Effects of a single injection of hCG or GnRH agonist on day 12 post mating on fetal growth and reproductive performance of sheep

Animal Reproduction Science 80 (2004) 81–90

Effects of a single injection of hCG or GnRH agonist on day 12 post mating on fetal growth and reproductive performance of sheep Mehmet Akif Cam∗ , Mehmet Kuran Ondokuz Mayis Universitesi, Ziraat Fakultesi, Kurupelit, 55139 Samsun, Turkey Received 5 September 2002; received in revised form 28 May 2003; accepted 20 June 2003

Abstract Reproductive performance and fetal growth were determined when hCG (150 i.u. Pregnyl; n = 44), GnRH (4 ␮g synthetic GnRH agonist, buserelin, Receptal; n = 43) or saline (control, n = 45) was administered (i.m.) to ewes on day 12 post mating during the breeding season. A total of 12 ewes was slaughtered on day 45 of pregnancy (four from each treatment group). Non-return rate and lambing rate were higher for ewes in the hCG (0.89 and 84%) and GnRH treated groups (0.86 and 79%) than for ewes in the control (0.69 and 62%) group (P < 0.05). The ewes in the hCG and GnRH groups also had more twins (P < 0.05). Birth weights of these twin lambs in the hCG and GnRH groups were heavier than those in the control group (P < 0.05), but this difference had disappeared at weaning 60 days later. Lamb mortality was similar among treatment groups resulting in a higher number of lambs weaned in the hCG and GnRH groups. The ovarian weights and the number of corpora lutea (CL) and luteal weights of ewes slaughtered on day 45 of pregnancy were greater (P < 0.05) in the hCG and GnRH treated groups than those measured in the control group. Administration of hCG on day 12 post mating increased gravid uterus weight, crown-rump-length (CRL), chorioallantois weight and total cotyledon weight (P < 0.05) of conceptuses recovered on day 45 of pregnancy compared to the control group. The weights of caruncules (P < 0.05) and placenta (P < 0.01) were higher in conceptuses of both the hCG and GnRH groups. The weights of fetuses in the hCG group were higher than those in both the GnRH and control groups (P < 0.05). In conclusion, both hCG and GnRH administration improved reproductive performance of ewes when administered on day 12 post mating. However, hCG and GnRH appeared to act differently on embryo survival because only hCG administration increased fetal growth. © 2003 Elsevier B.V. All rights reserved. Keywords: Sheep-endocrinology; hCG; GnRH; Reproductive performance; Fetal growth

∗ Corresponding author. Tel.: +90-362-457-6020x1364; fax: +90-362-457-6034. E-mail address: [email protected] (M. Akif Cam).

0378-4320/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-4320(03)00158-1

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1. Introduction Stimulation of ovulation by different methods can increase ovulation rate to improve reproduction by increasing the proportion of ewes having twin ovulations and thereby increase lambing percentage. However, the net improvement is limited by embryo loss (Wilkins, 1997). Preimplantation embryonic loss is the major factor limiting optimum reproductive performance in farm animals. In sheep, 30–40% of fertilised eggs are lost during the first 3 weeks of pregnancy (Bolet, 1986; Nancarrow, 1994; Michels et al., 1998). Of this total loss, 70–80% occurs between day 8 and 16 after insemination (Sreenan et al., 1996). One of the major causes of embryonic loss is likely to be the inadequate luteal function (Wilmut et al., 1986; Ashworth et al., 1989; Nancarrow, 1994). In attempts to reduce embryonic mortality and hence to improve reproductive performance, progesterone supplementation during early pregnancy has been employed in sheep and cattle (reviewed by Sreenan et al., 1996; Thatcher et al., 2001). There are studies showing that progesterone supplementation after breeding improves not only pregnancy rate (McMillan et al., 1986; Davies and Beck, 1992) but also stimulates subsequent fetal growth (Garrett et al., 1988; Kleemann et al., 1994). Progesterone supplementation after breeding to compensate for possible luteal insufficiency or to stimulate embryo development in order to amplify the embryonic signal, has been beneficial, but with variation between studies (Sreenan et al., 1996). Alternative approaches in reducing embryonic loss during early pregnancy have been the administration of hCG or GnRH resulting in a significant increase in systemic progesterone (Sreenan et al., 1996). Much of this rise in progesterone would be due to the induction of accessory CLs (Mann and Picton, 1995; Beck et al., 1996). Administration of GnRH on days 10, 11, 12 or 13 post mating has been shown to improve early embryo survival (Beck et al., 1994; Cam et al., 2002) and pregnancy rate (McMillan et al., 1986; Cam et al., 2002) in sheep. Although Nephew et al. (1994) reported that a single injection of hCG on day 11.5 post mating stimulated uterine secretions and conceptus growth, there has not been any detailed report on the effect of hCG administration on the reproductive performance of sheep. The timing of such hormonal treatments also may seem to be important. The administration of hCG or GnRH on the day of mating, on days 4, 5 and 12 post mating have been reported (Beck et al., 1994; Drew and Peters, 1994; Nephew et al., 1994; Peters, 1996; Thatcher et al., 2001; Cam et al., 2002; Khan et al., 2003). The administration on day 12 post mating is the critical period for maternal recognition of pregnancy in the ewe. It coincides with the beginning of regression of the CL in the natural estrous cycle (Bazer et al., 1998). Hormonal induction of progesterone production after or on day 12 may increase interferon-␶ production (Thatcher et al., 1995) and may in turn prevent luteolysis by preventing PGF2 ␣ secretion (Bazer et al., 1998). Our laboratory has shown previously that GnRH administration on day 12 post mating improved the reproductive performance of ewes through its beneficial effect on embryo survival by enhancing luteal function, but not through stimulating fetal growth (Cam et al., 2002). It has been reported that hCG administration can stimulate conceptus growth in utero, as determined on day 13 (Nephew ve ark., 1994) and on day 25 (Khan et al., 2003). It is not known however whether GnRH and hCG act similarly on fetal growth and/or embryo survival. Therefore, the objective of the present study was to investigate whether a

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single administration of hCG or GnRH before the time of maternal recognition of pregnancy would have similar effects on subsequent fetal growth and pregnancy rate, and on the overall reproductive performance of ewes.

2. Material and methods 2.1. Animals and treatments A total of 132 ewes (89 Karayaka and 43 Karayaka x Sakiz F2 crossbreds) from 2 to 6 years of age were used in an experiment that was conducted at Ondokuz Mayis University, Samsun field station (41.2◦ N) during the breeding season. The individually identifiable ewes in natural estrus were randomly allocated into three treatment groups. The ewes in each treatment group were mated during September to six fertile (four Karayaka and two Karayaka x Sakiz F2 crossbreds) rams. Estrus was detected by teaser rams. Ewes in estrus were mated naturally. On day 12 post mating (day of estrus = day 0), the ewes in one group (n = 44) were given an i.m. injection of 150 i.u. hCG (hCG group; Pregnyl, Organon, Istanbul, Turkey) while the ewes in second group (n = 43) were given an i.m. injection of 4 ␮g of synthetic GnRH agonist (GnRH group; buserelin, Receptal, Topkim, Istanbul, Turkey). Ewes in the third group (n = 45) were given a saline (i.m.) injection (control group). Returns to first service were determined using a teaser ram to calculate non-return rate. The ewes were maintained on pasture and supplemented with hay and concentrates during pregnancy. The date of birth, number and weight of each lamb were recorded at lambing. Weaning weight on day 60 post lambing and lamb mortality by weaning were also recorded. 2.2. Recovery of conceptus products A total of 12 ewes were slaughtered on day 45 of pregnancy (four ewes from each treatment groups). The number of corpora lutea (CL) and ovarian weights were recorded post slaughter. Luteal weights were determined after isolation of all CLs for each ewe using scissors and forceps. Curved crown-rump-length (CRL) and weights of gravid uterus, fetuses, chorioallantois, caruncle and cotyledone were determined. Placental weight was calculated using the weights of caruncle and cotyledon. Weight of fetal fluid was calculated from gravid uterus, fetus, chorioallantois, uterus and caruncle weights. The number of fetuses was also recorded post slaughter. 2.3. Statistical analysis Differences between treatment groups were analysed with χ2 -analysis for non-return rate, pregnancy rate and twinnings. Data for gestation length, birth weight, weaning weight, ovarian weight, numbers and weights of corpora lutea and products of conception were subjected to analysis of variance, and differences between means were tested by a LSD test.

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3. Results 3.1. Lambing performance The number of ewes returning to estrus, non-return rate and number of lambing in control, hCG and GnRH treatment (on day 12 post mating) groups are presented in Table 1. The number of ewes returned to estrus was lower and there was a higher non-return rate in hCG and GnRH treated ewes when compared to ewes in the control group (P < 0.05), but there was no difference between the hCG and GnRH groups (P > 0.05). Both the hCG and GnRH treatment on day 12 post mating increased lambing rate when compared to the control group (P < 0.05). The ewes in hCG and GnRH groups had more twins (P < 0.05) and consequently they had a higher total number of lambs born and a larger litter size than those in control group, but there was no difference between hCG and GnRH treatment groups (P > 0.05). There was no difference (P > 0.05) in gestation length between control, hCG and GnRH administered ewes. Administration of hCG or GnRH on day 12 post mating did not affect lamb birth weight of single births (P > 0.05) while birth weights of twin lambs were higher in the Table 1 Reproductive performance of ewes, and growth performance and mortality of lambs in groups of ewes injected with hCG, GnRH or saline (control) on day 12 post mating Control

hCG

GnRH

No. of ewes Mated/treated Returning to estrus

45 14

44 5

43 6

Non-return rate No. pregnant (%)

0.69 a 28 (62.2) a

0.89 b 37 (84.1) b

0.86 b 34 (79.1) b

No. lambing No. of lambs Singles Twins

24

33

30

20 8a

17 32 b

18 24 b

28

49

42

Total no. of lambs Litter size Gestation length (days) Singles Twins Birth weight of lambs (kg) Singles Twins

1.17 148.8 ± 0.5 147.9 ± 0.6 3.91 ± 0.13 2.78 ± 0.15 a

Weaning weight at day 60 post lambing (kg) Singles 18.9 ± 0.47 Twins 15.7 ± 0.56 Lamb mortality at weaning (%)

1.48 149.1 ± 0.9 148.4 ± 0.7

1.40 148.4 ± 0.3 148.1 ± 0.2

3.83 ± 0.12 3.47 ± 0.17 b

4.02 ± 0.12 3.39 ± 0.12 b

20.4 ± 0.46 16.0 ± 0.27

19.9 ± 0.63 15.2 ± 0.33

7.1

Values within rows with different letters differ significantly (P < 0.05).

2.1

4.8

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hCG and GnRH groups than those in the control group (P < 0.05). In all treatment groups, birth weights of single lambs were heavier than those of twins (P < 0.01). There were no significant differences between treatment groups in terms of the live weight of lambs at weaning on day 60 (P > 0.05). Weaning weights of single lambs were heavier than those of twins (P < 0.01) in all treatment groups. The lamb mortality rate at weaning was 7.1, 2.1 and 4.8% for control, hCG and GnRH groups, respectively, and was observed only in twins in all treatment groups. 3.2. Litter size, ovarian characteristics and products of conception at slaughter on day 45 The number of viable fetuses and litter size distribution in ewes slaughtered on day 45 post mating from control, hCG and GnRH administered groups are presented in Table 2. The 12 slaughtered ewes (four ewes from each treatment group) were all pregnant. More ewes had twins in the GnRH group (P < 0.05) and consequently they had a higher total number of viable fetuses and a larger litter size than those in the control group. There was a similar tendency in ewes in the hCG treatment group. Total ovarian weight was higher (P < 0.05) in the hCG and GnRH ewes at day 45 post mating than those in the control group (Table 2). The ovarian weights of ewes in the hCG and GnRH groups did not differ from each other (P > 0.05). The number of corpora lutea was 2.2 times higher in hCG and 2.4 times higher in GnRH administered ewes than those in control group (P < 0.05). Similarly, luteal weights were greater in the hCG and GnRH administered ewes than those in the control group (P < 0.05). Administration of hCG on day 12 post mating increased gravid uterial weight, CRL, chorioallantois weight and total cotyledon weight (P < 0.05) in conceptuses recovered on day 45 of pregnancy, when compared to the control group (Table 3). There was no difference between the hCG and GnRH treatment groups (P > 0.05). The weights of caruncles (P < 0.05) and placenta (P < 0.01) were higher in conceptuses of both the hCG Table 2 Number of viable fetuses, litter size distribution, numbers and weights of CLs of ewes at slaughter on day 45 post mating in ewes from the control, hCG or GnRH groups Control

hCG

GnRH

n No. of ewes pregnant

4 4

4 4

4 4

No. of viable fetuses Singles Twins

3 2a

1 6 ab

0 8b

Total no. of fetuses Litter size Ovarian weight (g) No. of CL Luteal weight (g)

5 1.25 2.47 ± 0.16 a 1.25 ± 0.25 a 0.98 ± 0.20 a

7 1.75 3.53 ± 0.17 b 2.75 ± 0.63 b 1.55 ± 0.27 b

8 2.00 3.37 ± 0.21 b 3.00 ± 0.41 b 1.74 ± 0.18 b

Values within rows with different letters differ significantly (P < 0.05).

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Table 3 Means (±S.E.M.) for products of conception at day 45 of pregnancy for fetuses from ewes in the control, hCG or GnRH groups

Gravid uterus (g) Fetal weight (g) Fetal fluids (g) Chorioallantois (g) Fetal crown-rump-length (mm) Total caruncle weight (g) Total cotyledon weight (g) Placental weight (g)

Control

hCG

GnRH

384.1 ± 57.3 a 8.68 ± 0.48 a 105.7 ± 47.2 103.8 ± 10.5 a 70.2 ± 1.1 a 13.3 ± 2.6 a 22.2 ± 1.7 a 35.5 ± 3.8 c

581.8 ± 45.3 b 11.73 ± 0.47 b 169.3 ± 50.2 155.4 ± 8.6 b 77.6 ± 2.0 b 22.0 ± 1.3 b 43.5 ± 3.0 b 65.5 ± 2.3 d

489.4 ± 73.0 ab 9.06 ± 0.39 a 164.3 ± 52.8 111.7 ± 19.8 ab 73.9 ± 1.1 ab 21.9 ± 2.7 b 33.2 ± 6.9 ab 55.3 ± 4.0 d

Values within rows with different letters differ significantly (a, b; P < 0.05) (c, d; P < 0.01).

and GnRH group than those of control group. Fetal weights of fetuses in hCG group were heavier than those in the GnRH and control groups (P < 0.05) and there was no difference between the weights of the fetuses in GnRH and control groups (P > 0.05). Administration of hCG or GnRH on day 12 post mating did not affect fetal fluid weights (P > 0.05).

4. Discussion The results of the present study showed that administration of either hCG or a GnRH agonist on day 12 post mating (before the time of maternal recognition of pregnancy) improved pregnancy rate, litter size and number of lambs born. Non-return rate (20%), lambing rate (22%) and twinning rate (31%) were higher with the hCG administration when compared to the control group. The equivalent rates in GnRH group were 17, 17 and 23%, respectively and were also higher than those in control group. This resulted in an improved reproductive performance in ewes given hCG and GnRH on day 12 post mating compared to control group. The difference between control and hCG or GnRH groups in terms of non-return rate reflected the effect of hormonal treatments on reducing the early embryonic mortality that accounted for 70–80% of the total embryonic loss (Sreenan et al., 1996). These results indicate that hCG or GnRH administration before the time of maternal recognition of pregnancy improved pregnancy rate, twinning rate and the number of lambs born by improving embryo survival. Kittok et al. (1983) reported that hCG administration before the time of maternal recognition of pregnancy increased pregnancy rate 20% in lactating, seasonal anestrous ewes. There are also studies reporting that GnRH administration improved reproductive performance in sheep during the breeding season (Macmillan et al., 1986; Beck et al., 1994, 1999; Khan et al., 1999; Cam et al., 2002). The results of the present study are in agreement with these findings. Nephew et al. (1994) also reported that hCG administration on day 11.5 post mating increases pregnancy rate in sheep. They showed that it increased interferon-␶ secretion, luteal weights and conceptus length determined on day 13 of pregnancy. The effect of the hCG on pregnancy rate and fetal weights could be attributed to its effects on progesterone production and uterine secretions which were embryotrophic. This may result in a stronger

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signal for maternal recognition of pregnancy from embryos in hormonal treatment groups which would degenerate otherwise. The effect of GnRH on embryo survival may occur through GnRH-stimulated LH surge (Cam et al., 2002) stimulating production of progesterone (Cam et al., 2002) by CL and/or causing ovulation and the formation of accessory CLs. Similarly hCG administration has also been reported to increase the number of CL (Beck et al., 1998) and plasma progesterone concentration (Nephew et al., 1994; Khan et al., 1999). The results of the present study provide evidence that hCG and GnRH caused ovulation and the formation of accessory CLs, since a higher number of CL was observed in ewes given hCG or GnRH and slaughtered on day 45 of pregnancy. This result is in agreement with the previous findings that GnRH (Mann and Picton, 1995; Beck et al., 1996; Cam et al., 2002) and hCG (Nephew et al., 1994; Beck et al., 1998) administration resulted in an increased number of CL. We have previously demonstrated (Cam et al., 2002) that GnRH administration does not affect fetal growth. The results of the present study also showed that GnRH administration did not affect fetal weights. The stimulatory effect of GnRH and hCG on products of conceptuses can be partly attributed to the litter size effects. However, hCG administration stimulated fetal growth observed by increases in fetal weights and fetal CRL on day 45 of pregnancy. The beneficial effect of hCG administration on embryo survival may be through the stimulatory effect of hCG-induced progesterone on fetal growth, because it has been shown that progesterone supplementation increased subsequent fetal growth (Garrett et al., 1988; Kleemann et al., 1994). The stimulatory effect of progesterone on fetal growth occurred by providing an enriched nutritive environment as well as providing growth stimulants in the form of growth factors (Barnes, 2000). If progesterone was stimulatory for fetal growth, then, both hCG and GnRH should have stimulated the fetal growth, as GnRH (Khan et al., 1999; Cam et al., 2002) and hCG (Nephew et al., 1994; Khan et al., 1999) administration has been shown to result in an increase in circulating plasma progesterone concentrations. It is plausible that while GnRH administration improved embryo survival by enhanced luteal activity, hCG administration not only improved embryo survival but also stimulated fetal growth. The stimulatory effect of hCG administration on fetal growth has been reported in sheep determined on day 13 (Nephew et al., 1994) and on day 25 (Beck et al., 1999; Khan et al., 2003) of pregnancy. It is noteworthy that the birth weights of lambs were higher in both hCG and GnRH group only in twins. However, we did not observe any stimulatory effect of GnRH on birth weights of single or twin born lambs in a previous study aiming to investigate the effect of GnRH administration on day 12 post mating on subsequent fetal growth and reproductive performance of ewes (Cam et al., 2002). The chance of embryo survival decreased as the litter size increased (Nancarrow, 1994). It is possible that hormonal treatments used in the present study prevented the mortality of the twin embryos by stimulating the fetal growth which resulted in an increased weight and number of twins at birth which otherwise would not have survived. The differential effect of hCG and GnRH on fetal growth may suggest that the mechanisms of hCG and GnRH effect on embryo survival are not the same, although both stimulated luteal activity. The difference between treatment groups in terms of birth weights of twin lambs disappeared at weaning on day 60 post lambing. The non-return rate and pregnancy rate in control group in the present study was approximately 60%. This was lower than reports in some other studies (e.g. Nephew et al., 1994).

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The effects of hormonal treatments in improving reproductive performance of ewes may be more profound in such conditions. The low pregnancy rate in the control group may have reflected the well-known genetic effect of breed on embryonic mortality (Nancarrow, 1994). The pregnancy rate of the breed used in the present study has been reported to be around 60% (Cam et al., 2002). The breeding season of sheep coincides with the summer or early autumn months in subtropic and tropic climates in the northern hemisphere. Heat stress can reduce the embryonic survival by its direct effect on embryo and possibly also by its indirect effect on plasma progesterone concentration. It has been reported in cattle that plasma progesterone concentration (Jonsson et al., 1997; Wolfenson et al., 2000) and in vitro progesterone production by luteal cells (Wolfenson et al., 2000) from heat stressed animals during summer months are lower than those during winter months. Delayed effect of heat stress on cattle fertility has also been reported during autumn (Roth et al., 2000). There is no information on such effects of heat stress on sheep fertility. The hormonal treatments in the present study resulted in 20% to 30% increases in reproductive performance of sheep when administered on day 12 post mating. Such an increase may have been due to advanced luteal activity in heat stressed ewes since we have reported previously that GnRH administration on day 12 post mating in the same breed during the breeding season increased plasma progesterone concentrations (Cam et al., 2002). The maximum air temperature during the breeding season was up to 37 ◦ C in the region where the study was carried out. Such heat stress effects on luteal activity may also contribute to the explanation of the variation between the results of different studies, on the effect of such hormonal treatments on reproductive performance of sheep and cattle, apart from other differences between experimental conditions. Some studies report improvements while some others report no effect of such hormonal treatments in reproductive performance of sheep and cattle (Jubb et al., 1990; Ryan et al., 1994; Tefera et al., 2001). It is possible that the supplementation of progesterone or administration of GnRH or hCG may be more effective in reducing embryonic mortality in tropical and sub-tropical climatic conditions.

5. Conclusion The results of the present study showed that both hCG and GnRH agonist administration improved reproductive performance of ewes effectively when administered on day 12 post mating. It appeared that the hCG and the GnRH acted differentially on embryo survival because only hCG administration increased fetal growth.

Acknowledgements This study was financially supported by Ondokuz Mayis University Department of Agriculture Research Fund. The authors are grateful for the support of the staff and facilities of the Department of Animal Science, University of Ondokuz Mayis, Turkey.

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References Ashworth, C.J., Sales, D.I., Wilmut, I., 1989. Evidence of an association between the survival of embryos and periovulatory plasma progesterone concentration in the ewe. J. Reprod. Fertil. 87, 23–32. Barnes, F.L., 2000. The effect of the early uterine environment on the subsequent development of embryo and fetus. Theriogenology 53, 649–658. Bazer, F.W., Ott, T.L., Spencer, T.E., 1998. Maternal recognition of pregnancy: comparative aspects—a review. Trophoplast Res. 12, 375–386. Beck, N.F.G., Peters, A.R., Williams, S.P., 1994. The effect of GnRH agonist (buserelin) treatment on day 12 post mating on the reproductive performance of ewes. Anim. Prod. 58, 243–247. Beck, N.F.G., Jones, M., Davies, B., Mann, G.E., Peters, A.R., 1996. The effect of GnRH analogue (buserelin) treatment on day 12 post mating on ovarian structure and plasma progesterone and oestradiol concentration in ewes. Anim. Sci. 63, 407–412. Beck, N.F.G., Green, A., Khan, T.H., Khalid, M., 1998. The effects of buserelin (GnRH analogue) or hCG treatment on luteal weight and embryonic development during early pregnancy in the ewe. J. Reprod. Fertil. Abstr. Ser. 21 (95) (abstract). Beck, N.F.G., Khan, T.H., Khalid, M., 1999. The effects of hCG treatment on day 12 post mating on conceptus growth and fertility in ewes and ewe lambs. J. Reprod. Fertil. Abstr. Ser. 23 (85) (abstract). Bolet, G., 1986. Timing and extent of embryonic mortality in pigs, sheep and goats: genetic variability. In: Sreenan, J.M., Diskin, M.G. (Eds.), Embryonic Mortality in Farm Animals. Martinus Nijhoff, The Hague, pp. 13–43. Cam, M.A., Kuran, M., Yildiz, S., Selcuk, E., 2002. Fetal growth and reproductive performance in ewes administered GnRH agonist on day 12 post-mating. Anim. Reprod. Sci. 72, 73–82. Davies, M.C.G., Beck, N.F.G., 1992. Plasma hormone profiles and fertility in ewe lambs given progestagen supplementation after mating. Theriogenology 38, 513–526. Drew, S.B., Peters, A.R., 1994. Effect of buserelin on pregnancy rates in dairy cows. Vet. Rec. 134, 267–269. Garrett, J.E., Geisert, R.D., Zavy, M.T., Morgan, G.L., 1988. Evidence for maternal regulation of early conceptus growth and development in beef cattle. J. Reprod. Fertil. 84, 437–446. Jonsson, N.N., McGowan, M.R., McGuigan, K., Davison, T.M., Hussain, A.M., Kafi, M., Matschoss, A., 1997. Relationships among calving season, heat load, energy balance and post partum ovulation of dairy cows in a subtropical environment. Anim. Reprod. Sci. 47, 315–326. Jubb, J.P., Abhayaratne, D., Malmo, J., Anderson, G.A., 1990. Failure of an intramuscular injection of gonadotrophin releasing hormone 11 to 13 days after insemination to increase pregnancy rates in dairy cattle. Aust. Vet. J. 67, 359–361. Khan, T.H., Beck, N.F.G., Khalid, M., 1999. The effects of GnRH or hCG treatment on day 12 of pregnancy on luteal function in ewes and ewe lambs. J. Reprod. Fertil Abstr. Ser. 23 (58) (abstract). Khan, T.H., Hastie, P.M., Beck, N.F.G., Khalid, M., 2003. hCG treatment on day of mating improves embryo viability and fertility in ewe lambs. Anim. Reprod. Sci. 76, 81–89. Kittok, R.J., Stellflug, J.N., Lowry, S.R., 1983. Enhanced progesterone and pregnancy rate after gonadotropin administration in lactating ewes. J. Anim. Sci. 56, 652–655. Kleemann, D.O., Walker, S.K., Seamark, R.F., 1994. Enhanced fetal growth in sheep administered progesterone during the first three days of pregnancy. J. Reprod. Fertil. 102, 411–417. Macmillan, K.L., Taufa, V.K., Day, A.M., 1986. Effects of an agonist of gonadotrophin releasing hormone (buserelin) in cattle. III. Pregnancy rates after a post-insemination injection during metoestrus or dioestrus. Anim. Reprod. Sci. 11, 1–10. Mann, G.E., Picton, H.M., 1995. Ovarian and uterine effects of a single buserelin injection on day 12 of the oestrous cycle in the cow. J. Reprod. Fertil. Abstr. Ser. 15 (61) (abstract). McMillan, W., Knight, T.W., Macmillan, K.L., 1986. Effects of gonadotrophin releasing hormone (buserelin) on sheep fertility. Proc. N. Z. Soc. Anim. Prod. 46, 161–163. Michels, H., Vanmontfort, D., Dewil, E., Decuypere, E., 1998. Genetic variation of prenatal survival in relation to ovulation rate in sheep: a review. Small Rum. Res. 29, 129–142. Nancarrow, C.D., 1994. Embryonic mortality in the ewe and doe. In: Zavy, M.T., Geisart, R.D. (Eds.), Embryonic Mortality in Domestic Species. CRC Press, London, pp. 79–97. Nephew, K.P., Cardenas, H., McClure, K.E., Ott, T.L., Bazer, F.W., Pope, W.F., 1994. Effects of administration of human chorionic gonadotropin or progesterone before maternal recognition of pregnancy on blastocyst development and pregnancy in sheep. J. Anim. Sci. 72, 453–458.

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Peters, A.R., 1996. Embryo mortality in the cow. Anim. Breed. Abstr. 64, 587–598. Roth, Z., Meidan, R., Braw-Tal, R., Wolfenson, D., 2000. Immediate and delayed effects of heat stress on follicular development and its association with plasma FSH and inhibin concentration in cows. J. Reprod. Fertil. 120, 83–90. Ryan, D.P., Snijders, S., Condon, T., Grealy, M., Sreenan, J., O’Farrell, K.J., 1994. Endocrine and ovarian responses and pregnancy rates in dairy cows following the administration of a gonadotrophin releasing hormone analogue at the time of artificial insemination or at mid-cycle post insemination. Anim. Reprod. Sci. 34, 179–191. Sreenan, J.M., Diskin, M.G., Dunne, L., 1996. Embryonic mortality: the major cause of reproductive wastage in cattle. In: Proceedings of the 47th Annual Meeting of the European Association of Animal Production. Lillihammer, August 1996. Tefera, M., Chaffaux, S., Thibier, M., Humblot, P., 2001. A short note: lack of effect of post-AI, hCG or GnRH treatment on embryonic mortality in dairy cattle. Livest. Prod. Sci. 71, 277–281. Thatcher, W.W., Meyer, M.D., Danet-Desnoyers, G., 1995. Maternal recognition of pregnancy. J. Reprod. Fertil. Suppl. 49, 15–28. Thatcher, W.W., Moreira, F., Santos, J.E.P., Mattos, R.C., Lopes, F.L., Pancarci, S.M., Risco, C.A., 2001. Effects of hormonal treatments on reproductive performance and embryo production. Theriogenology 55, 75–89. Wilkins, J.F., 1997. Method of stimulating ovulation rate in Merino ewes may affect conception but not embryo survival. Anim. Reprod. Sci. 47, 31–42. Wilmut, I., Sales, D.I., Ashworth, C.J., 1986. Maternal and embryonic factors associated with prenatal loss in mammals. J. Reprod. Fertil. 76, 851–864. Wolfenson, D., Roth, Z., Meidan, R., 2000. Impaired reproduction in heat-stressed cattle: basic and applied aspects. Anim. Reprod. Sci. 60–61, 535–547.