Interference with cyclical activity in the gilt by heat stress prior to oestrus.

Abstract

Animal Production in Australia Vol. 15 INTERFERENCE WITH CYCLICAL ACTIVITY IN THE GILT BY HEAT STRESS PRIOR TO OESTRUS D H Pett* SUMMARY A total of 40 gilts was used to study the effects of heat stress (applied for 5 days) beginning either 6 days or 3 days before the expected date of oestrus on reproductive performance. In both experiments heating gilts reduced their feed intake from 2.0 to 1.5 kg per day and increased their rectal Gilts exposed to heat stress 6 days before temperature by 1.8OC (P < 0.001). oestrus had longer oestrous cycles (P < 0.10) by an average of 1.4 days. There was a negative correlation (r = -0.52, P ( 0.05) between feed intake and length of the oestrous cycle. For gilts subjected to high temperatures 3 days before oestrus there was a negative correlation (r = -0.60, P < 0.05) between rectal temperature and the duration of oestrus, suggesting that heat stress shortens oestrus. Ovulation rate was not directly affected by high temperatures before The differences in ovulation rate, although not statistically oestrus. significant, reflected a nutritional interaction rather than a direct effect of heat. These results suggest there may be a critical period towards the end of the oestrous cycle in gilts where high temperatures may exert an effect on both the duration of oestrus and length of the oestrous cycle. However a study of the endocrinology of the system is required to elucidate the possible mechanisms. INTRODUCTION examined the incidence and ( 1978) Paterson, Barker and Lindsay characteristics of summer infertility in the field and found an increase in the number of extended irregular returns to oestrus, that is, sows taking 25 days or longer to return. Their conclusions were that heat stress around the time of mating may affect ovarian function resulting in temporary infertility and an endocrine imbalance which could result in delayed, irregular returns to oestrus. Sawyer (1977) investigated the effects of mating on Merino ewes and showed that Similar responses oestrus was disrupted. exposed to heat stress conditions proximal 1972). of high temperatures at the time the normal cyclical activity at have been cited in cattle when to oestrus (Bond and McDowell It is hypothesised that heat stress in the pig is similar to that in The notion is that high ambient temperatures or stress sheep and cattle. conditions preceding oestrus disrupt the typical endocrine changes associated with luteal regression and enhanced follicular growth and thereby prevent the normal progression of events in the oestrous cycle. Two experiments were conducted to determine firstly the effect of heat stress on gilts 6 days before the expected commencement of oestrus (late luteal phase) and, secondly, to determine the effect of heat stress 3 days before the expected date of oestrus (enhanced follicular growth and immediate behavioural oestrus phase). * Department of Animal Science and Production, University of Western Australia, Nedlands, WA 6009. 533 Animal Production in Australia Vol. 15 MATERIALS AND METHODS In the first experiment 15 large white landrace cross gilts were introduced to a hot room (32OC for 7 h and 38OC for 17 2;) six days before their expected dates of oestrus. After remaining in the hot room for five days they were moved to a control room maintained at approximately 20�C. Five gilts were maintained in the control room throughout the experiment. The second experiment was identical to the first except that the heat treatment commenced three days before the expected date of oestrus. All animals received 2.0 kg of dry sow ration daily. Feed intake and rectal temperature were recorded daily in the hot room. Oestrus was detected daily with vasectomised boars, commencing two days before the expected date of oestrus. A gilt was deemed to display oestrus when she allowed the boar to mount. Ovarian activity and ovulation rate were determined by laparoscopy. Oestrous behaviour, ovarian activity and ovulation rate were recorded at the following oestrus (post-heated oestrus) to determine whether there was any residual effect of heating in the next cycle. RESULTS Heating reduced feed intake in both experiments from 2.0 to 1.5 kg per day and increased rectal temperature by 1.8OC (P < 0.001). Gilts exposed to heat stress six daysbefore oestrus had longer oestrous cycles (P < 0.10) by an average of 1.4 days (Table 1). TABLE 1 There was a negative correlation (r = -0.52, P < 0.05) between feed intake and length of the oestrous cycle. The duration of oestrus and ovulation rate were not affected by heat treatment. Gilts subjected to high temperatures three days before oestrus had normal oestrous cycle length (Table 2). There was no difference in the mean duration of oestrus between the heated and control gilts, however there was a negative correlation (r = -0.60, P < 0.05) between rectal temperature and the duration of oestrus. Half of the gilts heated three days prior to oestrus were receptive to the boar for only one day. In the post-heated cycle there was a 30 per cent reduction in gilts displaying oestrus for only one day, or, conversely, a 30 per cent increase in animals 'standing' to the boar for three days. 534 Animal Production in Australia Vol. I5 TABLE 2 Effect of heat stress applied 3 days prior to the expected oestr on reproductive performance of gil ts (mean and s tandard error) # One animal with abnormally low ovulation rate was removed from analysis Ovulation rate was not directly affected by heating. The differences in ovulation rate reflected a nutritional interaction rather than a direct effect of heat. In the second experiment depressed feed intake resulting in substantial body weight losses during the heat treatment may have been responsible for the differences observed, although not statistically significant (Table 2). This response may be exaggerated in the case where body reserves are limiting or a severe nutritional stress has been incurred. The effects of heat stress observed in experiments 1 and 2 are confined to the oestrous cycle during which treatment was imposed. Where cyclical activity was disturbed, normal function was resumed in the next oestrous cycle. DISCUSSION The main response of exposure to heat stress before oestrus is apparently interference with cyclical activity, although the subsequent reproductive performance of the animals is not known. These results suggest that the timing and duration of oestrus can be affected by heat stress. Heating gilts in the late luteal phase delays oestrus by 1 to 2 days. 'Heat stress applied closer to oestrus in the follicular phase has no effect on the timing of oestrus but may reduce its duration. Measured effects of environmental stress are likely to be extremely variable (Reid and Mills 1962). When heat treatment is applied for several days it is difficult to predict the response of animals. Some animals adapt quickly and their rectal temperatures return to normal within one or two days, while other animals may have elevated rectal temperatures for the duration of the heat treatment. Interference with cyclical oestrous activity may well be a graduated effect, and this may be indicated by the length of time a gilt remains receptive to the boar after'heating. There may be several degrees of disruption to normal cyclical activity which seem to be a function of the timing of heat stress in relation to the stage of the oestrous or breeding cyclel its severity and, possibly, state of nutrition of the animals. The findings of this investi .gation are consistent expl ain, the field observations on seasonal infertility. with, and help A study of the 535 Animal Production in Australia Vol. 15 endocrinology of the system and subsequent reproductive performance are required to elucidate the possible mechanisms and differentiate between the responses. REFERENCES BOND, J. and MCDOWELL, R.E. (1972). J. Anim. Sci. 35: 820. Aust. J. exp PATERSON, A.M., BARKER, I. and LINDSAY, D.R.=(1978). P-P Agric. Anim. Husb. 18: 698. REID, R.L. and M1LLrS.C. (1962). Aust. J. Agric. Res. 13: 282. SAWYER, G.J. (1977). PhD Thesis, University of Western AEtralia. 536