The method of mass killing are quite brutal too. In the last outbreaks, primarily these were the two primary methods:

Ventilation shutdown (VSD) is a means to kill livestock by suffocation and heat stroke in which airways to the building in which the livestock are kept are cut off. It is used for mass killing — usually to prevent the spread of diseases such as avian influenza. Animal rights organizations have called the practice unethical. The addition of carbon dioxide or additional heat to the enclosure is known as ventilation shutdown plus (VSD+).[1][2][3][4][5]

https://en.wikipedia.org/wiki/Ventilation_shutdown

Foam depopulation or foaming is a means of mass killing farm animals by spraying foam over a large area to obstruct breathing and ultimately cause suffocation.[1] It is usually used to attempt to stop disease spread.[2] Foaming has also been used to kill farm animals after backlogs in slaughtering occurred during the COVID-19 pandemic.[3] Foam depopulation has been used on poultry and pigs and has seen initial research for use on cattle.[4] It has faced criticism from some groups. Some veterinarians have called it inhumane,[5] along with many animal rights and animal welfare organizations who cite the pain caused by suffocation or the harm experienced by the stray survivors.[6][7]

https://en.wikipedia.org/wiki/Foam_depopulation