Nitrate Levels Could Be Unsafe In Heavily Fertilized Fields and Pastures
Livestock Update, September 2002
John B. Hall, Extension Animal Scientist, Beef, VA Tech
Many hay fields, pastures and corn crops may contain excessive amounts of nitrates due to drought conditions across the state. Fields fertilized with biosolids or heavy applications of poultry litter are especially dangerous. Nitrate levels in excess of 0.44 % nitrate ion or 1000 ppm nitrate N can be hazardous especially to pregnant cows. High nitrate concentrations in feeds can result in abortion of calves. If nitrate levels are high enough, death of cows or growing cattle can occur.
Animals that are poisoned by high nitrate levels die from inability of the red blood cells to carry oxygen. Animals usually die within 3 hours of ingesting lethal amounts of nitrate containing forage. Hemoglobin in red blood cells binds the nitrite ion (formed from nitrate) more readily than oxygen, which forms methemoglobin. Methemoglobin will not carry oxygen. Fetuses of pregnant cows are even more susceptible than their dam to the toxic effects of nitrates. Fetal hemoglobin binds nitrite ions more readily, so nitrate concentrations that would not hurt cows will kill the fetus.
Deciding whether to test or feed a forage depends on the probability of high nitrate levels. The following forages should be tested:
The Virginia Tech Forage Testing laboratory will analyze forages for nitrates for $ 5.00 per sample and provide feeding recommendations. Other commercial forage testing laboratories offer this service as well. Producers should consult with their county extension agent on using high nitrate forages.
There are few options with high nitrate forages except to limit intake. This is extremely difficult when feeding round bales. Rolling out limited amounts or limiting access to bales to only a few hours a day may work for forages in the 1000 - 1500 ppm nitrate N range. However, all it takes is one animal to eat too much and its too late.
The ensiling process reduces nitrate concentrations by 50 to 70 %. One management tool to reduce nitrate problems is to ensile forages instead of haying them. However, producers still need to test ensiled forages after the ensiling process is complete before making feeding decisions.
Clearly, the nitrate concentrations in forages will vary greatly from location to location due to the highly scattered nature of rainfall this summer. Fields fertilized the same way, but getting different amounts of rain may vary considerably in concentrations of nitrate. Producers should be extremely cautious this year with feeding hay or grazing pasture or crop residues from heavily fertilized fields. For more information or technical assistance with sampling procedures or interpreting nitrate tests, contact your local extension office.