Grazing oats or wheat after a frost can result in high nitrates. The recommendation is to wait 1 week after a killing frost to graze oats or wheat regrowth. Nitrates don’t seem to be an issue with alfalfa and frost.
Bloat blocks will be a good idea. The bloat risk goes up somewhat right after a killing frost, when the cell damage makes the plant more digestible. 1-2 weeks after a killing frost the plant gets safer and the bloat risk is less. For alfalfa, bloat seems to be the only real issue.
The big issue with early frosts is prussic acid, and that’s typically in sorghum – sudan grass plants. Nitrates somewhat in this type of plant too.
From what we can tell, the changes in the plant still take a little while, so getting them off the morning after a hard freeze should still be OK.
There are only a small number of plants that can contain prussic acid, and they are in the sorghum family. Common forages include sudangrass, sorghum, sorghum x sudangrass hybrids and Johnsongrass. The prussic acid is concentrated in the leaves of these plants, therefore grazing immature plants, or immediately following a frost, can be detrimental. The HCN is highly concentrated in young, immature and leafy plant growth, so beware of regrowth in fields that have been hayed or drought-stressed fields where growth has been stunted.
A general recommendation is that sudangrass should not be grazed until it reaches a minimum height of 18 to 20 inches-tall and hybrids should not be grazed until they reach 24 to 30 inches-tall; sorghums are generally unsafe for grazing until after plants are harvested or fully mature (dormant). Nitrogen fertilizer can also increase HCN levels, similar to nitrates.
If you plan to graze any of the plants in the sorghum family and the plants have suffered a killing frost, wait 10 days to two weeks before grazing. This will allow for the prussic acid to dissipate. If it was only lightly frosted, ensure that the plant height exceeds the recommendations previously discussed and beware of regrowth.
Russ Daly DVM, MS, DACVPM
State Public Health Veterinarian
Department of Veterinary and Biomedical Science
South Dakota State University