Forage Nitrate Toxicity a Major Concern as Drought Worsens

(Image Source: University of Missouri Extension)

Weather conditions across Ohio have been challenging this growing season with some areas of the state reaching a D3 drought status. Other areas of the state may not currently be under drought status but are drier than normal and at risk of quickly experiencing a flash drought. These adverse growing conditions can cause unforeseen challenges with forages. We have had multiple reports of high nitrate levels this year in early harvested summer annual forages as producers needed feed.

Plants readily take up nitrates from the soil, even under dry or cool conditions.  Once in the plant, nitrate is converted to nitrite, then ammonia, and finally into amino acids and plant protein. Any environmental stress that significantly slows down plant photosynthesis and metabolism can lead to excessive nitrate levels in the plant because the nitrate uptake from the soil will be faster than its metabolism into plant protein. Such stresses include drought, frost, extended cold weather, cloudy conditions, or hail damage. While frost is a concern for increasing nitrates in forage a few months from now, the sorghum family also has prussic acid concerns when plants die quickly because of a frost. Prussic acid and nitrate poisoning are not the same.

The highest level of nitrate accumulation in corn occurs from V6 through pollination. While all drought-stressed corn can be at risk of high nitrate levels the greatest risk is in corn that was not drought-stressed from V6 through pollination and then became drought-stressed. This corn took up much of the nitrogen that was applied to the field but did not produce grain to utilize that nitrogen.

When ruminants consume excessive levels of nitrate in their diet, the nitrate is converted to nitrite by rumen microbes faster than it can be converted to ammonia, amino acids, and eventually plant protein. Accumulated nitrite in the rumen is then absorbed into the bloodstream where it prevents oxygen transport, which leads to death. Livestock sensitivity to nitrates ranked from highest to lowest is: pigs > cattle > sheep > horses. Older or sick animals are generally more sensitive than young healthy animals. The fetus in pregnant animals is very sensitive to high nitrates ingested in the diet.

One of the common solutions for forages that have slightly elevated levels of nitrates is to mix them with another forage source that is low in nitrates. The best way to do this is to truly mix the two forages so that your cattle eat both at once as a balanced lower nitrate diet. When this is not possible, feed the low nitrate forage first, allowing them to fill up on it, then offer the higher nitrate forage keeping them full for the day alternating forages each feeding. This year it may be important to test those dilution forages to be sure they are truly low in nitrates. Nitrate levels in forage are commonly reported in 3 different ways, ppm NO₃ DM, percent NO₃, and ppm NO₃-N (DM). Table 1 below summarizes how to interpret the results.

Table 1: Interpretation of nitrate forage test results.

Any time forage growth has been significantly slowed due to dry conditions, extended cold nights, cloudy weather, or premature plant death, nitrates may be an issue. All these stresses can lead to higher nitrate levels in plants due to slowed growth. Nitrogen fertilizer or manure applications made to forages increase the risk for higher nitrate levels in plant tissue, especially if excess nitrogen is available and forage growth is slow.

Nitrate accumulation is possible in many forage species, including all cool-season perennial forage grasses, alfalfa, all cereal forages (oat, rye, triticale, wheat, barley, spelt, etc.), and brassicas (might be present in cover crop mixes). Nitrates can also accumulate in warm season annuals (corn, sorghum species, millet, and many weeds). Weed species are heavy nitrate accumulators, including lambsquarter, pigweed, dock, some mustard species, johnsongrass, horse nettle, nightshade, quackgrass, and jimsonweed. Heavy infestations of those weeds when harvested with the forage will increase the risk of nitrate toxicity.

Nitrate levels are generally higher in younger than more mature growth. Delaying forage harvest to the dough stage and other forages to flowering/heading stages can significantly reduce nitrate levels. Cutting height can also affect levels as nitrates accumulate in the lower one-third of plants more than in the upper two-thirds.

Plant nitrate concentrations are higher in the morning than later in the day (plant metabolism during daylight drives the conversion of nitrate to plant protein). Mowing hay late in the afternoon on a sunny day can reduce nitrate levels in forage. Once hay is mowed, nitrate levels do not change much during the drying process, so dry hay levels will be similar to levels at the time you mow. Prior to mowing, nitrate levels vary across the field based on plant growth and variable soil nitrogen. This variability increases even more in a field based on mowing time. If we start in the morning and mow all day, the evening mowed forage should have lower nitrate levels.

However, ensiling can reduce nitrate levels from 10-65% provided fermentation is good. But if the forage is initially very high in nitrates, the silage could still contain toxic nitrate levels, so this is not an automatic fail-safe option. Be very cautious as high nitrate forages ferment, the bacteria break down the nitrate and release deadly nitrogen gas. Nitrogen oxide gases are heavier than air, may be reddish or yellow-brown in color, and have a bleach-like smell. Nitrogen oxide gases will accumulate in low-lying places, such as around the base of a silo or in the feed room below a tower. When ensiling forage that may have high nitrate concentrations, do not enter the silo for at least three weeks after harvest. If you must enter the silo to level or cover the silage, do it immediately after filling and leave the blower running while anyone is in the silo. If you usually run the blower for an hour prior to entering the silo, it may be necessary to run it for 2 hours to be sure the gas is cleared and fresh air is present.

Silage must be harvested at the proper moisture for complete fermentation (Table 2). When forages are harvested too dry, they do not ferment properly and nitrate reductions will be less. Baleage is often harvested on the drier side, and even when it is harvested in the ideal moisture range, reductions will not be as much as well-packed silage. Baleage densities are much lower than properly packed silage, so the additional oxygen slows fermentation. If nitrate levels are reduced in silage in about 3 weeks, it will take 6 or more weeks for levels to be reduced in baleage. Since nitrate levels can vary across a field, the harvested forage can be quite variable in nitrate concentration.

Table 2. Ideal moisture content for forage stored as silage or baleage.
The bottom line is that if you suspect the forage could be high in nitrate, the safest thing to do is to sample the forage and have it tested before it is harvested, because if levels are high, you can delay harvest to reduce the levels. You should certainly sample the stored forage before feeding it if you suspect higher levels! Call your forage lab and follow their guidelines closely for sampling the forage, packaging, and shipping the sample to them.

Author: Jason Hartschuh, Dairy Management and Precision Livestock, Field Specialist

Originally Published on the OSU Sheep Newsletter (https://u.osu.edu/sheep/)