The tolerance of three temperate breeds of broilers to tropical heat condition was evaluated at the poultry unit of Department of Animal and Environmental Biology, Adekunle Ajasin University, Akungba-Akoko, Nigeria. One hundred and fifty (150) day-old broiler chicks comprising of Arbo acre, Cobb and Marshall breeds were raised under the same management condition for 8 weeks.Thermotolerance traits were measured and mortality of the broilers were recorded. Analysis of variance showed that Cobb broiler strain had the least cloacal temperature (40.89±0.03oC) followed by Arbo acre (41.06±0.05oC) while Marshall Broiler strain had the highest cloacal temperature (41.55 ± 0.06oC). Cobb strain also had the lowest respiratory rate (51.16 ± 0.15 breath/min).The respiratory rate of Marshall strain (56.80± 0.45 breath/min) was significantly higher than that of Arbo acre strain (54.63 ± 0.24 breath/min) . Marshall Strain had the highest pulse rate (73.50 ± 0.70 beats/min) while the least pulse rate (66.40 ± 0.47 beats/min) was found in Cobb breed. The least heat stress index was recorded for Cobb breed. The effect of strain was also significant on the mortality levels of the broilers. Marshall Strain had the highest total mortality percentage followed by Arbo acre while the least total mortality percentage was recorded for Cobb breed. The effect of age was significant on all the parameters studied (p<0.05).Young broilers were less tolerant to heat stress than older broilers. Mortality percentage also reduces with advance in age. The effect of sex was also significant on the thermotolerance traits. Females’ broilers were less tolerant to thermal stress. In conclusion, there was genetic variation in the thermotolerance traits of broilers. Cobb strain tolerated heat stress better than Arbo acre and Marshall Strain in the humid tropics.
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