Studies aimed at producing stronger birds and with better growth performance are being developed involving artificial incubation. To that extent, thermal stimulation by the heat of 1.39 ?oC and 1?oC above the standard, and by cold of 36 ?oC fixed, were applied in the last week of the embryonic development (day 14 to day 18) to Cobb ?? strain birds of 33 to 53 weeks age. With the purpose of evaluating the behavior of the embryos facing these temperature stimuli to identify what is the best amplitude, frequency and the most appropriate period to perform temperature variation, obtaining more adapted birds to the field. It has been observed that heat or cold stimulation did not cause embryo mortality and did not affect the hatching and the quality of the chick for this strain negatively. The weight of the pullet, the residual weight of the yolk, and the peak of birth were not modified. The thermal stimulation during the last week of the birds' incubation is an available tool to improve hatchery production, quality and performance. However, several variables must be tested.
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