For three or four days after a frost, sudan grass and sorghum-sudan crosses should not be pastured. A poisonous substance called prussic acid is released from the leaves when the cell walls of the plant are frozen. Grazing can begin again as long as the plants do not send out new shoots from the base. These new shoots would be high enough in prussic acid to kill cattle.
Since cattle will frequently prefer new shoot over older green or dried materials, it is recommended that grazing not be permitted until new shoots reach 15 inches, or until three to four days after the regrowth is also killed by a freeze.
Frost has little effect on alfalfa. Hungry cattle might be more prone to bloat when turned out on fresh alfalfa, but that is easily managed by feeding the cattle before turning then loose on a new food source.
Jeff Burbrink is an Extension educator in agriculture and natural resources. Write to him at 17746 E. C.R. 34, Goshen, IN 46528; or call 533-0554.