138 EFFECTS OF CLIMATIC HEAT STRESS ON EMBRYO MORTALITY OF GRAZING CATTLE IN AZORES

Abstract

Seasonal high temperatures and humidity negatively affects reproduction in cattle. This negative effect is caused by high body temperature, which influences endocrine status other than thyroid. In fact, heat stress is known to negatively affect ovarian function, the expression of estrus, the quality of oocytes, and embryo development. The aim of this study was, therefore, to relate the temperature and humidity index (THI) to the embryo mortality rate in grazing cattle in the Azores summer. For such purpose, the success of 2139 artificial inseminations in different breeds was evaluated from October 1, 2007 to March 31, 2009, performed by an artificial insemination company. The climacteric data were obtained by different meteorological stations located near the farms and were further transferred to the meteorological center. The embryonic mortality was calculated using the dates of artificial inseminations of the cows. Those who demonstrated estrus after 26 days, as observed by visual observation, were admitted as occurrence of embryonic death. For climatic values, relative humidity and temperature were recorded every hour and the THI calculated for each day between 1100 to 1700 hours. In the warmer months (July to September), the THI ranged between 68 and 79 (× = 73). In this period, THI was 18% higher than that observed in January to March, which ranged between 58 and 68 (× = 62). The embryonic mortality in summer was 51%, which was almost double that observed for the winter period. By linear regression analysis, a positive correlation between the monthly average values of the THI and the rate of embryo mortality was observed (R² = 0.78). August was the most critical month and daily THI has always been up to 70, which is considered as the limit of cattle thermal comfort. The THI started decreasing in October (× = 72), but in this month embryo mortality rate was just like in August (57%). Results of the present study clearly demonstrated that in the conditions of the Azores, heat and humidity stress negatively affect the reproductive performance as evaluated by embryonic mortality. Although the THI decreased in October (× = 72), the rate of embryo mortality was still high (57%), which may have been a result of heat stress that the animals suffered during summer.

The first author is supported by the Regional Foundation for Science and Technology of the Azores Government. This study was supported by the IBBA Institute grant number M2.1.2/I/022/2008. CITA-A is fully acknowledged.