Effects of High Incubation Temperature on the Body Weight and Yolk Consumption of Two Commercial Broiler Strain*

Background: Temperature is the most important factor controlling embryonic development. Hyperand hypothermic effects depend on the degree of temperature, duration of exposure and the stage of the development. From the middle of incubation period, embryo temperature is greater than air temperature of the incubator because of the increased metabolic activity. Thus elevated temperature especially in the second half of incubation has a major impact on hatchability and chick quality. The aim of this work was to investigate and compare the effects of high incubation temperature applied from the 10th day to the end of incubation on the body weight and yolk consumption of Ross 308 and Hybro embryos. Materials, Methods & Results: In the experiment, a total of 500 eggs, 250 fertile eggs from each of Hybro and Ross 308 broiler strains were used. The eggs of each strain were weighed and then divided into 2 groups as control and high incubation temperature exposed (heat-stress) groups. The control eggs of both strains were maintained under optimal conditions (incubation temperature: 37.8C) during the whole incubation period, whereas heat stress imposed eggs were maintained under 37.8C until the 10th day of incubation and then continuously exposed to high temperature (38.8C) in the incubator. Other environmental conditions were standardized. At the 11, 13, 15, 18, 20 and 21 days of incubation, randomly sampled eggs from each group were opened until 10 living embryos were obtained from each group. The embryos and their yolk sacs were weighted at the 11, 13, 15, 18 and 20 of incubation and chick weight with yolk sac were determined on hatching day (d 21 of incubation) of the groups. Mean relative embryo weights [(embryo weight/egg weight at setting) x 100] and relative yolk sac weights [(yolk sac weight/egg weight at setting) x 100] of each groups were calculated. In both strain, the heat stress group had a significantly lower yolk-free embryo weight than the control group. However relative yolk weights in heat-stress groups were found to be significantly higher when compared to those in the control groups. There was no significant difference between mean embryo and yolk sac weights of the control groups both strains, except for embryo weight at 20 d of incubation. We also observed that the chicks were heavier at the day of hatch in heat-stress groups than the control groups. Discussion: These observations suggest that high incubation temperature (38.8C) from the 10th of incubation decreased yolk consumption and depressed mean embryonic weights. Maternal antibodies are passively transferred from the hen to the offspring through the yolk and albumin. Therefore decreased consumption of the yolk due to high incubation temperature will not only affect the body weight, but may also negatively affect the immune status of the newly hatched chick. We also observed that body weight of the chicks at the hatch day was higher in the heat-stress group of both strains in comparison to their controls. Greater amounts of remaining of unused yolk in the heat-stress groups may be responsible for this difference. Results of the present study have revealed that the efforts to increase the body weight of broilers should not be limited to management and care at post hatch period, incubation factors affecting the performance should be determined and taken necessary precautions.