Frost warnings are being issued for tonight for several counties in northern Wisconsin. A frost this late in May is unusual and has many alfalfa, clover and grass growers concerned about damage to their crops. However, Dan Undersander, University of Wisconsin-Madison Extension and research forage agronomist says, a killing frost (32 degrees) will have no permanent affect on the hay crop.

"Alfalfa and clover would not be expected to have frost damage unless air temperature has been below 26 to 24 degrees for 4 hours or longer," Undersander says. He adds:

* Frost damage will be terminal to buds (tops) of the alfalfa and clover stems. If frozen, tips will wilt in a day or two and then gradually turn brown.

* Further growth of the stand will cease so it might as well be harvested to allow regrowth of second cutting.

Grasses will suffer frost damage in the same temperature range however yield response will be minimal, Undersander notes.

"The leaf tips, if frosted will gradually turn brown but, since the growing point is at the leaf base near the stem, future growth will not cease unless the entire leaf is frozen," he says. "Seed set will be affected but hay or haylage should be harvested before this is a consideration."

So watch for wilting of the alfalfa or clover one to two days after the last frost.

"If it occurs, harvest as soon as possible," Undersander says. "If frost occurs in grasses, do not harvest unless the entire top growth is frozen."