Effect of thermal comfort index on reproductive performance of Brangus cows in tropical conditions

In tropical environments, high temperature and high humidity associated with air radiation, reduces the efficiency of heat loss and thus, increases the animal's stress (Silva et al., 2007 Brazilian Journal of Zootecnia.v.36, p.1192-1198), limiting development, production and reproduction (Vasconcelos et al., 2006 Theriogenology, v.65, p.192-200). According to HAHN (Stress physiology in livestock, v.2, 1985), for domestic animals in general, a value of temperature and humidity index (THI) at or below 70 indicates normal condition; and above 71 values characterize stress condition. For Armstrong (Journal of Dairy Science, v.77, p.2044-2050, 1994) THI above 72 is considered heat stress. The aim of this study was to evaluate the effect of thermal comfort index, using THI, on the reproductive performance of Brangus cows submitted to TAI in wetland savannah. Brangus females (n = 421) were submitted to estrus synchronization and TAI protocols between September and October 2013, in two separate groups. The protocol used was as follows: on a random day of the estrous cycle, vaginal devices with 1.9 g of progesterone (CIDR®) were inserted and 2mg of estradiol benzoate IM was applied (BE; Estrogin®). On D9 device removal was performed and 300 IU of eCG (SincroeCG®), 0.8 mg of estradiol cypionate (ECP®) and 12.5 mg of dinoprost (Lutalyse®) IM was applied. The TAI was performed 48-54 hours after device withdrawal. At the same time the THI was calculated hourly during the program, using the methodology proposed by Thom (Weatherwise, v.12, p.57-60, 1959). The THI of the whole TAI program was 72 for the first group of animals and 73 for the second one. On the day of the TAI the THI was 72 and 74 respectively, classified as mild stress index, what according Hahn, is sufficient to cause reduction in reproductive performance. However, the TAI pregnancy rate was 54.4% (143/263) and 63.3% (100/158) for the first and second groups, both results considered suitable for TAI programs. It is suggested that THI up to 73 does not characterize heat stress for the Brangus breed. Further comprehensive studies should be conducted to define the range of thermal comfort for cattle breeds adapted to the tropics.