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Understanding behavior patterns of cattle adaptation to heat stress
Changes in climate may have negative effect on livestock that will have to adapt to more extreme environment in the near future. Therefore understanding how different breeds developed mechanisms to thrive under extreme conditions may help livestock production. This research compared the movement, ruminating time and weight gain between two breeds, Nellore (indicine, n=24, 210±15 kg) and Angus (taurine, n=24, 227±25kg) aging 10 months and kept for 80 days at pasture during tropical spring (average temperature 26°C, maximum 32.2°C, minimum 21.5°C). Animals were sons of the same bull (Nellore or Angus) to minimize genetic variation. Concentrate supplement was provided twice a day (1.5% LW/day), netted shade was available (80% sun block, 10 m2/animal). Animal motility (using an accelerometer, measured relative movement-RM) and rumination time (minutes/day, using a sound sensitive sensor) were evaluated through a collar-sensor (SCR, Systems Heat time) by radio telemetry. Live weight (kg) was measured with a chute scale every 15 days. Data were compared by unpaired t test, two way Anova and Pearson correlation analysis.
We observed that Angus remained under the netted shade during daytime and grazed after sunset, Nellore on the other hand stayed on the sun despite the available shade. Nellore (781±16 RM) moved less (P < 0.001) than Angus (919±17 RM) but ruminated for longer periods (P <0.0001; Nellore: 496±9 min/day) compared to Angus (370±8 min/day). Nellore (268±27 kg) were lighter (P = 0.0003) than Angus 303±24 kg) at the end of the 80 days period, average daily gain was lower (P = 0.029) in Nellore (0.65±0.04 kg/day) compared to Angus (0.85±0.05 kg/day).
In summary Nellore moved less and ruminated more compared to Angus. It is possible that a reduction on movement is part of the Nellore adaptation to heat stress. By being less active Nellore produces a lower amount of heat to be dispersed but in order to move less, it selects the grass ingested in a worst way. The consequence is a need to ruminate more that can be done without much muscle activity and less heat production. Despite of moving more and ruminating less, Angus animals were more efficient than Nellore considering average daily gain. Samples from blood, muscle, skin, liver and semen were collected to be evaluated for metabolic, gene expression and epigenetic modifications induced by heat stress.
Keywords: Nellore, Angus, Time ruminating