Comments
Feed costs usually constitute between 50% to 70% of the total livestock production costs. Reducing the costs of farming crops used for feed (like soybeans and corn) would likely make animal farming more efficient and hence increase the number of farmed animals.
I guess that increasing the efficiency of crops farmed directly for human consumption would barely impact the number of farmed animals. Maybe people would produce and consume more non-meat products because they are more profitable and/or cheaper. Hence, they might consume a bit fewer animal products. But maybe people would afford to buy more animal products because other food is cheaper. I guess that the former effect would be a bit higher but ¯\_(ツ)_/¯ Whether it's net-positive would likely mostly depend on other factors that have little to do with the welfare of farmed animals in the short term. E.g., it might increase the human population a little bit.
Rather than farmers investing more profits from growing plants into animal farming, I think the main avenue of harm is that animal feed is an input to meat production, so if the supply of feed increases, production of meat would increase.