Effect of frost on green feed

An Alberta Agriculture specialist says that producers need to be wary of an early frost and the effect it can have on green feed.
"Nitrates accumulate over time in a frost damaged crop," says Barry Yaremcio, beef/forage specialist, Alberta Agriculture and Rural Development. "Typically, the highest accumulations occur four to five days after frost. It is best to cut or harvest the crop within one day of a light or injuring frost."
Nitrate is the form of nitrogen that plant roots take up from the soil. It is transported to the leaves where it is eventually converted into protein. Frost can interrupt this process allowing excess nitrates to accumulate in an injured plant.
"The plant's root system doesn't realize the top has been injured, and continues to push up nitrogen as normal" explains Yaremcio. "However, the damaged top cannot use these nitrates, and they begin to accumulate in the plant."
Nitrate levels will gradually decline 10 to 14 days after the injury as the plant resumes growth and repairs itself. Plants killed by the injury or stress will not be able to decrease their nitrate levels.
"A killing frost completely destroys the entire system that moves nutrients up into the plant," says Yaremcio. "As such, a killing frost is better than a light one, as it eliminates the problem of nitrate accumulation."
Yaremcio says the same rules apply for swath grazing. "If the crop is not mature enough for combining and it's a light frost, you want to swath it within a day or two. If it's a killing frost, watch the leaves. You want to delay the cutting if the weather is poor. You need to cut the crop when you see the leaves starting to drop off, in order to save the quality."
For more information, Alberta Agriculture has a number of documents on its website at www.agriculture.alberta.ca, including Nitrate Risk in Forage Crops. Producers can also call the Ag-Info Centre at 310-FARM.