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.
Russ Daly DVM, MS, DACVPM
State Public Health Veterinarian
Department of Veterinary and Biomedical Science
South Dakota State University