High temperatures during regression of the corpus luteum disrupt hormone levels and decrease reproductive efficiency in the gilt.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 17 HIGH TEMPERATURES DURING REGRESSION OF THE CORPUS LUTEUM DISRUPT HORMONE LEVELS AND DECREASE REPRODUCTIVE EFFICIENCY IN THE GILT. D.H. PETT * SUMMARY Twenty-four gilts were used to study of high temperatures during regression of exposed to 32�C for 7h and 38�C for 17 (where d0 is the first day of oestrus). controls. the effect the corpus h from d13 Six gilts on reproductive performance luteum. Eighteen gilts were to d18 of the oestrous cycle were maintained at 20�C as The normal decline in progesterone concentrations in plasma expected at this stage of the oestrous cycle was delayed by one to two days in treated gilts, while the pre-oestrous peak of oestradiol and preovulatory surge of LH were delayed by 1.4d(P< 0.05) and 2.0d (PSO.025) respectively. The onset of oestrus was delayed by 1.4d(P<O.lO),but ovulation rate and oestrous duration were not affected by high temperature. There was a higher proportion of non-pregnant animals in the heated group compared with the control group (P< 0.01). Embryo mortality was 22% in the control gilts and 48% in the treated gilts. Key words : high temperatures, gilts, luteal regression, progesterone, pregnancy rate INTRODUCTION When sows and gilts are mated during the summer months reproductive performance is often depressed (Paterson et a1.1978;Claus and Weiler 1985). Pregnancy rate is up to 30% lower but litter size often remains unchanged.The incidence of ovulation without oestrus generally increases, oestrous cycles are often extended and the interval between weaning and mating increases. The evidence that litter size often remains unchanged suggests the effects of temperature occur before oestrus and not during early gestation. Studies in the sow when exposed to high temperature in early gestation have found increased embryo mortality and decreased litter size (Edwards et al. 1968;Omtvedt et al. 1971): When sheep and cattle are exposed to high temperatures before oestrus abnormally high concentrations of progesterone are often observed (Gwazdauskas et a1.1973;Abilay et a1.1975; Sawyer 1977) This experiment was conducted to test the hypothesis that high ambient temperatures preceding oestrus delay the normal fall in progesterone associated with regression of the corpus luteum. *School of Agriculture, University of Western Australia, Nedlands, W.A. 6009 Present Alddress : Dairy Industry Authority of Western Australia, P.O. Box 75, Claremont W.A. 6010 Proc. Aust. Soc. Anim. Prod. Vol. 17 MATERIALS AND METHODS 279 Twenty-four,Large White X Landrace gilts of similar age (196+14d),bodyweight(ll2+10Kg) and sexual maturity (first oestrus) were fitted with indwelling venous catheters. Gilts were allocated at random into a control group (n=6) and a treatment group (n=18). Gilts in the treatment group were introduced to the hot room (32�C for 7h and 38�C for 17h, at 55% RH with 12h light and 12h dark) on d13 of their third oestrous cycle. On d18 they were removed from the hot room and placed in the control room, which was maintained at ZOOC. All animals received Z.OKg/d- of a diet which contained 18.2% crude protein, 13.4MJ of digestible . energy/Kg and a mineral and vitamin supplement. Feed intake and rectal temperature were recorded daily. Blood samples were collected every 12h from d13 of the oestrous cycle, increasing to every 4h on d18 and continued until 48h after the first detection of oestrus. Samples were analysed for progesterone(P),luteinising hormone(LH) and'oestradiol(E2). Gilts were checked every 12h for oestrus, beginning on d18. All gilts were mated at the first detection of oestrus and again 12 hours later. Ovarian activity and ovulation rate were measured by laparoscopic examination seven days after mating. Thirty days after mating all animals were slaughtered, ovaries sectioned and corpora lutea counted. RESULTS The normal decline of P associated with the regression of corpora lutea was delayed by I-2d in those gilts exposed to high temperatures. The mean concentration of P between dl5 and d17 was greater in the heated group(4.2+0.8 vs 1.8kO.6 ng/ml,P,( 0.05) which delayed the pre-oestrous peak of E2 (Figure 1). The day of maximum E2 (DME) was 18.6dkO.4 in the control animals and 20.0dk0.4 in the heated animals (P<.O.O5). The preovulatory surge of LH in gilts subjected to high temperatures showed a 2d delay compared to control gilts (P&0.025). 280 Proc. Aust. Soc. Anim. Prod. Vol. 17 High temperatures during the regression of corpora lutea disrupted the balance between E2 and P before oestrus. P was higher and E2 lower on d18 and d19 in the heated gilts while the balance between the two steroids was reversed by d20 and d21. The reproductive performance of the gilts is summarised in Table 1. High temperatures extended the length of the oestrous cycle and delayed the onset of oestrus by an average of 1.4d(PG0.10). Ovulation rate and the duration of oestrus were not significantly different between the heated and control animals. Pregnancy rate was 28% lower in the heat stressed gilts. Three of the five nonpregnant gilts subjected to heat failed to return to oestrus within 30d after mating. Cystic corpora lutea were found in one heated animal that showed no preovulatory surge of LH at oestrus and had failed to conceive. The heat treatment decreased feed intake from 1.5 to Z.OKg/d(P,(O.Ol) and increased rectal temperature by l.l�C(P~O.OO1). More than half of the heated gilts had 25% or greater reductions in feed intake for the 5 days of the heating. Most animals resumed a daily intake of 2.OKg at the end of the treatment. Table 1 Effect of heat treatment from day 13 to day 18 on reproductive performance of gilts (mean and standard error). superscript differ values within rows followed by different significantly (sO.10) * value does include non-pregnant animals, assuming all foetuses lost a, DISCUSSION Results of this study support the hypothesis that high temperatures during the regression of corpora lutea delay the normal decline in P associated with luteolysis and delay the pre-oestrous rise in E2 and the onset of behavioural oestrus in gilts. These results concur with the theory that high temperatures during the week of mating may affect ovarian function resulting in an endocrine imbalance which could decrease pregnancy rates and delay returns to oestrus(Paterson et a1.1978). The balance between E2 and P before oestrus is an important stimulatory cue for the pre-ovulatory surge of LH and the display of behavioural oestrus. Changes in the balance of ovarian steriods may also affect sperm transport and early embryonic death(Polge 1972). Proc. Aust. Soc. Anim. .@od. Vol. 17 281 High P in the late luteal phase may be caused by interference with regression of corpora lutea resulting in an extended luteal phase, by P released Elevated from the adrenal gland or changed metabolic clearance of P. concentrations of adrenal androgens and progestagens have been found in stressed pigs. Moody(l964)measured increased P of adrenal origin in cattle exposed to high temperatures. He claimed that increased adrenal P.combined with ovarian P may act together to inhibit LH secretion. corticoid concentrations of adrenocorticotrophic(ACTH) and Elevated hormones have been measured in animals at high temperatures(Collins and Weiner 1968; Marple et a1.1972). Increased concentrations of ACTH may have been responsible for the endocrine abnormalities and imbalances observed in several heat stressed gilts. The failure of all animals to respond in the same manner to a similar stress could reflect differing adrenal thresholds to ACTH and heat stress. ACKNOWLEDGEMENTS The work reported in this paper was funded by the Australian Pig Industry Research Committee (APIRC) under an APIRC postgraduate studentship. REFERENCES ABILAY, T.A., JOHNSON, H.D. and MADAN, M. (1975). J. Dairy Sci. 58:1836. CLAUS, R. and WEILER, U. (1985). J. Reprod. Fert. Suppl. 33:1857 COLLINS, K.J. and WEINER, J.S:(1968). Physiol. Rev. 48(4)785, EDWARDS, R.L., OMTVEDT, I.T. TURMAN, E.J., STEPHENS, KF. and MAHONEY, G.W.A. (1968). J. Anim. Sci. 27:1634-. GWAZDAUSKAS, F.C., THATCHER,?W. and WILCOX, C.J. (1973). 3. Dairy Sci. 56:873. MARPLE, D.N., ABERLE, E.D., FOREST, J.C., BLAKE, W.H. and JUDGE, M.D. (19Fa) J. Anim. Sci. 34:809, MOODY, L.L. (1964) KSC. Thesis, University of Arizona, Tuscan. OMTVEDT, I.T., NELSON, R.E., EDWARDS, R.L., STEPHENS, D.F. and TURMAN, E.J.' (1971). J. Anim. Sci. 32:3X PATERSON, A.M., BARKER, I. and LINDSAY, D.R.(1978). -Aust. 3. Exp. Agric. Anim. Husb. 18:698. POLGE7.r (1972) Pig Production ed. D.J.A. Cole (Butterworths,) SAWYER, G.J. (1977) Ph.D Thesis. School of Agriculture. University of Western Australia