stress is a very general term.
It could mean in swine farming as anything that the animal do which uses energy that are unnecessary.
Things to consider:
In feeding time, do it quick as much as possible. Put the feeds in the feeding area as instant as possible. So they would not have time to battle for position.
Mouth breathing means the animal is exhausted from the heat or physically exhausted. Try to make the pen more cooler. Put some insulation on the ceiling etc.
Salamat po sa reply doc, napanood ko kasi sa Discovery channel na malaki pala ang epekto sa utak isang baboy kapag ito ay nai-stress. Hindi ko po kasi naumpisahan, ang inabot ko lang ay iyong experiment na nila na isang stress at isang hindi stress na baboy na pinalalangoy sa mini pool.