Rvc Open Access Repository – Copyright Notice Title: Higher Levels of Co2 during Late Incubation Alter the Hatch Time of Chicken Embryos Higher Levels of Co2 during Late Incubation Alter the Hatch Time of Chicken Embryos Introduction

15 1. It has been reported that the increasing CO2 tension triggers the embryo to pip the air cell and 16 emerge from the egg. However, the mechanism by which higher CO2 concentrations during the 17 last few days of incubation affect chick physiology and the hatching process is unclear. This study 18 investigates the effect of CO2 concentrations up to 1% during pipping, on the onset and length of 19 the hatch window and chick quality. 20 2. Four batches of Ross 308 broiler eggs (600 eggs per batch) were incubated in two small scale 21 custom built incubators (Petersime NV). During the final three days of incubation, control eggs 22 were exposed to a lower CO2 concentration (0.3%), while the test eggs experienced a higher CO2 23 concentration program (peak of 1%). 24 3. There were no significant differences found in blood values, select organ weight and body weight. 25 There was also no difference in hatchability between control and test groups. However, a small 26 increase in the chick weight and the percentage of first class chicks was found in the test groups. 27 Furthermore, plasma corticosterone profiles during hatching were altered in embryos exposed to 28 higher CO2; however they dropped to normal levels at day 21. Importantly, the hatching process 29 was delayed and synchronised in the test group, resulting in a narrowed hatch window (HW) which 30 was 2.7 hours shorter and 5.3 hours later than the control group. 31 4. These results showed that exposing chicks to 1% CO2 concentration during pipping did not have 32 negative impact on physiological status of newly hatched chicks. In addition, it may have 33 significant impact on the physiological mechanisms of controlling hatching and have benefits for 34 health and welfare of chickens by reducing the waiting time after hatching. 35