Relationship of weight gain with infrared temperatures in Nelore and F1 (Nelore x Angus) heifers reared in two forage production systems.

Intensive production systems require high-yield genetics as obtained in Bos taurus x Bos indicus crossbreeding. Generally, high-producing taurine cattle are more susceptible to parasites and heat stress. This study evaluated animal performance, heat-stress measurement (infrared temperatures) and internal parasite infection with daily weight gain in heifers from two genetic groups (Nelore and F1 - Nelore x Angus) reared in two forage production systems (with or without crop-livestock system) during one year. The main objectives were determine the relationship between infrared measures and animal performance and whether it differ between genetic groups and environments. Thirty-six heifers were randomly assigned to two forage production systems, one considered as high-input system with crop-livestock system and other exclusive livestock system considered as low-input. At each 28 days, infrared temperatures (IR), weight and internal parasite infection (fecal egg count) were measured. The temperatures of the eye, snout, forehead, dewlap, body, ground and squeeze chute were determined. F1 heifers had higher weight gain than Nelore (P<0.05) and both did not differ in internal parasite infection (P>0.05). F1 heifers had higher IR than Nelore (P<0.05). The main body points that differentiate between genetic groups were dewlap, forehead and eye. Higher dewlap IR temperature (DW) was associated with higher average daily gain (ADG) during dry season (independently of genetic groups) (ADG = -0.755 + 0.032*DW; R2=0.44). Otherwise, the IR temperatures had a negative relationship with ADG during rainy season and low forehead IR temperature was related to higher average daily gain (ADG = 1.81 - 0.033*forehead; R2=0.12 for F1 animals and ADG = 1.46 - 0.025*forehead; R2=0.07 for Nelore). The infrared temperatures were more related to animal performance during the dry season, which had high temperature and low humidity. The infrared temperatures were able to identify the animal response to the environment challenge. Animals with higher temperatures (dewlap and forehead) had higher daily gain during the dry season.