Nitrate toxicity from hay & forages is also a consideration
Drought, excessive soil nitrogen, shade, frost, certain herbicides, acid soils, low growing temperatures and nutrient deficiencies can contribute to high nitrate levels in plants. Stems usually have higher nitrate content than leaves.
I know in our parts we sometimes get early frosts sometimes before people finish their haying. The frost causes spikes in nitrogen levels of the alfalfa,grasses or greenfeed crops.The high nitrates can cause problems with pregnant mares