Opposite monkey paw situation, as cows are no longer farmed due to the poison risk, global emissions are massively reduced. And the huge amounts of land dedicated to feeding cows is returned to forestry, further reducing emissions.
The poisonous cows solve climate change long before the vegans can.
According to the USDA, the US cattle industry contributes about 3.7% of US greenhouse gas emissions, and livestock as a whole is around 14.5% globally on a cursory search