[IMR 4350]

If it was in the lungs it was probably mold from something they were fed.

Cows and horses are no different than people when they breath in mold and grain dust.

Lungs fill with fluid.

One of the reasons for putting molasses on oats for horse feed, keeps the dust down so they don’t breath it in.

[PA Engineer]

If it was in the lungs it was probably mold from something they were fed.

Very high probability.

My first suspicion would be clover or moldy hay.

Hydrogen Sulfide would be low on my list. Haven't heard of issues in the states, however this was a big problem in Saudi Arabia.