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Fertility trial in Zebu cattle after a natural or controlled estrus with prostaglandin F2 Alpha, comparing natural mating with artificial insemination
THERIOCENOLOGY FERTILITY TRIAL Iti ZEBU CATTLE AFTER A NATURAL ESTRUS WITH PROSTAGLANDIN F2 ALPHA, COMPARING WITH ARTIFICIAL INSEMINATION. Landivar,
C.,
Departments Veterinary D.F.
Received
Galina,
C.S.,
Duchateau,A.
of Reproduction and Medicine, University
for
publication: Accepted:
OR CONTROLLED NATURAL MATING
and Navarro-Fierro R1
BiostatisticsI, Faculty of Mexico, 04510 MExico,
February
20,
1984
December
14,
1984
of
ABSTRACT With the object of comparing reproductive efficiency obtained by natural mating and by artificial insemination a natural estrus but also after an (AI), not only following induced estrus with PGF2Alpha in Zebu cattle in the tropics, 244 adult cows were divided into 4 groups. Group I (N = 69) and Group III (n= 62) were injected with 25 mg of PGF2Alpha when a functional CL was found on rectal examination. Group I was inseminated and group III was served by natural mating, both groups within five days after injection.Groups II (n = 57) and IV (n = 56) were left untreated, group II being AI and group IV ran with a fertile bull for 22 days. Estrus detection was carried out only in the injected groups (I and III) for 15 minutes every three hours between 0600 and 1800. All information was analyzed by linear trigonometric models. The onset of estrus occurred on average 68.7 h after injection in group I and 59.5 h in group III. However only 46.3% and 54.8% of animals were detected in estrus in the difference being significant roup I and III respectively, Conception rates were 18.6%, 29.8%, 19.3% and 9 Pd 0.10). 33.9% for groups I, II, III, and IV respectively. A significant difference (Pd 0.10) existed between the injected groups and the untreated ones. Artificial Insemination, prostaglandins Key words: Zebu, tropics, ACKNOWLEDGMENTS The authors are grateful to financial support and J. Galina manuscript.
MARCH
1985VOL.23NO.3
Upjohn International for revision of the
for
421
THEFUOGENOLOGY INTRQDUCTION The products of cattle in the tropics (meat and milk) supply only approximately 35% of the requirements for these nutrients for the ever growing population (1). One of the reasons for this is poor reproductive efficiency (2). However, one has to remember that the bovine pupulation in the tropics (mostly Bos indicus) has been managed in areas where other chance even
types of cat(like to survive due to
Bos adverse
taurus) have conditions.
little
One continuing problem with Bos indicus cattle is the lack of information on their basicreproductive physiology. Consequently, knowledge generated using temperate breeds has been used to manage reproduction in Bos indicus cattle. This may explain the poor results obtainXfrom programs like artificial insemination at a synchronized estrus in Bos indicus where pregnancy rates seldom exceed 30% ( 3K We have reported previously that Zebu cattle have peculiarities with respect to their expression of estrus: animalsin estrus do not allow themselves to be repeatedly mounted, averaging only one mount per hour in estrus as compared to 2.8 mounts per hour by Charolais cattle (6). In addition 85% of the riding behavior is performed only by cows in estrus (7). Furthermore, when cows are kept in a their behavior is altered and corral to observe estrus, estrus is not expressed (8). it is thought that the timing From these observations, of events from estrus expression until the animal is inseminated has to be reconsidered, as Zebu cattle are reported to have shorter periods of estrus (g ,lO). Chenoweth (11) Blockey (12) and Falcon (13) evaluated serving capacity and fertility of Zebu bulls and libido, reported that their fertility and sexual interest are inferior to bulls of Bos taurus breeds. The objective of the fertility in Zebu cattle natural mating following
present study was to measure to artificial insemination or natural and PGF2*-induced estrus.
MATERIAL
& METHODS
Six hundred and sixteen lactating and non-lactating palpated and time of palpation Zebu cows were preselected, recorded on six private farms located in the state of This region is located 20"27' latitude Veracruz, Mgxico. North and 97"5' longitude West with a climate type A according to Garcia (14).
422
MARCH
1985VOL.23NO.3
THERIOCENOLOGY
A total of 429 (69.6%) of these cows were not used because 267 (43.3%) were pregnant and 162 (26.3%) had no corpus luteum. Group I (n=6!) and Group III (n=62) were injected with when a functional corpus luteum (CL) was 25 mg. of PGF2ti found on rectal examination. Group I except for ten cows (see Table 2) was artificially inseminated (AI) eight hours after being observed in estrus and those not showing estrus were bred at 80 h after PGF2& injection. Group III was served by natural mating by one bull with proven fertility per group within five days after injection. In the case of natural mating the control groupconsisted see Table 2) which ran se arately of 56 cows (17,19,20 with three different bulls for 21 days (Group IV P . In order to make up the AI control group 57 additional cows inseminated 15 days before and after prostaglandintreatment were selected (Group II). Recording of estrus detection was carried out only in the injected groups (I and III) for 15 minutes every three hours between 0600 and 1800 for five days after injection. Pregnancy was verified by rectal palpation 60 days after mating. _ Data on estrus detection and fertilitywere compared between the homosexual groups (only cows) and heterosexual (with a bul 1 present) andwere analyzed by logarithmic linear mode s as described previously (15). RESULTS Table 1 contains the estrus detection data in three of the farms. In the homosexual group of the Indobrazil animals, only 29.6% of the cows were observed in estrus, contrasting with 60% where the bull was present, the difference being it was possible to significant (P< 0.05). However,overall detect only 46.3% and 54.8% of animals in estrus in the homosexual (Group I) and heterosexual (Group III) respectively. Furthermore, by analysing the information of the two groups in a cumulative frequency test (Fig. 1) it could be seen that a suitable time for AI for the majority appears to be around 70 hours after PGFP#injection when 60% of the cows were already in estrus.
a) Tham
MARCH
Salt,
Upjohn
1985VOL.23NO.
Co. Kalamazoo
3
MI.
423
THERIOGENOLOGY
TABLE ANALYSIS
OF THE RESULTS
PGF20c'APPLICATION
OBTAINED
IN THREE
IN ESTRUS
OF THE FARMS
Homosexual FARM BREED
1
groups 2 GYR
1NDO;RAZIL
DETECTION
AFTER
STUDIED.
Heterosexual
3 GYR
groups
1NDO:RAZIL G:R
3 GYR
NUMBER OF ANIMALS IN ESTRUS
8
11
13
9
13
12
NUMBER OF ANIMALS WITHOUT ESTRUS
19
6
12
6
11
11
TOTAL
27
17
25
15
24
23
29.62 a
64.70
PERCENTAGE OF COWS IN ESTRUS AVERAGE FOR GROUPS a and c and
424
b differ d differ
52.00
54.16 52.17
54.84d
46.38'
significantly significantly
60.00 h
PL 0.05 PCO.10
MARCH
1985 VOL. 23 NO. 3
THERIOGENOLOGY
-8
-8
-%
-8
MARCH
1985 VOL. 23 NO. 3
425
THERIOGENOLOGY
Table 2 contains the fertility results. Notice that fertility was poor in all groups and, as expected, there was considerable variation among farms and bulls in particular. Performing a logarithmic test, a significant difference was found at the 10% level between the injected (19.0%) and non-injected (31.9%) groups. However,it has to be emphasized that AI in the control group was performed in cows in estrus, whereas in the treated group, most probably some animals were not in estrus at the time of insemination by appointment (see Table 2). DISCUSSION The pregnancy data agree with reports in the literature (3,5). It is evident that fertility using a bull at either a synchronized or natural estrus is low, which indicates the need to establish a suitable breedin soundness evaluation in Zebu bulls as already recommended (11 4 . In the present study fertility varied from 8.7 to 47.3%. No such variation has been found in the groups using AI. Since estimates of fertility are of critical importance in breeding programs, further work on this aspect should receive high priority, not only under Mexican conditions, but elsewhere in the tropics. Our data on estrus detection (around 50% of animals detected in estrus) confirm what has been observed by others that riding behavior is a very reliable sign of estrus this is not seen very often when observed, unfortunately as Zebu animals average only one mount per hour in estrus (6,7). Even so, of the cows that showed estrus most of them (85%) were detected by 80 hours after injection, indicating to estrus is similar that the interval from PGF2 ~injection between Zebu and Bos taurus cattle. The number of animals that could not be used for the fertility trial is disturbing; as palpation for ovarian structures is slow only 21.3 animals were palpated per hour, so sorting out pregnant from non-pregnant can be too time consuming and particularly difficult in Zebu as the corpus luteum tends to protrude less from the ovarian surface compared to European breeds (9,16). in palpation can also result in pregnant animals injected with the lytic drug with abortion resulting.
Error
being
426
MARCH
1985 VOL. 23 NO. 3
b
a
Of
difficulties
total
15
of
3
12
33.3
15
10
cows
and
this
20.8
24
19
NM
PGF2d
injected
before
forming
12.4
25
22
days
in
29.4
number
inseminated
to
the
cows
Due
17.3
17
17b
TOTAL
PERCENT PREGNANT
12
14
NUMBER OF COWS NON-PREGNANT
353552656928
NUMBER OF PREGNANT
COWS
12
FARM
AI
SERVICE
TYPE
OF
INDUCED
(NM)
8.7
23
21
3
WITH
19
14
is
33.3
18
12
3
17 were
PGF2d
AI.
20
18
NM
9
3
17
ESTRUS
from treatment.
only
47.3 10.0 47.0
19
10
12
NATURAL
NATURAL
analysed
29.3
after
30.0
20
only
days (27)
15
12
AIa
PGF2wAND
14
group,data
TABLE 2 INDUCED ESTRUS
GROUP
MATING
FOLLOWING
NATURAL
OR
AFTER
AI
RESULTS
PREGNANCY
THERIOGENOLOGY REFERENCES 1.
Chicco, C., Shultz, E. El uso de 10s recursos para la alimentaci6n de 10s bovinos. X Congreso National de Buiatria Mem., Mexico, 605 (1978).
2. Vaccaro, L. Some aspects of the performance and crossbred dairy cattle in the tropics. Anim. Breed. Abs. -41;571-591 (1973).
tropicales D.F.,
pg.
of purebred
3. Mucciolo, R.G., Barnabe, R.C., Barnabe, V.H. Utilizacao da prostaglandina F2 alfa (PGF2 alfa) na Sincronizasao do Ciclo Estral en Bovinos III. Inseminasoes Artificiais practicas em horarios Prg-Determinados, Sem Observasao de Sintomas de Cio. Rev. Fat. Med. Vet. Zootec. Univ. S. Paul0 -14:45-51 (1977). 4. Barnabe, V.H., Mucciolo, R.G., Barnabe, R.C. Utilizacao da prostaglandina F2 alfa (PGF2 alfa) na sincronizasao do ciclo estral en bovinos II. Inseminasoes artificais practicados em horarios Prg determinados, corn observasao de sintomas de cio. Rev. Fat. Med. Vet. Zootec. Univ. S. Paulo -13:367-376 (1976). 5. Hardin, D.R., Warnick, A.C., Wise, T.H., Schultz, R.H., Fields, M.J. Artificial Insemination of subtropical commercial beef cattle following synchronization with Cloprostenol (ICI 80996). I. Fertility. Theriogenology (1980). -14:249-258 M. Detection of 6. Galina, C.S., CalderBn, A., McCloskey, signs of estrus in the Charolais cow and its Brahman cross under continuous observation. Theriogenology (1982). -17:485-498 7. Orihuela, A., Galina, C., Escobar, J., Riquelme, E. Estrous behavior following prostaglandin F2@ injection in Zebu cattle under continuous observation. Theriogenology (1983). -19:795-809 S., Escobar, J., 8. Vaca, L.A., Galina, C.. Fern%ndez-Baca, levels and relationship with Ramirez, B. Progesterone the diagnosis of a corpus luteum by rectal palpation during the oestrous cycle in Zebu cows. Theriogenology -10:67-76 (1983).
428
MARCH
1985 VOL. 23 NO. 3
THERIOGENOLOGY
9. Anderson, J. The periodicity and duration of oestrus Zebu and grade cattle. Agric. Sci. 34:57-67 (1944). 10. Zakari, A.Y., Molokwu, season on the oestrous indigenous to Northern (1981).
in
E.C.I., Osorio, D.I.K. Effect of cycle of cows (Bos indicus) Nigeria. Vet. Rec. 110:213-215
11. Chenoweth, P.J. Libido and mating befiavior in bulls, boars and rams. A review. Theriogenology -16:155-177 (1981). 12. Blockey, M.A. The influence on herd fertility. J. Anim.
of serving capacity of bulls Sci. 46:589-595 (1978).
13. Falcon, C. The relationship of breeding soundness and libido evaluation to subsequent fertility in beef bulls. MSc. Thesis, University of Florida, U.S,A. (1981). 14. Garcia, E. Veracruz modificaciones al sistema de clasificaci6n clim6tica de Koppen. Universidad National Aut6noma de MGxico, Instituto de Geografia, Mexico, D.F., Cap. 15 (1973). 15. Everitt, B.S. The analysis Chapman and Hall. p. 1-128 16. Lauderdale, J.W. Effects estrous cycle in cattle.
MARCH
1985 VOL. 23 NO. 3
of contigency (1977).
tables.London:
of PGF2 alfa in pregnancy and J. Anim. Sci. 35:246 (1972).
429
C.,
Departments Veterinary D.F.
Received
Galina,
C.S.,
Duchateau,A.
of Reproduction and Medicine, University
for
publication: Accepted:
OR CONTROLLED NATURAL MATING
and Navarro-Fierro R1
BiostatisticsI, Faculty of Mexico, 04510 MExico,
February
20,
1984
December
14,
1984
of
ABSTRACT With the object of comparing reproductive efficiency obtained by natural mating and by artificial insemination a natural estrus but also after an (AI), not only following induced estrus with PGF2Alpha in Zebu cattle in the tropics, 244 adult cows were divided into 4 groups. Group I (N = 69) and Group III (n= 62) were injected with 25 mg of PGF2Alpha when a functional CL was found on rectal examination. Group I was inseminated and group III was served by natural mating, both groups within five days after injection.Groups II (n = 57) and IV (n = 56) were left untreated, group II being AI and group IV ran with a fertile bull for 22 days. Estrus detection was carried out only in the injected groups (I and III) for 15 minutes every three hours between 0600 and 1800. All information was analyzed by linear trigonometric models. The onset of estrus occurred on average 68.7 h after injection in group I and 59.5 h in group III. However only 46.3% and 54.8% of animals were detected in estrus in the difference being significant roup I and III respectively, Conception rates were 18.6%, 29.8%, 19.3% and 9 Pd 0.10). 33.9% for groups I, II, III, and IV respectively. A significant difference (Pd 0.10) existed between the injected groups and the untreated ones. Artificial Insemination, prostaglandins Key words: Zebu, tropics, ACKNOWLEDGMENTS The authors are grateful to financial support and J. Galina manuscript.
MARCH
1985VOL.23NO.3
Upjohn International for revision of the
for
421
THEFUOGENOLOGY INTRQDUCTION The products of cattle in the tropics (meat and milk) supply only approximately 35% of the requirements for these nutrients for the ever growing population (1). One of the reasons for this is poor reproductive efficiency (2). However, one has to remember that the bovine pupulation in the tropics (mostly Bos indicus) has been managed in areas where other chance even
types of cat(like to survive due to
Bos adverse
taurus) have conditions.
little
One continuing problem with Bos indicus cattle is the lack of information on their basicreproductive physiology. Consequently, knowledge generated using temperate breeds has been used to manage reproduction in Bos indicus cattle. This may explain the poor results obtainXfrom programs like artificial insemination at a synchronized estrus in Bos indicus where pregnancy rates seldom exceed 30% ( 3K We have reported previously that Zebu cattle have peculiarities with respect to their expression of estrus: animalsin estrus do not allow themselves to be repeatedly mounted, averaging only one mount per hour in estrus as compared to 2.8 mounts per hour by Charolais cattle (6). In addition 85% of the riding behavior is performed only by cows in estrus (7). Furthermore, when cows are kept in a their behavior is altered and corral to observe estrus, estrus is not expressed (8). it is thought that the timing From these observations, of events from estrus expression until the animal is inseminated has to be reconsidered, as Zebu cattle are reported to have shorter periods of estrus (g ,lO). Chenoweth (11) Blockey (12) and Falcon (13) evaluated serving capacity and fertility of Zebu bulls and libido, reported that their fertility and sexual interest are inferior to bulls of Bos taurus breeds. The objective of the fertility in Zebu cattle natural mating following
present study was to measure to artificial insemination or natural and PGF2*-induced estrus.
MATERIAL
& METHODS
Six hundred and sixteen lactating and non-lactating palpated and time of palpation Zebu cows were preselected, recorded on six private farms located in the state of This region is located 20"27' latitude Veracruz, Mgxico. North and 97"5' longitude West with a climate type A according to Garcia (14).
422
MARCH
1985VOL.23NO.3
THERIOCENOLOGY
A total of 429 (69.6%) of these cows were not used because 267 (43.3%) were pregnant and 162 (26.3%) had no corpus luteum. Group I (n=6!) and Group III (n=62) were injected with when a functional corpus luteum (CL) was 25 mg. of PGF2ti found on rectal examination. Group I except for ten cows (see Table 2) was artificially inseminated (AI) eight hours after being observed in estrus and those not showing estrus were bred at 80 h after PGF2& injection. Group III was served by natural mating by one bull with proven fertility per group within five days after injection. In the case of natural mating the control groupconsisted see Table 2) which ran se arately of 56 cows (17,19,20 with three different bulls for 21 days (Group IV P . In order to make up the AI control group 57 additional cows inseminated 15 days before and after prostaglandintreatment were selected (Group II). Recording of estrus detection was carried out only in the injected groups (I and III) for 15 minutes every three hours between 0600 and 1800 for five days after injection. Pregnancy was verified by rectal palpation 60 days after mating. _ Data on estrus detection and fertilitywere compared between the homosexual groups (only cows) and heterosexual (with a bul 1 present) andwere analyzed by logarithmic linear mode s as described previously (15). RESULTS Table 1 contains the estrus detection data in three of the farms. In the homosexual group of the Indobrazil animals, only 29.6% of the cows were observed in estrus, contrasting with 60% where the bull was present, the difference being it was possible to significant (P< 0.05). However,overall detect only 46.3% and 54.8% of animals in estrus in the homosexual (Group I) and heterosexual (Group III) respectively. Furthermore, by analysing the information of the two groups in a cumulative frequency test (Fig. 1) it could be seen that a suitable time for AI for the majority appears to be around 70 hours after PGFP#injection when 60% of the cows were already in estrus.
a) Tham
MARCH
Salt,
Upjohn
1985VOL.23NO.
Co. Kalamazoo
3
MI.
423
THERIOGENOLOGY
TABLE ANALYSIS
OF THE RESULTS
PGF20c'APPLICATION
OBTAINED
IN THREE
IN ESTRUS
OF THE FARMS
Homosexual FARM BREED
1
groups 2 GYR
1NDO;RAZIL
DETECTION
AFTER
STUDIED.
Heterosexual
3 GYR
groups
1NDO:RAZIL G:R
3 GYR
NUMBER OF ANIMALS IN ESTRUS
8
11
13
9
13
12
NUMBER OF ANIMALS WITHOUT ESTRUS
19
6
12
6
11
11
TOTAL
27
17
25
15
24
23
29.62 a
64.70
PERCENTAGE OF COWS IN ESTRUS AVERAGE FOR GROUPS a and c and
424
b differ d differ
52.00
54.16 52.17
54.84d
46.38'
significantly significantly
60.00 h
PL 0.05 PCO.10
MARCH
1985 VOL. 23 NO. 3
THERIOGENOLOGY
-8
-8
-%
-8
MARCH
1985 VOL. 23 NO. 3
425
THERIOGENOLOGY
Table 2 contains the fertility results. Notice that fertility was poor in all groups and, as expected, there was considerable variation among farms and bulls in particular. Performing a logarithmic test, a significant difference was found at the 10% level between the injected (19.0%) and non-injected (31.9%) groups. However,it has to be emphasized that AI in the control group was performed in cows in estrus, whereas in the treated group, most probably some animals were not in estrus at the time of insemination by appointment (see Table 2). DISCUSSION The pregnancy data agree with reports in the literature (3,5). It is evident that fertility using a bull at either a synchronized or natural estrus is low, which indicates the need to establish a suitable breedin soundness evaluation in Zebu bulls as already recommended (11 4 . In the present study fertility varied from 8.7 to 47.3%. No such variation has been found in the groups using AI. Since estimates of fertility are of critical importance in breeding programs, further work on this aspect should receive high priority, not only under Mexican conditions, but elsewhere in the tropics. Our data on estrus detection (around 50% of animals detected in estrus) confirm what has been observed by others that riding behavior is a very reliable sign of estrus this is not seen very often when observed, unfortunately as Zebu animals average only one mount per hour in estrus (6,7). Even so, of the cows that showed estrus most of them (85%) were detected by 80 hours after injection, indicating to estrus is similar that the interval from PGF2 ~injection between Zebu and Bos taurus cattle. The number of animals that could not be used for the fertility trial is disturbing; as palpation for ovarian structures is slow only 21.3 animals were palpated per hour, so sorting out pregnant from non-pregnant can be too time consuming and particularly difficult in Zebu as the corpus luteum tends to protrude less from the ovarian surface compared to European breeds (9,16). in palpation can also result in pregnant animals injected with the lytic drug with abortion resulting.
Error
being
426
MARCH
1985 VOL. 23 NO. 3
b
a
Of
difficulties
total
15
of
3
12
33.3
15
10
cows
and
this
20.8
24
19
NM
PGF2d
injected
before
forming
12.4
25
22
days
in
29.4
number
inseminated
to
the
cows
Due
17.3
17
17b
TOTAL
PERCENT PREGNANT
12
14
NUMBER OF COWS NON-PREGNANT
353552656928
NUMBER OF PREGNANT
COWS
12
FARM
AI
SERVICE
TYPE
OF
INDUCED
(NM)
8.7
23
21
3
WITH
19
14
is
33.3
18
12
3
17 were
PGF2d
AI.
20
18
NM
9
3
17
ESTRUS
from treatment.
only
47.3 10.0 47.0
19
10
12
NATURAL
NATURAL
analysed
29.3
after
30.0
20
only
days (27)
15
12
AIa
PGF2wAND
14
group,data
TABLE 2 INDUCED ESTRUS
GROUP
MATING
FOLLOWING
NATURAL
OR
AFTER
AI
RESULTS
PREGNANCY
THERIOGENOLOGY REFERENCES 1.
Chicco, C., Shultz, E. El uso de 10s recursos para la alimentaci6n de 10s bovinos. X Congreso National de Buiatria Mem., Mexico, 605 (1978).
2. Vaccaro, L. Some aspects of the performance and crossbred dairy cattle in the tropics. Anim. Breed. Abs. -41;571-591 (1973).
tropicales D.F.,
pg.
of purebred
3. Mucciolo, R.G., Barnabe, R.C., Barnabe, V.H. Utilizacao da prostaglandina F2 alfa (PGF2 alfa) na Sincronizasao do Ciclo Estral en Bovinos III. Inseminasoes Artificiais practicas em horarios Prg-Determinados, Sem Observasao de Sintomas de Cio. Rev. Fat. Med. Vet. Zootec. Univ. S. Paul0 -14:45-51 (1977). 4. Barnabe, V.H., Mucciolo, R.G., Barnabe, R.C. Utilizacao da prostaglandina F2 alfa (PGF2 alfa) na sincronizasao do ciclo estral en bovinos II. Inseminasoes artificais practicados em horarios Prg determinados, corn observasao de sintomas de cio. Rev. Fat. Med. Vet. Zootec. Univ. S. Paulo -13:367-376 (1976). 5. Hardin, D.R., Warnick, A.C., Wise, T.H., Schultz, R.H., Fields, M.J. Artificial Insemination of subtropical commercial beef cattle following synchronization with Cloprostenol (ICI 80996). I. Fertility. Theriogenology (1980). -14:249-258 M. Detection of 6. Galina, C.S., CalderBn, A., McCloskey, signs of estrus in the Charolais cow and its Brahman cross under continuous observation. Theriogenology (1982). -17:485-498 7. Orihuela, A., Galina, C., Escobar, J., Riquelme, E. Estrous behavior following prostaglandin F2@ injection in Zebu cattle under continuous observation. Theriogenology (1983). -19:795-809 S., Escobar, J., 8. Vaca, L.A., Galina, C.. Fern%ndez-Baca, levels and relationship with Ramirez, B. Progesterone the diagnosis of a corpus luteum by rectal palpation during the oestrous cycle in Zebu cows. Theriogenology -10:67-76 (1983).
428
MARCH
1985 VOL. 23 NO. 3
THERIOGENOLOGY
9. Anderson, J. The periodicity and duration of oestrus Zebu and grade cattle. Agric. Sci. 34:57-67 (1944). 10. Zakari, A.Y., Molokwu, season on the oestrous indigenous to Northern (1981).
in
E.C.I., Osorio, D.I.K. Effect of cycle of cows (Bos indicus) Nigeria. Vet. Rec. 110:213-215
11. Chenoweth, P.J. Libido and mating befiavior in bulls, boars and rams. A review. Theriogenology -16:155-177 (1981). 12. Blockey, M.A. The influence on herd fertility. J. Anim.
of serving capacity of bulls Sci. 46:589-595 (1978).
13. Falcon, C. The relationship of breeding soundness and libido evaluation to subsequent fertility in beef bulls. MSc. Thesis, University of Florida, U.S,A. (1981). 14. Garcia, E. Veracruz modificaciones al sistema de clasificaci6n clim6tica de Koppen. Universidad National Aut6noma de MGxico, Instituto de Geografia, Mexico, D.F., Cap. 15 (1973). 15. Everitt, B.S. The analysis Chapman and Hall. p. 1-128 16. Lauderdale, J.W. Effects estrous cycle in cattle.
MARCH
1985 VOL. 23 NO. 3
of contigency (1977).
tables.London:
of PGF2 alfa in pregnancy and J. Anim. Sci. 35:246 (1972).
429