Drought stressed crops & nitrate toxicity

As drought conditions continue to expand across South Dakota, a number of producers will be considering harvesting failed grain crops as forage as a way to salvage some value from those fields. Besides considering crop insurance and other considerations, another factor that has to be considered during the process is the potential for nitrate toxicity.

All plants contain nitrates, but when plant growth is slowed because of drought or hail, nitrates can accumulate to levels high enough to cause losses, especially when high rates of nitrogen fertilizer have been applied. Rumen microorganisms can convert nitrates into microbial protein, but if the intake of nitrates is too high, then toxic levels of an intermediate chemical (nitrite) are absorbed into the bloodstream. At high enough levels of nitrite, the oxygen carrying capacity of the red blood cells is reduced to the point that the animal will asphyxiate.

High-nitrate forage management

So can high nitrate forages be utilized? The answer is yes, if we know what the levels of nitrates are in the feedstuffs in question. Once the nitrate level is known, management decisions can be made about the best harvesting and feeding techniques to best utilize that roughage.

In most cases, laboratories will report nitrate test results as percent nitrate nitrogen on a dry matter basis. The table below contains usage guidelines for nitrate levels in forage for mature cattle.

Table 1. Guidelines for nitrate levels in forage for mature cattle.

Percentage content of nitrate nitrogen
(dry matter basis)

Comments

0.0 - 0.1

Safe to feed if adequate feed and water are available.

0.10 - 0.15

Safe for non-pregnant animals. Limit to 50% of ration dry matter for pregnant animals.

0.15 - 0.20

Limit to 50% of total ration dry matter for all animals, may experience some symptoms of nitrate toxicity.

0.20 - 0.35

Limit to 35 to 40 % of ration dry matter.
Do not feed to pregnant animals.

0.35 – 0.40

Limit to 25% of total ration dry matter.
Do not feed to pregnant animals.

Greater than 0.40

Toxic. DO NOT FEED.

Adapted from Nitrate Poisoning of Livestock: Causes & Prevention, 2012.

 

Sometimes there is confusion regarding the differences between nitrate toxicity and prussic acid poisoning. Sorghum, sudangrass, and sorghum-sudangrass hybrids can contain prussic acid as well as potentially nitrates. The greatest risk of prussic acid poisoning typically occurs following a frost, but could be an issue in the case of drought-stunted plants.

Additional considerations

Some other factors to consider when dealing with high nitrate feeds include:

  • The highest concentrations of nitrates are in the lower parts of the plant, so cutting higher than normal will help reduce the amount of nitrates in harvested forage.
  • Pregnant cattle are much more sensitive to high nitrates. If possible, save low nitrate feeds for cattle in late gestation and direct high nitrate feeds to growing or finishing cattle.
  • When diluting high nitrate feeds, make sure that the forages are mixed evenly to minimize the chance of individual animals ingesting too much nitrate. Self-feeding high nitrate hay is dangerous for the same reason.
  • Increased feeding frequency and including grain in the ration are both ways to reduce the toxic effects of nitrates.
  • Give cattle the opportunity to adapt high nitrate feedstuffs by gradually increasing the amount in the diet.
  • Consider ensiling the feedstuffs when possible. Research has shown that fermentation process can reduce nitrate concentrations by about one third. Because this response can vary, it is a good idea to check ensiled forages at least 4 weeks after harvest to determine the final nitrate levels.