September 6, 2013 15:21
It may be hard to believe with the current warm temperatures but the first fall frost is lingering. After a first frost, there are concerns about nitrates in forage and other livestock feeds. So what are nitrates anyway, where do they come from and why can they be a concern in a feeding program?
Plant roots take up nitrogen from the soil in the form of nitrate (NO3). The nitrate is transported into the leaves where it is converted into protein. Immature plants usually contain more nitrate as they are still growing rapidly. Annual forage crops like oats and millet also normally contain more nitrates compared to perennial forages. Alfalfas, vetches, trefoils, peas and clovers generally do not accumulate nitrates. Under normal growing conditions, nitrates are continuously taken up and converted to protein to fuel plant growth and seed production.
Nitrates can accumulate in a plant following a sudden interruption of growth such as after a frost. The root system is often not affected by a fall frost and continues taking up nutrients from the soil which includes nitrates. As nitrates are being pumped into the plant, the aboveground tissue is not able to process those nutrients and hence nitrates can start to accumulate in stems and leaves. The highest accumulation typically occurs two to three days following a frost. Nitrates will decrease 10 to 14 days after the injury if the plant was not killed and is able to resume growth. If a plant is killed during a frost, the nitrates have no place to go and will remain in the plant tissue. To conserve forage quality it is best to harvest the forage crop within one day of damage. Forage should be tested to know how much nitrates it contains and to be able to adjust the feeding plan accordingly.
The nitrates themselves are not toxic to livestock. However, in ruminants such as cattle, sheep and goats, the rumen bacteria convert nitrate to nitrite (NO2). Nitrite is then converted to ammonia. Nitrate poisoning occurs when the conversion of nitrate to nitrite exceeds the animals’ capacity to convert nitrite to ammonia. Nitrite causes toxicity by reducing the capability of blood to carry oxygen which leads to internal suffocation of the animal. Sub-lethal doses may result in loss of appetite, reduced milk production, slow growth, and abortion.
For more information please contact me at the Watrous Ministry of Agriculture office 306 946-3219, call the Agriculture Knowledge Centre at 1-866-457-2377 or visit our website http://www.agriculture.gov.sk.ca/.
by Nadia Mori
Regional Forage Specialist