Abstract
The purpose of the study was to determine the effect of thermal stress on the reproductive efficiency of buffalo heifers. This study was carried out on 100 buffalo heifers, during two different environmental seasons (winter and summer). Heifers were mature, free from pathological and congenital affections, and fed on balanced rations. Animals were divided into two groups (25 per each) and each group was divided into two subgroups. In addition, 10 buffalo heifers were kept indoors as a control group in a closed building and 15 heifers kept outdoors exposed to direct environmental stress (ambient temperature and humidity). Temperature humidity index (THI) was calculated. Estrus synchronization using two doses of PGF2α 11 days' interval was carried out. Heifers were examined by ultrasonography per rectum and blood samples were collected on the 1st and 15th day of the synchronized estrous cycle to accurate detection of progesterone, estrogen, cortisol, and glucose. Mating was done naturally using fertile buffalo bulls introduced to heifers after the 2nd PGF2α injection. Pregnancy diagnosis was carried out through ultrasonography and conception rates were determined. The results indicated that THI outdoors was significantly higher than indoors in the summer season and not significantly different during winter. Blood levels of estradiol, progesterone and glucose were lower during summer when compared to winter season, while cortisol levels were higher. Also, conception rates were significantly higher indoors than outdoors during winter and summer seasons. respectively. In conclusion, environmental stress resulted in lowering reproductive performance in buffalo heifers, especially in the summer season, and indoor housing significantly improved reproduction.

Key words: buffalo heifers, reproductive performance, thermal stress, hormonal assay