Nitrate accumulation can become a problem when crops experience light frosts of -1 C to -2 C for even only a few hours during the night.
These conditions damage the leaves of the plants, but not the roots. Over the next three to four days, the roots continue to send nutrients up the plant, and the damaged plant is unable to use those nutrients, resulting in nitrate accumulation.
“When we get a heavy frost of -5 C to -6 C for six or seven hours, the internal working system of the plant is completely destroyed and it can no longer move water or nutrients,” said beef and forage specialist Barry Yaremcio. “A killing frost means that the plants are dead and therefore nitrates won’t accumulate.
“It’s the light frosts experienced for only a few hours that damage the plant but not the internal bundles that are still able to move water and nutrients up the stem. The injured leaves can’t use the nutrients effectively, and that’s when there is the greatest chance of nitrate accumulation.”
Annual crops are the most susceptible to nitrate accumulation, oats being the worst, but also barley and wheat. Immature salvage canola crops cut for silage or greenfeed also has a tendency to accumulate nitrates.
“Alfalfa is a legume and the nodules attached to the roots tend to hoard the nitrogen and only release as much as the plants require,” said Yaremcio. “Nitrate accumulation is extremely rare in alfalfa.”
Application of nitrogen fertilizer or manure also increases the risk of nitrate accumulation.
“If fields have been used for swath grazing, winter feeding areas, have had high manure applications or high amounts of fertilizers applied in the spring, those fields are more susceptible to accumulate nitrate in the plants,” said Yaremcio. “If there’s been no fertilizer applied, or if it’s an old grass field, those fields are not typically a problem.”
After a frost, the timing for cutting the field is a key factor in managing nitrogen accumulation. “If producers can get out in the field the day after the frost and cut it as quickly as they can, there shouldn’t be a problem,” said Yaremcio. “Nitrate levels increase and peak on the third or fourth day after a frost.”
If there is time for the plants to recover, and no subsequent killing frost, then nitrate levels will decline and return to normal after 14 days.
“It’s either get out there the day after the frost and cut very quickly, or wait 10 to 14 days before cutting the field,” said Yaremcio.