Plant-based diets have the potential to dramatically reduce farm animal suffering, but the welfare impact of dietary choices depends on which animals are affected and how.
The welfare arithmetic of dietary change is powerful: reducing demand for animal products directly reduces the number of animals born into production systems. A person who eliminates chicken from their diet prevents the suffering of the most numerically significant farm animal. However, diet change is not binary — partial reductions, shifting to higher-welfare products, and choosing animal products with lower welfare impact (bivalves over chicken) all represent meaningful welfare improvements. Evidence-based welfare-motivated dietary choices can have substantial aggregate effect.