Frost effect on plants
Some deciduous leaves can be deadly after a frost or after they have wilted due to broken branches, fall leaf shed or storm damage. Leaves that tend to be most toxic are those of red maple and cherry trees. Identify all such seasonally toxic trees on your property, and keep horses from their fallen or frost damaged leaves for at least 30 days.
Even though these leaves are not commonly eaten, horses can accidentally ingest them, especially if hungry or bored. Nitrate toxicity can also be an issue after frost with some nitrate-accumulating plants. Generally, this is only a concern with some grass species where high nitrogen has been used and with some weeds that are know to be nitrate accumulators like lambsquarter and pigweed. It is recommended that horse owners wait up to a week after a killing frost before grazing areas where nitrate toxicity is a concern. Prussic acid accumulation can also be an issue after a frost with some specific warm-season annual grasses like sorghum and sudan grasses, but these grasses aren't commonly grazed by horses or fed in horse quality hay.