SEQUENTIAL HORMONAL PROFILES THROUGHOUT GESTATION PERIOD AND RESUMPTION OF OVARIAN ACTIVITY IN POSTPARTUM DAIRY COW IN UPPER EGYPT

Megahed, G.A.* and Daghash, H.A.**

Dept. of Theriogenology, Faculty of Vet. Med.* and Dept. of Animal Production,

Faculty of Agriculture**, Assiut University, Egypt.

Summary

This study included the levels of progesterone (P₄), estradiol 17b (E₂) and prostaglandin F₂a (PGF₂a) in dairy cows during pregnancy till parturition. Also, the relationship between resumption of ovarian activity with subsequent fertility and levels of these hormones were evaluated.  Twenty-four pregnant animals were used in this study.  Blood samples were collected from each pregnant cow monthly throughout gestation till parturition. Eight cows from normally calved and normal postparturient animals were chosen. The blood samples were obtained from these animals every 4 days. The animals were observed twice daily for behavioural estrus. Cows were inseminated naturally at eat as well as observed again for return of heat, then examined at 45-60 days for pregnancy. The results suggest that gestation period had a significant (P< 0.01) effect upon P₄, E₂ and PGF₂a.  Out of 24 calved animals, 5 showed the ovarian activity between 41-57 days postpartum as well as, and 15 animals showed a prolonging interval from calving to conception. However, 4 cows became anoestrum and had not any hormonal indicator to ovarian activity. Four cows out of six have a fertile estrus at the first heat which was exhibited between 45-57 days postpartum as well as the hormonal values that ranged between 0.15-0.16 ng/ml (P₄), 32.18-36.72 pg/ml (E₂) 137.4-198.7 pg/ml (PGF₂a). According to this study, it is suggested that the synchronization of animals may be very important under our environmental conditions to avoid bad atmospheric conditions with high temperatures, which can effect post partum resumption of ovarian activity.

Introduction

          Ruminants play an important role in world agriculture. As the population increases, the food required to feed a hungry world also increases greatly.  The dairy industry is one of the most important agriculture industries in the world as well as milk production can play an important role (Foote, 1996).  Reproduction performance of cows affects the efficiency of milk production in the herds because of its influence on the calving to service interval which is affected by efficiency of resumption of ovarian activity postpartum (Roche, et al., 1992).

          Early reestablishment of cyclic ovarian activity and post parturient recovery of the uterus are essential in order to maintain an acceptable calving to conception interval and therefore, economic efficiency (Beckett and Lean, 1997).  However, dairy cows are exposed to considerable stress factors during early postpartum period, which had deleterious effects on female reproductive efficiency (Torres, et al., 1997).  These factors are: onset of lactation, negative energy balance (Granfield and Bulter, 1991), loss of body condition (Heinonen, et al., 1988) and periparturient disorders (Nakao, et al., 1992).

          Prostaglandin F_2a is released from the cotyledon in cows around parturition and plays an important role in postpartum uterine involution (White and Dobson, 1990 and Kindahl, et al., 1992). The increasing of PGF_2a metabolite (PGFM) is negatively correlated with the time for uterine involution in cows (Kindahl, et al., 1992).  Postpartum PGF_2a is a by-product of uterine involution, which in turn activates the hypothalamic-pituitary hormones, and subsequently initiate ovarian activity (Savio, et al., 1990).  There is no available literature upon this point on dairy animals reared in upper Egypt, meanwhile, this paper reports a study to investigate the levels of P₄, E₂ and PGF_2a during pregnancy till parturition.  This study aimed to also evaluate the relationship between resumption of ovarian activity in postpartum cows with subsequent fertility and levels of the hormones previously mentioned.

Material and Methods

Cows and their management:

          Twenty-four pregnant Friesian cows were used in this study.  The age of the animals ranged from 4-6 years and their parity was 2-3.  They were housed in Assiut village under the same feeding and management conditions.  Moreover, the animals were given a free access to drinking water. They calved in the period between March and May.  Following parturition, the cows were allowed for suckling.  Calves were removed about 72 hours after birth. Milking was carried twice daily. Eight cows from normally calved animals were chosen. These animals had a normal puerperium (without retained placenta and endometritis).  The animals were observed twice daily for estrus.  The annual average values of air temperature and relative humidity were 39.7^oC and 69.5%, respectively.  All cows were inseminated naturally at the first detectable estrus after calving as well as observed again for return heat them examined 45-60 days after insemination for pregnancy.  The animals, which returned again to heat, were reinseminated.

Blood collection:

          Blood samples were collected from each pregnant animal monthly throughout gestation till parturition via jugular venipuncture.  Furthermore blood samples were obtained from chosen parturient animals (n = 8) every 4 days.  The time of collection of all samples was subsequently standardized from the time of parturition taken as day 0. Blood samples were allowed to clot for 30-60 min. at room temperature (20-25^oC) and centrifuged at 3000 rpm for 20 min. Sera were pipetted into glass vials and stored at –20^oC until assay for hormones.

Hormonal assay:

          Progesterone (P₄) and estradiol-17b (E₂) concentrations in blood serum were measured by commercial ELISA kit (BIOSURCE, EUROPS S.A) and ELISA automatic reader.

Prostaglandin F_2a concentrations in blood serum were measured by an ELISA kits cod. No. DE 1100 (R7D system, Inc. USA).

Statistical analysis:

          Data were expressed as the mean + SD for all the hormonal concentration during gestation period, then analyzed by using analysis of variance and means + SD were tested at least significant difference (LSD). All tests were done by using SAS, (1987) computer program. Results were considered significant at P< 0.05 or less.

Results

The mean concentrations (+ S.E) of serum P₄, E₂ and PGF_2a throughout gestation period are shown in Table 1 and Fig. 1. Gestation period had a significant (P< 0.01) effect on P₄, E₂, and PGF_2a. During first and second trimesters, serum concentrations of P₄ increased gradually until the 7^th month of gestation but this elevation is not significant.  At the third trimester, P₄ level decreased significantly (P< 0.01). The decline of this hormone during the 7^th month of gestation (Fig. 1A) was significant but at the 8^th and the 9^th was not significantly.

          Concentration of E₂ started to increase slightly at 5^th month of gestation (Fig. 1B) but this elevation was not significant. After this, it was followed by an abrupt increase to 20.36 + 0.44 pg/ml (P< 0.01) and continued to increase steeply and significantly till the 9^th month of gestation to 24.0 + 0.78 pg/ml (P< 0.01).

          The level of serum PGF_2a ranged between 218.7+7.1 to 352.9+9.9 pg/ml during gestation period (Fig. 1C). Its concentration increased progressively to reach a peak value (352.9+9.9 pg/ml) at the 9^th month.  There was a slight non-significant increase in mean serum PGF_2a concentrations during the first 3 months, then decreased to 220+12.4 pg/ml at 4^th month.  During the remaining months of gestation, the concentrations of PGF_2a rose non-significantly different as well as increased significantly (P< 0.01) at the last month of pregnancy.

          Table 2 illustrates the interval from calving to conception in the experimental animals. There are eight animals that conceived after 90-120 days postpartum, as well as, seven cows exhibited heat then conceived after 65-70 days postpartum. However, 4 animals had not any rectal finding or external signs of heat.

          The return of ovarian activity during postpartum in chosen cows is illustrated in Table 3 and Figures (2-5). Six out of eight cows showed ovarian activity between 41-57 days postpartum. However, two cows did not show any external signs of heat.  Furthermore, the ovaries of these cows had no palpable structures which is characteristic of inactive ovaries.  These two animals had not any hormonal indicator to ovarian activity (Fig. 5).  Four animals out of six have a fertile estrus at the first heat which was exhibited between 45-57 days postpartum, as well as the hormonal concentrations were ranged between 0.15-0.16 ng/ml (P₄), 32.18-36.72 pg/ml (E₂) and 154.8 -198.7 pg/ml in case of PGF_2a.

          Cow No. 87, showed the ovarian activity at 41 days postpartum.  Unfortunately, it returned to heat again at 65 day of postpartum (Fig. 4). After mating, the cow did not return to heat and became pregnant. There is a variation in the concentrations of P₄ during fertile heat (0.15 ng/ml) than that at infertile heat (0.18 ng/ml).  Moreover, E₂ concentrations rise during fertile heat (33.71 pg/ml) than that at infertile heat (30.62 pg/ml), as well as decreasing value (165.5 pg/ml) of PGF_2a during fertile heat than infertile heat (178.8 pg/ml). Cow No. 998 failed to conceive in the first and second post-partum heat inspite of exhibiting the ovarian activity via hormonal profile and external signs of heat at 49 and 80 postpartum (Fig. 4). The third heat (came on day 101 post-partum) was a fertile one.  The lowest values of P₄ and PGF_2a (0.15 ng/ml and 137.4 pg/ml), respectively observed at fertile heat but the highest value of E₂ (35.65 pg/ml) estimated during fertile heat than during the other two heat (30.13 and 32.75 pg/ml).

Discussion

1- Throughout gestation period:

          The present results indicated that serum P₄ concentrations increased significantly during the first and the second trimester, then decreased significantly (P< 0.01) at 7^th month of gestation.  These findings are in agreement with that reported by Gordon (199a).  However, Eissa et al. (1995) and Eissa and El-Belely (1990) concluded that the plasma concentrations of P₄ increased until the third month of gestation, then decreased significantly at the fourth month.

          The higher values of P₄ during the first trimester of gestation might be associated with the formation of accessory luteal tissue arising from an ovulation which frequently occurs within the gestation period (Robertson, 1972 and Essia and El-Belely, 1990).  The obtained results were in contrast to those previously reported in the pregnant buffalo cows (Virakul, 1987).  They did not find any differences in plasma P₄ concentrations during the months of gestation.

          The finding that levels of estradiol-17b increased steeply from the 4^th to 6^th month of gestation within a narrow range was in agreement with the finding of Eissa and El-Belely (1990).  Patel et al. (1995) reported that estradiol-17b concentrations throughout gestation were related to the stage of gestation.  The present findings agreed well with Harada (1980) and Eissa et al. (1995) that serum estradiol-17b concentration showed a steeply elevation till maximal concentration at the last month of gestation. The gradual increase in estradiol-17b throughout gestation to peak at last month of pregnancy is characteristic of calving cows. This progressive increase, especially from 4^th month of gestation, probably reflects the maturation of placenta (Dobson, et al., 1993 and Patel, et al., 1995).

          Furthermore, the higher serum values of estradiol-17b during the last 30 days of pregnancy might be a result of the increased corticosteroid level at this time which stimulate the secretion of placental estradiol-17b (Pimental et al., 1986; Hung and Prakash, 1990 and Eissa, et al., 1995).  Moreover, our interpretation for the placental estradiol was supported by the formation of follicular waves which occurs throughout pregnancy except during the last 30 days before parturition and the presence of FSH surges without an apparent follicular response during this time (Dominnguez, 1995 and Ginther, et al., 1996).

          The serum PGF_2a concentrations showed trifle changes during the first 4 months of gestation, then markedly increased until the last 30 days of pregnancy, to reach peak levels. The low level of PGF_2a during pregnancy might be attributed to the inhibitory capacity in the endometrium, which is seen in both cytoplasm and microsomes of endometrial tissue.  The increased level of this hormone at the last 30 days of pregnancy could be attributed to the elevation of corticosteroid hormone at this time (Eissa and El-Beley, 1990) which result in increased release of PGF_2a by the endometrium (Pimental, et al., 1986). Furthermore, the increase of corticosteroid level stimulates the increase secretion of placental estradiol (Wendorf, et a l., 1983), which is responsible for development of endometrial receptors of oxytocin as well as interacts with receptors in the uterus to stimulate secretion of PGF_2a (Bazer, et al., 1991). However, White an Dobson (1990) reported that near the parturition a massive amount of PGF_2a is released from the cotyledons in cows.

2- Postpartum period:

          From the economic point of view, early re-establishment of cyclic ovarian activity after calving is essential in order to maintain an acceptable calving to conception interval.  In the present study, six of eight cows showed behavioral estrus after 41-57 days postpartum. This finding agree with earlier findings showing that the interval from calving to first behavioral estrus was 43+10 days (Mukasa – Mugerwa, et al., 1991), 42 days (Mcleod and Williams, 1991) or 69-77 days (Shipka et al., 1998).  The obtained results are somewhat in agreement with that reported by Narasimha et al. (1995) who concluded that the first estrus in cows was manifested between 31 and 60 days of calving.  Other studies recorded the postpartum ovarian activity and estrus behavioral at 30 days longer in the rainy season than in the dry one (Dawuda, et al., 1988). Moreover, in the tropical area, the cows resumed ovarian activity on day 60-160 postpartum (Bolanos, et al., 1996).

          In this study, the first postpartum behavioral estrus was found with significantly lower P₄ concentration.  A similar finding has been previously reported by Darwash and Lamming (1995). They reported that P₄ level was fluctuating and gradual rise took place prior to the first postpartum estrus.  The source of this P₄ has not been elucidated. Since, there are apparently no functional corpora lutea present, it is convential that either follicular luteinization or adrenal secretion may be responsible for the pre-estrual progesterone secretion (Souza et al., 1997). However, Meredith (1995) concluded that the resultant corpora lutea formed after the first ovulation often produce progesterone.  Furthermore, he added that, this diminished P₄ is often sufficient to result in better functioning of the hypothalamus pituitary axis, as well as perhaps improved intra ovarian communication to provide ideal conditions during the next wave of follicular growth that result in normal luteal life-span.

          Estradiol-17b serum concentrations obtained in the present study were similar to those recorded by Dimmick et al. (1991) who found low E₂ concentrations during the postpartum period and its increasing can be associated with follicular growth.  The obtained results which show gradual rise of E₂ till maximum level during fertile heat were attributed to the development, which involved growth and regression of the follicles less than 8 mm in diameter until detection of dominant follicle (Savio, et al. 1990 and Roche, et al., 1992). The high or peak level of E₂ has a positive feedback action on gonadotrophin surge release resulting in ovulation and subsequent fertilization (Gyawu and Pope, 1990).  Another interpretation to the obtained results as events leading to resumed ovarian activity after calving included the following: (i) some GnRH is secreted immediately after calving but not is sufficient amounts to cause gonadotrophin release; (ii) plasma FSH concentrations rise rapidly after parturition, stimulating follicular development; (iii) there is a gradual increase in the frequency of LH pulses and in plasma LH concentration, (iv) gonadotrophin secretion stimulates follicular growth and the production of estradiol and perhaps inhibin, and (v) concurrent with such endocrine changes, there is a gradual recovery of the positive feedback mechanism so that ovarian cycles commence after parturition (Gordon, 1996 b).

          After parturition, in the studied animals, high levels of the PG_2a were estimated and returned relatively slow to reach a low concentration during heat. This finding is somewhat similar to those of Kindahl et al. (1992). PGF_2a has an important role during the bovine postpartum period. A massive PGF_2a release is negatively correlated to uterine involution in normal calving as well as can stimulate onset of ovarian cyclicity (Kindahl et al., 1992).  This means that animals with more rapid uterine involution also normalize ovarian activity earlier.  The source of PGF_2a production during the postpartum period is the uterus and a luteolytic release of this hormone occurs terminating the life-span of the corpus luteum during the short cycle postpartum (Peter et al., 1989).

In this study, the number of cows, which have a prolonged postpartum anoestrus period (65-120 days) was 15 out of 24.  This is in agreement with McDougall et al. (1998).  These long periods of postpartum anoestrus are associated with a low LH pulse frequency and a low intrafollicular E₂ concentration (McDougall and Macmillan, 1994 and Prado, et al., 1990). It is hypothesized that low LH pulse frequency results in reduced E₂ synthesis so that a positive feedback loop does not occur (McDougall and Macmillan, 1994). Additionally, there is evidence of enhanced negative feedback of E­₂ on the hypothalamus resulting in suppression of GnRH release (McDougall, et al., 1998).

          Moreover, among the animals in this study, four animals were in true anoestrus and the concentrations of P₄, E_­2 and PGF_2a in serum were very low compared with the active animals. This results are similar to those reported by Souza et al. (1997).  The obtained results, which show postpartum true anoestrus are attributed to failure of ovulation of the majority of first dominant follicle, rather than a delay in development of dominant follicle (Roche, et al., 1992).  Moreover, the GnRH content of the hypothalamus is normal but the anterior pituitary is less sensitive to GnRH-induced LH release in the early postpartum period as well as FSH concentrations are low for a short period after calving (Schallenberger, 1985). In addition, an interaction of chronic undernutrition and bad atmosphere lead to inhibits GnRH secretion with consequent effects on pulsatile LH release and follicular development (Fitzpatrik, 1994).  In adequent LH pulse frequency results in low production of androgens in the follicle with less estrogen biosynthesis activity (Prado, et al., 1990) and thus the post-estrus estrogen rise does not occur and the dominant follicle which is the final stages terminal differentiation, undergoes atresia (Roche, et al., 1992). Furthermore, the obtained results, which shows prolonging interval from calving to conception might be attributed to the time of year when calving occurred as well as the exposure to a high environmental temperature had very bad effect upon post partum ovarian activity (Gordon, 1996 a)

          In conclusion, the present investigation revealed particularly in dairy cows that, gestation period had a significant effect upon progesterone, estradiol-1-17b and PGF_2a increased significantly at the last month of pregnancy. Moreover, the fertile estrus occurred between 45 – 57 days postpartum. During fertile heat, there is a significant decrease in progesterone and PGF_2a as well as increasing in estradio1-17b. The high environmental temperature had a bad effect upon the animals after parturition, which leads to prolonging the interval from calving to conception and prolonged anoestrum. According to this study, it is suggested that extrafeeding in late gestation period for 6-8 weeks may reduce the post partum interval as well as the synchronization of animals may be very important under our environmental condition to avoid calving during a summer season.

REFERANCES

Bazer, F.W.; Thatcher, W.W.; Hansen, P.J.; Mirando, M.A.; Ott, T.L. and Plate, C. (1991): mechanisms of pregnancy in ruminants. J. Reprod. Fertil.(Suppl.43): 39-47.

Beckett, S.D. and Lean, I.J. (1997): Gonadotrophin-releasing hormone in postpartum dairy cattle: a meta-analysis of effects on reproductive efficiency. Anim. Reprod. Sci., 48:93-112.

Bolanos, J.M.; Meneses, A. and Forsberg, M. (1996): Resumption of ovarian activity in zebu cows (Bos indicus) in the humid tropics: Influence of condition and levels of certain blood components related to nutrition. Trop. Anim. Hlth. Prod., 28:237-246.

Darwash, A.O. and Lamming, G.E. (1995): To define and quantify atypical ovarian function in untreated postpartum cows. Biol. Reprod., 52 (Suppl. 1), 72-76.

Dawuda, P.M.; Esievo, K.A. and Molokwu, E.C. (1988): Interval between calving and first observable oestrus in postpartum period of indigenous Nigerian Zebu cows. Br. Vet. J. 144:253-257.

Dimmick, M.A.; Gimenez, T. and Spitzer, J.C. (1991): Ovarian endocrine activity and development of ovarian follicles during the post partum interval in beef cows. Anim. Reprod. Sci., 173-183.

Dobson, H.; Rowan, T.G.; Kippax, S. and Humblot, P. (1993): Assessment of fetal number and placental viability throughout pregnancy in cattle. Theriogenology, 40:411-425.

Dominnguez, M.M. (1995): Effects of body condition, reproductive status and breed on follicular population and oocyte in cows. Theriogenology, 43:1408.

Eissa, H.M. and El-Belely, M.S. (1990): Sequential changes in plasma progesterone, total oestrogens and corticosterioid in the cow throughout pregnancy and around parturition. Br. Vet. J., 146:24-29.

Eissa, H.M.; El-belely, M.S.; Ghoneim, I.M. and Omaima, H. Ezzo, (1995): Concurrent changes of plasma progesterone, oestradiol-17b, oestrone sulphate, corticosteroid and a metabolite of PGF2a throughout pregnancy, around parturition and during postpartum in buffaloes cows. Vet Med., J., 43 (2): 247-256.

Fitzpatrik, L.A. (1994) : Advances in the understanding of post partum anoestrum in Bos indicus cows. Inter. Atomic Energy Agency (LAEA) Report, pp. 19-35.

Foote, R.H. (1996): Review: Dairy catte Reproductive Physiology Research and management – Progress and Future Prospects. J. Dairy Sci., 79: 980-990.

Ginther, O.J.; Kot, K.; Kulick, L.J.; Martin, S. and Wiltbank, N.C. (1996): Relationships between FSH and ovarian follicular waves during the last six-month of pregnancy in cattle. J. Reprod. Fertil., 108:271-279.

Gordon, I. (ed.) (1996 a): Controlling the calving interval In: Controlled Reproduction In Cattle and Buffaloes. CAB international, Wallingford, uk, pp. 215-223.

Gordon, I.(ed.) (1996 b): Physiological and endocrinological changes in early pregnancy in the cow: In: Controlled Reproduction in cattle and Buffaloes. CAB international, Wallingford, UK, pp. 169-171.

Granfield, R.W. and Buiter, W.R. (1991): Energy balance, first ovulation and the effect of naloxone on LH secretion in early postpartum dairy cows. J. Anim. Sci., 69: 740-746.

Gyawu, P. and pope, G.S. (1990): Postpartum ovarian function in dairy cows as revealed by concentrations of oestradiol-17b and progesterone in defatted milk. Br. Vet. J., 146: 194-204.

Harada, T. (1980): Serum adrenocortical and sex steroid hormones in dairy cows and their young and foetus in perinatal stage. J. Jap. Vet. Med. Ass., 33: 529-533.

Heinonen, K.; Ettola, E. and Alankko, M. (1988): Effect of postpartum live weight less on reproductive functions in dairy cows. Acta Vet. Scand., 29: 249-254.

Hung, N.N. and Prakash, B.S. (1990): Influence of gestation on blood plasma concentrations of oestrone and oestrone sulphate in Korea swiss cows and murrah buffaloes. Br. Vet. J., 146: 449-459.

Kindahl, H.; Odensvik, K.; Aiumlamai, S. and Fredriksson, G. (1992): Utero-ovarian relationships during the bovine postpartum period. Anim. Reprod. Sci., 28: 363-369.

McDougall,S. and Macmillan, K.L. (1994): Anovulatory dairy cows have lower LH pulse frequency and intrafollicular concentrations of estradiol (E₂) and t(T) than cyclic cows. Aust. Soc. Rep. Bio., 4: 20-31.

McDougall, S.; Macmillan, K.L. and Williamson, N.B. (1998): Factors associated with a prolonged period of postpartum in pasture-fed dairy cattle. XX World Buiatrics Cong., AACV Sydney, 6-10 July (1998), pp. 657-662.

Mcleod, B.J. and Williams, M.E. (1991): Incidence of ovarian dysfunction in postpartum dairy cows and the effectiveness of its clinical diagnosis and treatment. Vet. Rec., 128(6): 121-124.

Meredith, M.J. (ed) (1995): Cattle breeding and infertility. In: Animal breeding and infertility. Blackwell Sci., Ltd., pp. 149-161.

Mukasa-Mugerwa, E.; Tegegne, A. and Franceshini, R. (1991): Influence of sucking and continuous cow-calf association on the resumption of postpartum ovarian function in Bos indicus cows monitored by plasma progesterone profiles. Reprod. Nutr. Dev., 31 (2): 214-247.

Nakao, T.; Moriyoshi, M. and Kawata, K. (1992): The effect of postpartum dysfunction and endometritis on subsequent reproductive performance in high and medium producing dairy cows. Theriogenology, 37: 341-349.

Narasimha, Rao, A.V.; suri Babu, M. and Rama Nohan Rao, H. (1995): Evaluation of fertility traits in Jersey cows. Ind. Vet. J.; 72: 939-942.

Patel, O.V.; Takahashi, T.; Hirako, M.; Tomizuka, T.; Kojima, T.; Sasaki, N. and Mika, I. (1995): Estrone and estradiol concentrations throughout gestation in cows with single and twin pregnancies. J. Reprod. Develop., 41: 29-35.

Peter, A.T.; Bosu, W.T.K.; Liptrap, R.M. and Cummings, E. (1989): Temporal changes in serum prostaglandin F2a and oxytocin in dairy cows with short Iuteal phase after the first postpartum ovulation. Theriogenology, 32: 277-284.

Pimental, C.A.; Vijaykumar, R.; Weston, P.G.; Pimental, S.M.; Wagner, W.C. and Hixone, J.E. (1986): Effects of estrone infusion on secretion of F series prostaglandins and function of the corpus luteum during late gestation in cattle. A.J. Vet. Res., 47: 1972-19977.

Prado, R.; Rhind, S.M.; Wright A.; Russel, A.J.; McMillen, S.R.; Smith, A.J. and McNeilly, A.S. (1990): Ovarian follicle populations, steroidogenicity and micromorphology at 5 and 9 weeks postpartum in beef cows in two levels of body condition. Anim.Prod., 51: 103-108.

Robertson, H.A. (1972): Sequential changes in plasma progesterone in the cows during the oestrous cycle, pregnancy, at parturition and postpartum. Can. J. Anim. Sci., 52: 645-658.

Roche, J.F.; Crowe, M.A. and Boland, M.P. (1992): Postpartum anoestrum in dairy and beef cows. Anim. Reprod. Sci., 28: 371-378.

SAS (1987) : SAS user’s Guide, Statistics (6^th ed.) Cary NC, SAS Institute INC.

Savio, J.D.; Boland, M.P.; Hynes, N. and Roche, J.F. (1990): Resumption of follicular activity in the early postpartum period of dairy cows. J. Reprod. Fertil., 88: 569-579.

Schallanberger, E. (1985): Gonadotrophins and ovarian steroids in jugular vein postpartum. Acta Gndocrinol., 109: 37-43.

Shipka, M.P.; Campbell, C.B.; Wierenga, T.L.; Panter, K.E. and Lamb, R.C. (1998): use of radiolemetery, visual observation and progesterone assay determine true silent ovulation. ADSA-ASAS Joint Meeting, July 28-31, 1998, Colorado, USA, P. 216.

Souza, H.E.; Silva, A.O.; Sousa, J.S. and Vale, W.G. (1997): Progesterone profiles through the postpartum period in milk buffaloes under poor management condition. 5^th World Buffalo Cong., Italy, October 13-16, pp. 716-721.

Torres, E.; Nakao, T.; Osawa, T.; Moriyoshi, M.; Kawata, K. And Nakada, K. (1997): Postpartum adrenal, pituitary and ovarian functions in dairy cows. J. Vet. Med., A 44: 133-142.

Virakul, P. (1987): Conception, gestation and parturition in swamp buffalo. Swamp Buff. Reprod., 3: 212-224.

Wendorf, G.l.; Lawyer, M.S. and First, N.L. (1983): Role of the adrenal in the maintenance of pregnancy in cows. J. Reprod. Fertil., 68: 281-287.

White, A.J. and Dobson, H. (1990): Effect of prostaglandin F_{2 a} on the fertility of dairy cows after calving. Vet. Rec., 127: 588-592.

Table 1. Concentrations of progesterone, estradiol-17b and prostaglandin F_2a during gestation period in cows.*

* Mean with the different superscript letter at the same row are significant statistically (P< 0.01).

Table 2. Distribution of experimental cows.

*     From 8 of them, the blood samples were collected every 4 days till conception.

** One due to retained placenta and other due to endometritis after help during calving.

Table 3. Subsequent fertile postpartum in cows under hot climatic conditions.

* Infertile heat which return to heat after mating and inseminated again next heat.