As drought conditions intensify, some North Dakota farmers are harvesting drought-stressed crops for livestock forage. In some areas, small grains already have been harvested as hay.
While these crops can provide valuable forage for livestock producers, North Dakota State University Extension specialists warn that drought-stressed forages can have elevated nitrate levels that may pose a risk of poisoning.
Nitrate accumulation is influenced by factors including moisture, soil conditions and plant type. Oats, barley and corn account for most nitrate poisoning cases in North Dakota, but wheat, sudangrass, sorghum-sudan hybrids, turnips and pearl millet also can accumulate nitrates.
Producers considering low-yielding crops as livestock forage should have them tested for nitrates before feeding.
Not all drought conditions result in high nitrate levels. Plants need some soil moisture to absorb and accumulate nitrate. Plants that survive drought may have high nitrate levels for several days following the first rain.
Severely drought-stressed areas, such as hilltops with very sandy soils, may contain plants that look unhealthy but are not accumulating significant nitrate unless a recent rain event has provided moisture. Nitrate hot spots tend to occur in areas that have received moisture and contain stunted but lush, green plants.
Stress between plant jointing and heading is associated with greater nitrate accumulation. Fields that have received nitrogen fertilizer also are more susceptible to nitrate accumulation.
Nitrate toxicity generally is not a concern on rangelands. However, pastures containing nitrate-accumulating broadleaf plants such as kochia, pigweed and Russian thistle can pose a risk because the plants typically remain green and may be consumed by livestock.
Plant parts closest to the ground contain the highest nitrate concentrations. Leaves generally contain less nitrate than stalks and stems, while seeds and flowers typically contain little or no nitrate.
The risk can be reduced during haying by raising the cutter bar above 6 inches. When grazing, livestock should be monitored and removed when plant stubble reaches 6 inches.
Ruminants such as cattle and sheep are particularly susceptible to nitrate poisoning. Their digestive systems convert nitrate to nitrite, which is then converted to ammonia. When nitrate levels are high, nitrite can accumulate and be absorbed by red blood cells, reducing their ability to transport oxygen.
A similar reaction occurs in horses' hindgut, but to a much lesser extent. As a result, nitrate poisoning is very rare in horses.
Clinical signs of nitrate poisoning are associated with a lack of oxygen in the blood. Acute poisoning can occur 30 minutes to four hours after livestock consume toxic levels of nitrate. Symptoms can include bluish or chocolate-brown mucous membranes, rapid or difficult breathing, a rapid pulse of more than 150 beats per minute, salivation, bloating, tremors, staggering, dark chocolate-colored blood, weakness, coma and death.
If nitrate toxicity is suspected, cattle should not be moved or stressed because their blood oxygen levels may be dangerously low. In many cases, dead animals may be the first indication of a problem, making it important to contact a veterinarian to rule out other potential causes of sudden death.
Pregnant females that survive nitrate poisoning may abort because of a lack of oxygen to the fetus. Abortions generally occur 10 to 14 days after exposure.
Laboratories can analyze suspected plants for nitrate levels, but samples need to be representative of the field or bales in question. Samples should be packaged in a clean plastic bag and shipped to a laboratory. Producers can contact an NDSU Extension agent for information about nitrate testing.
The best way to prevent nitrate poisoning is to control the type and quantity of forage offered to livestock. Forages with potentially toxic nitrate levels should be avoided or diluted with feeds that are low in nitrate.
When grazing, producers should feed dry roughage first to reduce the number of affected plants that hungry livestock consume. For baled forages, representative core samples should be taken from bales that represent the entire field.
Producers also should avoid overstocking pastures when grazing high-nitrate forages. Overstocking can increase the amount of high-nitrate plant parts, particularly stems and stalks, consumed by livestock.
Nitrate content in small-grain forages can be reduced by up to 50% through chopping and ensiling. Silage should be allowed to ferment for 21 days and should be tested before feeding. Producers should dilute high-nitrate silage with other feeds as needed.
Source: North Dakota State University, "Drought brings increased risk of livestock nitrate poisoning"