVA . Stage
PC.05
Pe.05
PC.01
P-c.01
N.S.
N.S.
N.S.
N.S.
(2~1~1~10)(11 DAYSPOSTESTRUS
Figure 1.
672
Serum progesterone profiles in control, CLP and CLP + HCG COWS.
JUNE 1982 VOL. 17 NO. 6
THERIOGENOLOGY
REFERENCES 1.
Inskeep, E. L. Potential uses of prostaglandins in control of reproductive cycles of domestic animals. J. Anim. Sci. 36: 1149-1157. (1973)
2.
Manx,
3.
Gordon, I. Controlled breeding in cattle: Part 1. Hormones in the regulation of reproduction, oestrus control and set-time artificial insemination. Animal Breeding Abst. 44~265-275. (1976)
4.
Gordon, I. Controlled breeding in cattle: Part 2. Pregnancy testing, control of calving, reduction of the calving interval, induction of twinning breeding at younger ages, and future developments. Animal Breeding Abst. 44:451-460. (1976)
5.
Beverly, J. R. The outlook for estrous synchronization. 12th Conf. AI Beef Cattle NAAB, Denver. (1978)
6.
Louis, T. M., H. D. Hafs and B. E. Seguin. Progesterone, LH, estrus and ovulation after prostaglandin F2a in heifers. Sot. Exp. Biol. Med. 143:152-155. (1973)
J. G. and H. D. Hafs. Controlled breeding in cattle: review. Can. J. Anim. Sci. -56:121-130. (1976)
A
Proc.
Proc.
7.
Lauderdale, J. W., B. E. Seguin, J. N. Stellflug, J. R. Chenault, W. W. Thatcher, C. K. Vincent and A. F. Loyancano. Fertility of cattle following PGF2e injection. J. Anim. Sci. -38:964967. (1974)
8.
Cooper, M. J. Control of oestrus cycles of heifers with a synthetic prostaglandin analogue. Vet. Rec. -95:200-203. (1974)
9.
Schultz, R. H., D. R. Beerwinkle, C. J. Bierschwal, M. J. Fields, R. A. Godke, D. C. Kraemer, J. G. Manns, B. E. Seguin and R. J. Vatthauer. Synchronizing beef cows with cloprostenol. Proc. 69th ASAS, Madison, WI. (1977)
10.
Nancarrow, C. D., H. Hearnshaw, P. E. Mattner, P. J. Connell and B. J. Restall. Hormonal changes occurring in cattle following the administration of prostaglandin F,a. .I. Reprod. Fertil. -36:484-485. (1974)
11.
Nancarrow, C. D., H. M. Radford, P. J. Connell and P. E. Mattner. Hormonal changes occurring in cattle around normal oestrus and following administration of several luteolytic prostaglandins. J. Steroid Biochem. -5:402-403. (1974)
JUNE
1982 VOL. 17 NO. 6
673
THERIOGENOLOGY
12.
Hardin, D. R:, A. C. Warnick, T. H. Wise, R. H. Schultz and M. J. Fields. Artificial insemination of subtropical commercial beef cattle following synchronization with cloprostenol (ICI 80996): I. Fertility. Theriogenology -14:?49-257. (1980)
13.
Hardin, D. R., A. C. Warnick and 11.J. Fields. Artificial insemination of subtropical commercial beef cattle following synchronization with cloprostenol (ICI 80996): II. Estrus. Theriogenology -14:259-268. (1980)
14.
Rhodes, R. C. III, R. D. Randel. Reproductive studies of Brahman cattle I. Behavioral effect of various dose levels of estradiol178 upon ovariectomized Brahman, Brahman x Hereford and Hereford 9:429-435. (1978) cows. Theriogenology -
15.
Griffin, J. L. and R. D. Randel. Reproductive studies of Brahman cattle. II. Luteinizing hormone patterns in ovariectomized Brahman and Hereford cows before and after injection of gonadotropin releasing hormone. Theriogenology 9:437-447. (1978)
16.
Rhodes, R. C. III, R. D. Randel and P. G. Harms. Reproductive studies of Brahman cattle. IV. Luteinizing hormone levels in ovariectomized Brahman. Brahman x Hereford and Hereford cows following a 20 mg dose'of estradiol-17B. Theriogenology 10: 429-437. (1978)
17.
Hansel, W. and W. E. Beal. Ovulation control in cattle. In Animal Reproduction. H. Hawk (Ed). BARC Symposium-3, Beltsxlle, NJ. (1978)
18.
Veenhuizen, E. L., J. F. Wagner and L. V. Tonkinson. Corpus luteum response to 6-chloro-A6- 17acetoxy-progesteroneand HCG in the cow. Biol. Reprod. _6:270-276. (1972)
19.
Wagner, J. F., E. L. Veenhuizen, L. V. Tonkinson and R. P. Rathmaker. Effect of placental gonadotropin on pregnancy rate in the bovine. J. Anim. Sci. 36:1129-1136. (1973)
20.
Randel, R. D. Increasing artificial insemination conception rates in Brahman cattle by injecting 1500 IU Human Chorionic Gonadotropin at insemination. Proc. Southern Sect. Ann. Meeting Am. Sot. Anim. Sci., New Orleans, LA. (1979)
21.
Armstrong, D. T., J. O'Brien and R. 0. Greep. Effects of luteinizing hormone on progestin biosynthesis in the luteinized rat ovary. Endocrinology 75:488. (1964).
22.
Erb, R. E., E. L. Monk, T. A. Mollett, P. V. Malven and C. J. Callahan. Estrogen, progesterone, prolactin and other changes associated with bovine lactation induced with estradiol-17B and progesterone. J. Anim. Sci. 42~644-654. (1976)
674
JUNE 1982 VOL. 17 NO. 6
THERIOGENOLOGY
23.
Steel, R. G. D. and J. H. Torrie. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York. (1960)
24.
Henricks, D. M., S. L. Oxenreider and L. L. Anderson. Ovarian function after hypophyseal stalk transection in the cow. Amer. J. Physiol. 216:1213 (1969)
25.
Jackson, P. J., C. T. Johnson, D. C. Bulman and R. J. Holdsworth. A study of cloprostenol-inducedoestrus and spontaneous oestrus by means of the milk progesterone assay. British Vet. J. 135:578-590. (1979)
26.
Henricks. D. M., R. D. Roland and J. R. Hill. Progesterone secretion in the cycling and pregnant cow. Fed. Proc. Abstr. 28~771. (1969)
27.
Erb, R. E., H. A. Garverick, R. D. Randel, B. L. Brown and C. J. Callahan. Profiles of reproductive hormones associated with fertile and nonfertile inseminations of dairy COWS. Theriogenology -5~227-242. (1976)
JUNE
1982 VOL. 17 NO. 6
675
THE EFFECT OF CLOPROSTENOL AND CLOPROSTENOL + HCG‘bN CORPORA LUTEA AND SERUM PROGESTERONE IN BRAHMAN COWS1,2,3 D. R. Hardin and R. D. Randel Texas A@1 University Agricultural Research and Extension Center, Overton 75684 Received for publication: Accepted: ABSTRACT
January 12, 1982 Plarch 25, 1982
Two independent trials were conducted to evaluate 1) the effect of cloprostenol (CLP; ICI 80996) on subsequent corpus luteum size and progesterone content and 2) the effect of CLP and CLP followed by HCG (1500 IU) at estrus on daily serum progesterone levels in Brahman cows. In Trial 1, cows were assigned as untreated controls (n=8) or to receive 500 ng CLP intramuscularly on day 8-12 postestrus (n=9). Corpora lutea were removed surgically and weighed on day 13 after the spontaneous or CLP induced estrus. CL progesterone (P4) was also monitored. In Trial 2, cows were assigned as untreated controls (n=15), to receive 500 vg CLP on day 9, 10 or 11 postestrus (n=lO), or to receive CLP as above, plus 1500 IU HCG 12 hr after the CLP-induced estrus (n=lOI. Daily blood samples were collected from all cows from day 2 postestrus through the second estrus, thus encompassing the period of CL development and regression. The data generated in Trial 1 indicated that CLP depressed CL weight (2.7 vs 4.7 mg; Pc.05) and CL P4 content (220.88 vs 367.43 vg; Pc.05) as compared to untreated controls. Serum P4 during the time period corresponding with CL development was lower (Pc.05) in CLPtreated cows in Trial 2. The most distinct reduction occurred from day 7 through 10. Postestrus treatment with 1500 IU HCG appeared to increase CL steroidogenic capacity, however, a significant (Pc.05) difference was not detected between CLP + HCG and CLP or control groups.
lJourna1 paper TA 17196 , Texas Agr. Exp. Sta. 2This study was a contribution to the Western Regional Research Project w-112, "Reproductive Performance in Domestic Ruminants". 3The authors acknowledge the generous gift of Cloprostenol and additional support by ICI, Americas, Inc., Wilmington, DE.
JUNE 1982 VOL. 17 NO. 6
669
TMERIOGENOLOGY
INTRODUCTION The potential benefits offered by control of the estrous cycle with PGF20 are numerous. The advantages of controlled breeding are the subject of several review articles (1, 2, 3, 4, 5). However, the economic feasibility of a controlled breeding program is initially dependent upon the response of the animals to PGF2a and secondly on the conception rate achieved at the synchronized breeding. The efficacy of PGF2a (6, 7) and CLP (8, 9) has been established. In fact, the hormonal changes which occur following treatment with PGFZo (10) or CLP (11) have been considered to be normal based on investigations in Bos taurus females. The response of the Brahman-crossbred (Bos indicus xos taurus) females to synchronization attempts with ??% hasn p= (12, 13) and below the results of contemporary groups of straightbred Bos taurus females (9). The endocrine signaling process for behavioral estrus and LH release differ between the Brahman (Bos indicus) and Hereford (Bos taurus females (14, 15, 16). The possibilityexists that the differencethat have been demonstrated (vide supra) may be responsible for asynchronies in hormone patterns similarto those reported by Hansel and Beal (17) which were indicated as major causes of synchrony failures. The intensive processing of Brahman females immediately prior to estrus has deleterious effects on the behavioral (13) and endocrine (Randel, unpublished data) changes associated with the periestrous period. The physiological cause-effect relationship has been presumed to be stress/adrenal oriented, but remains poorly documented. Until the mechanism for the phenomenon has been established, data collected by intensive sampling of the proestrous ______ Bos indicus female should be scrutinized closely. Conversely, the stage of the estrous cycle encompassing luteal development may be monitored by frequent blood sampling without altering the reproductive cycle of the Brahman female (Randel, unpublished data). Therefore, efforts to evaluate the factors responsible for the low response of the Bos indicus type female to estrous synchronization attempts wi-chprostaglandins have been confined to the effects on the subsequent cycle. Human Chorionic Gonadotropin (HCG) has been demonstrated to exert a luteotrophic effect in the bovine by increasing CL size and total CL P4 (18). The improvement in ovarian CL development and steroidogenic capacity has been presumed to be responsible for the improved pregnancy rates following HCG treatment at breeding (19,2(l). The possibility of using the luteotrophic activity of HCG to improve the fertility response of the Bos indicus female to synchronization with prostaglandins seemed realistic. ~ Hence, in addition to evaluating the effects of CLP on CL weight, CL P4 content and daily serum P4, the following studies were designed to determine if HCG could affect the luteal P4 production during the CLP-induced cycle.
670
JUNE
1982 VOL. 17 NO. 6
THERIOGENOLOGY
EXPERIMEWAL PROCEDURE Multiparous, nonlactating Brahman cows served as experimental animals. Estrus was confirmed at least once immediately prior to assignment of treatment groups. Estrous detection was intensified to at least 4 x daily during critical periods and continued at a minimum of 2 x daily throughout the trials. Estrous detection was conducted by visual observation aided by surgically altered bulls equipped with chin-ball marking devices. Estrus was confirmed by observation of behavioral estrus followed by the presence of CL detected by palpation E rectum. In Trial 1, cows were assigned to control (n=8) or CLP (n=9) treatments. Corpora lutea (CL) were removed on day 13 after either the second spontaneous estrus or the CLP induced estrus. CL were removed by exposure of the CL bearing ovary through a paralumbar incision. CL weights were recorded immediately after removal. CL P4 content was determined according to a procedure by Armstrong -et al. (21). In Trial 2, animals were assigned to either control (n=l5), CLP (n=lO), or CLP + HCG (n=lO) treatment groups. CLP cows received 500 ug CLP on either day 9, 10 or 11 postestrus. CLP + HCG cows received 500 ug as above followed by 1500 IU HCG 12 hr after detection of the CLP-induced estrus. Blood samples were obtained from day 2 through either the subsequent estrus or day 45 whichever occurred earlier. Blood samples were collected via tail vessel puncture, processed to yield serum and stored at -20 C. Serum P4 content was determined by the procedure of Erb -et al. (22). CL weights and total CL P4 data were analyzed by analysis of variance (23). Serum P4 data were pooled over days where no difference in P4 level was detected and pooled means analyzed by analysis of variance. Pooling was done to increase degrees of freedom for each treatment subgroup and provide a more reliable test.
RESULTS AND DISCUSSION The data collected in Trial 1 indicated that a single luteolytic dose of CLP administered at midcycle reduced (Pc.05) weight and total P4 content of the subsequently developed CL (Table I). The apparent effect on the steroidogenic capacity of the developing CL was supported by the daily serum P4 levels observed in Trial 2 (Figure l), since CL size and peripheral P4 have been shown (24) to be highly correlated et al. (25) observed a similar suppression of milk (r = .89). Jackson -P4 following CLP in one of five herds in England. The impaired development of the subsequent CL and serum P4 production were reminiscent of the lower P4 levels of infertile or nonpregnant females reported by Henricks -et al. (26) and Erb et al. (27). These data collectively would suggest that a direct suppreGi= effect of CLP on the ovary may beresponsible for the lower fertility of Brahman type females reported (9, 12).
JUNE 1982 VOL. 17 NO. 6
671
THERIOGENOLOGY The need for development of normal luteal function at the time of second PGF2ffinjection (day 8-10) is a basic premise for the theoretical response in a 10 to 12 day double injection regime. The reduced serum P4 by day 8 through 10 postestrus, as observed in Trial 2, provides sufficient justification to suspect that impaired CL formation may be one causative factor in the poor estrous response of the Brahman-type female to PGF2e synchronization (13). The postestrus treatment with 1500 IU HCG increased (Pq.05) serum P4 levels slightly above P4 levels in CLP cows. The timing and/or dose of HCG following the CLP-induced estrus may have impaired improvement of CL development to control levels of P4 production. Additional research directed toward elucidation of the causative factors of the depressed CL and serum P4 parameters seems warranted.
TABLE I. EFFECT OF CLOPROSTENOL (CLP) ON CORPUS LUTEUM WEIGHT AND PROGESTERONE CONTENT IN BRAHMAN COWS. Corpus luteum Treatment
Weight
Progesterone
CLP
2.70 + 0.42 ga
220.88 + 38.85 ngc
Control
4.71 2 0.21 gb
367.43 + 38.86 ngd
"b'c'dMeans
+ SE with different superscripts differ (Pc.05).
SERUM PROGESTERONE IN BRAHMAN COWS ANOVA . Stage
PC.05
Pe.05
PC.01
P-c.01
N.S.
N.S.
N.S.
N.S.
(2~1~1~10)(11 DAYSPOSTESTRUS
Figure 1.
672
Serum progesterone profiles in control, CLP and CLP + HCG COWS.
JUNE 1982 VOL. 17 NO. 6
THERIOGENOLOGY
REFERENCES 1.
Inskeep, E. L. Potential uses of prostaglandins in control of reproductive cycles of domestic animals. J. Anim. Sci. 36: 1149-1157. (1973)
2.
Manx,
3.
Gordon, I. Controlled breeding in cattle: Part 1. Hormones in the regulation of reproduction, oestrus control and set-time artificial insemination. Animal Breeding Abst. 44~265-275. (1976)
4.
Gordon, I. Controlled breeding in cattle: Part 2. Pregnancy testing, control of calving, reduction of the calving interval, induction of twinning breeding at younger ages, and future developments. Animal Breeding Abst. 44:451-460. (1976)
5.
Beverly, J. R. The outlook for estrous synchronization. 12th Conf. AI Beef Cattle NAAB, Denver. (1978)
6.
Louis, T. M., H. D. Hafs and B. E. Seguin. Progesterone, LH, estrus and ovulation after prostaglandin F2a in heifers. Sot. Exp. Biol. Med. 143:152-155. (1973)
J. G. and H. D. Hafs. Controlled breeding in cattle: review. Can. J. Anim. Sci. -56:121-130. (1976)
A
Proc.
Proc.
7.
Lauderdale, J. W., B. E. Seguin, J. N. Stellflug, J. R. Chenault, W. W. Thatcher, C. K. Vincent and A. F. Loyancano. Fertility of cattle following PGF2e injection. J. Anim. Sci. -38:964967. (1974)
8.
Cooper, M. J. Control of oestrus cycles of heifers with a synthetic prostaglandin analogue. Vet. Rec. -95:200-203. (1974)
9.
Schultz, R. H., D. R. Beerwinkle, C. J. Bierschwal, M. J. Fields, R. A. Godke, D. C. Kraemer, J. G. Manns, B. E. Seguin and R. J. Vatthauer. Synchronizing beef cows with cloprostenol. Proc. 69th ASAS, Madison, WI. (1977)
10.
Nancarrow, C. D., H. Hearnshaw, P. E. Mattner, P. J. Connell and B. J. Restall. Hormonal changes occurring in cattle following the administration of prostaglandin F,a. .I. Reprod. Fertil. -36:484-485. (1974)
11.
Nancarrow, C. D., H. M. Radford, P. J. Connell and P. E. Mattner. Hormonal changes occurring in cattle around normal oestrus and following administration of several luteolytic prostaglandins. J. Steroid Biochem. -5:402-403. (1974)
JUNE
1982 VOL. 17 NO. 6
673
THERIOGENOLOGY
12.
Hardin, D. R:, A. C. Warnick, T. H. Wise, R. H. Schultz and M. J. Fields. Artificial insemination of subtropical commercial beef cattle following synchronization with cloprostenol (ICI 80996): I. Fertility. Theriogenology -14:?49-257. (1980)
13.
Hardin, D. R., A. C. Warnick and 11.J. Fields. Artificial insemination of subtropical commercial beef cattle following synchronization with cloprostenol (ICI 80996): II. Estrus. Theriogenology -14:259-268. (1980)
14.
Rhodes, R. C. III, R. D. Randel. Reproductive studies of Brahman cattle I. Behavioral effect of various dose levels of estradiol178 upon ovariectomized Brahman, Brahman x Hereford and Hereford 9:429-435. (1978) cows. Theriogenology -
15.
Griffin, J. L. and R. D. Randel. Reproductive studies of Brahman cattle. II. Luteinizing hormone patterns in ovariectomized Brahman and Hereford cows before and after injection of gonadotropin releasing hormone. Theriogenology 9:437-447. (1978)
16.
Rhodes, R. C. III, R. D. Randel and P. G. Harms. Reproductive studies of Brahman cattle. IV. Luteinizing hormone levels in ovariectomized Brahman. Brahman x Hereford and Hereford cows following a 20 mg dose'of estradiol-17B. Theriogenology 10: 429-437. (1978)
17.
Hansel, W. and W. E. Beal. Ovulation control in cattle. In Animal Reproduction. H. Hawk (Ed). BARC Symposium-3, Beltsxlle, NJ. (1978)
18.
Veenhuizen, E. L., J. F. Wagner and L. V. Tonkinson. Corpus luteum response to 6-chloro-A6- 17acetoxy-progesteroneand HCG in the cow. Biol. Reprod. _6:270-276. (1972)
19.
Wagner, J. F., E. L. Veenhuizen, L. V. Tonkinson and R. P. Rathmaker. Effect of placental gonadotropin on pregnancy rate in the bovine. J. Anim. Sci. 36:1129-1136. (1973)
20.
Randel, R. D. Increasing artificial insemination conception rates in Brahman cattle by injecting 1500 IU Human Chorionic Gonadotropin at insemination. Proc. Southern Sect. Ann. Meeting Am. Sot. Anim. Sci., New Orleans, LA. (1979)
21.
Armstrong, D. T., J. O'Brien and R. 0. Greep. Effects of luteinizing hormone on progestin biosynthesis in the luteinized rat ovary. Endocrinology 75:488. (1964).
22.
Erb, R. E., E. L. Monk, T. A. Mollett, P. V. Malven and C. J. Callahan. Estrogen, progesterone, prolactin and other changes associated with bovine lactation induced with estradiol-17B and progesterone. J. Anim. Sci. 42~644-654. (1976)
674
JUNE 1982 VOL. 17 NO. 6
THERIOGENOLOGY
23.
Steel, R. G. D. and J. H. Torrie. Principles and Procedures of Statistics. McGraw-Hill Book Co., New York. (1960)
24.
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