Responses of pigs divergently selected for cortisol response or feed efficiency to an ACTH challenge

Selection for feed efficiency can impair the animal ability to respond to stress. A key driver of this response is the hypothalamo–pituitary–adrenal (HPA) axis, which releases cortisol in response to stressors. Injecting a normalized dose of adrenocorticotropic hormone (ACTH) to stimulate the cortisol release by the adrenal cortex is a standardized methodology to evaluate the activity of the HPA axis independently from the animal perception of a stress. It has been used to select during three generations two lines (HighCortisol and LowCortisol) with divergent cortisol levels one hour (H1, peak of the response) after injection. A trial was set up to compare the responses to ACTH of these two Cortisol lines and those of two lines divergently selected for residual feed intake (RFI, measure of net feed efficiency) during nine generations. A total of 48 pigs per line was tested at six weeks of age. Blood samples were collected at H0, H1, and four hours after injection (H4). In the Cortisol lines, the cortisol was 20% higher in the highCortisol pigs than in the lowCortisol pigs at the 3 times, and at H1 cortisol was 25% higher compared to H0, whereas H4 was not significantly different from H0, as in earlier studies. The H0 level was numerically higher in lowRFI (more efficient) pigs (P=0.08) than in the highRFI line (less efficient). The highRFI line had a similar response to the ACTH injection as the Cortisol lines. The lowRFI line had higher cortisol at H1 than highRFI pigs (P=0.0002). This difference was higher at H4 (P<0.0001), due to greater H4 than H0 cortisol levels in lowRFI pigs (P<0.0001). Higher cortisol levels in the lowRFI line (P<0.0001) do not validate the hypothesis of decreased HPA axis activity associated to increased feed efficiency, and different dynamics of responses were observed. Blood counts, urea, glucose, free fatty acids (FFA) and IGF1 (H0 only) will be used to describe the metabolic responses of the lines to ACTH. This study is part of the Feed-a-Gene Project, funded from the European Union’s H2020 Programme under grant agreement no 633531.