By: Kelli Anderson
Although much of North Dakota has received adequate moisture this summer, it is important to remember that there is still potential for nitrate toxicity in many forage crops, including small grains, brassicas, millet, sorghum/sudangrass and corn harvested for hay.
“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,” says Janna Block, North Dakota State University Extension livestock systems specialist at the Hettinger Research Extension Center. “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.”
“Many producers only associate nitrate toxicity issues with drought,” Block says. “However, drought is not the only environmental factor that can lead to nitrate accumulation. For example, high temperatures combined with adequate moisture can impact plant metabolism and cause nitrate to build up in plants.”
Frost, hail and disease can cause nitrates to accumulate due to reduced leaf area, which limits the rate of photosynthesis. The risk of nitrate toxicity also increases when high levels of nitrogen fertilizer have been applied.
Although nitrates typically are not an issue on rangelands, pastures with nitrate-accumulating weeds such as kochia, lambsquarter, pigweed, quackgrass and thistle also may be a problem. Controlling these weeds in grazing situations is one way to reduce risks, says Block. Nitrate toxicity is most commonly a problem in ruminants, with cattle more susceptible than sheep.
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 of nitrite in the rumen, 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. Cases of lower-level, chronic toxicity also can occur. In those cases, producers may observe weight loss, night blindness and abortions in their cattle.
“Producers should test nitrate-accumulating forages every year prior to haying or grazing,” Block says.
Many NDSU Extension offices have access to a Nitrate QuikTest, which is a screening tool to assess whether nitrate is present in standing forage. Extension agents who have been certified can conduct the test in a field or office setting. Producers should provide a representative sample of at least 20 stems by clipping them to ground level while traveling in a zigzag pattern across the field.
“If nitrates are present in the sample, producers should delay grazing or harvesting for several days and then re-test,” Block says. “Samples also can be submitted to a laboratory for quantitative analysis to further assist with management decisions.”
If planning to graze, it is a good idea to provide a full feeding of hay before turnout and observe cattle frequently for the first several days. Avoid turnout in the morning, when nitrate levels are highest. Sick or thin animals are more susceptible to nitrate issues and should not graze high risk forages. In addition, pastures should be stocked lightly enough that animals are not forced to eat the lower portions of stems, where nitrate accumulation is greatest. Providing several pounds of an energy supplement can help rumen bacteria convert nitrate to protein more efficiently.
“When harvesting forages for hay, one suggestion is to raise the cutter bar because the majority of nitrates accumulate in the lower one-third of the stem,” Block says. “Nitrate levels are typically greatest in early growth stages, so delaying harvest and allowing plants to mature can help reduce nitrate levels. However, this strategy must be balanced with obtaining desired forage quality and yield. In addition, species such as oats may maintain high nitrate levels up to and through maturity.”
The Nitrate QuikTest is not designed to evaluate nitrate content in harvested forages. The best testing strategy for forages that have already been cut and baled is to use a bale probe to collect core samples and submit them to a laboratory for analysis. Ideally, 10% of bales or at least 20 core samples per lot of forage should be collected. A lot is defined as hay harvested within 48 hours from the same field.
Nitrate concentrations do not decrease through time in stored forages because photosynthesis is required for conversion of nitrates in the plant. Ensiling can decrease nitrate content through fermentation, but samples still should be submitted for analysis after the fermentation process has taken place to determine accurate levels.
“Producers need to understand the potential risks of nitrate toxicity and the factors leading to nitrate accumulation in plants,” Block says. “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.”