What happens to all those animals?

Covid-related slaughterhouse shutdowns in the US are leading to fears of meat shortages and price rises, while farmers are being forced to consider “depopulating” their animals.
More than 20 slaughterhouses have been forced to close and at least two million animals have already reportedly been culled on farm. That number is expected to rise. 
It has been reported that veterinarians and government officials would be ready to assist with culls, or “depopulation”, if alternatives could not be found.
But how do you use "euthanase", "depopulate", "kill" millions of animals? Take poultry for example, approved methods for slaughtering poultry includes slow suffocation by covering them with foam, or by shutting off the ventilation into the barns.
"Foaming" means covering hens with a layer of foam that blocks their airways, gradually suffocating them over several minutes. " Ventilation shutdown", meanwhile, although described by the AVMA as “not preferred”, is  "one of the cruellest, but cheapest options".
"Depopulation" (or whatever you choose to call it) or starvation – neither of them are great options for the animals, or their human caregivers.
But if you were the decision-maker, how would you do it differently?
And how would the current situation be different if, instead of just focusing on preventing and animals justified pain, distress or suffering ("negative states/experiences"), the law required people to take account of their positive states (e.g. comfort) as well? – would the "cheapest option" still trump?