Forages typically are the major component of beef cattle diets, and some may potentially contain toxic levels of nitrate, according to North Dakota State University (NDSU).
“Although these forages are a great option for haying or grazing, they could pose a risk of nitrate toxicity for livestock,” said Janna Block, livestock systems specialist at the NDSU Hettinger Research Extension Center.
Nitrate toxicity is a potential issue for livestock consuming small-grain forages (wheat, oats, rye, triticale and barley), brassicas, millet, sorghum and sudangrass and standing corn or corn used for hay, she said. Although nitrates typically are not an issue on rangelands, pastures with nitrate-accumulating weeds such as kochia, lambsquarters, pigweed, quackgrass and thistle also may be a problem. Nitrate toxicity is most commonly a problem in ruminants, with cattle being more susceptible than sheep.
“Nitrate is a common form of nitrogen found in the soil, which is taken up by plants and converted to protein through the process of photosynthesis,” Block explained. “Under normal growing conditions, nitrate does not accumulate in the plant. However, when plants encounter stressful growing conditions, photosynthesis is inhibited, and the potential for accumulation of nitrates is increased.
“In general, people associate an increased risk of nitrate toxicity with drought,” she added. “The current U.S. Drought Monitor for North Dakota indicates that over half of the state is abnormally dry for this time of year, mainly in the western counties.”
Block noted, “It is important to recognize that drought is not the only condition that can lead to nitrate accumulation. Prolonged cool temperatures and cloudy conditions also can disrupt the conversion process and cause nitrate to build up in plants. Additionally, nitrates may accumulate due to conditions that reduce leaf area and limit photosynthesis, such as frost, hail or disease.”
When beef cattle consume increased quantities of nitrate, it overwhelms the ability of rumen microbes to convert nitrate to protein. This results in a buildup in the rumen of nitrite, which is 10 times more toxic than nitrate.
Excess nitrite is absorbed into the bloodstream, which removes the blood’s ability to carry oxygen and causes the animal to suffocate, Block said. Cases of lower-level, chronic toxicity also can occur. In those cases, producers may observe weight loss, night blindness and abortions in their cattle.
Block provided several strategies to reduce the risk of nitrate toxicity:
- If applying nitrogen fertilizer, divide the total application into two or more treatments.
- Control potential nitrate-accumulating weeds in pastures.
- Avoid cutting forage or allowing cattle to graze it in the morning, when nitrate levels are at their highest.
- Consider raising the cutter bar when harvesting forage, because the majority of nitrates accumulate in the lower one-third of the stem.
- Consider delays in harvesting to allow plants to mature, because nitrate levels are typically greatest in young plants, although keep in mind that mature plants still can contain excess nitrate, and this strategy can result in decreased forage quality.
“Producers planning to graze nitrate-accumulating forages should take additional steps to minimize risks,” Block said. “Nitrate concentration can be extremely variable within areas of a field, and predicting and managing grazing animals’ intake is difficult.”
Additional steps to help reduce nitrate risk include:
- If possible, avoid grazing by pregnant, sick or thin animals due to increased susceptibility.
- Stock lightly so animals can select leaves and are not forced to eat the lower portions of stems.
- Ensure that cattle receive a full feed of hay before turnout, and observe cattle frequently for the first week or so of grazing.
- Provide energy supplements to help rumen bacteria convert nitrate to protein.
“The most important recommendation is to test for nitrates prior to grazing or haying,” Block said.
“If nitrates are present in the sample, producers should delay grazing or harvesting for several days and then re-test,” Block said. “Samples also can be submitted to a laboratory for quantitative analysis to further assist with management decisions.
“Producers need to understand the potential risks of nitrate toxicity and the factors leading to nitrate accumulation in plants,” she said. “Determining actual levels of nitrate present in grazed and harvested forages hay is critical to be able to utilize these feedstuffs in a safe manner.”