The Enid News and Eagle, Enid, OK

July 20, 2013

Test for nitrates and prussic acid in forage

By Rick Nelson, Extended Forecast
Enid News and Eagle

ENID, Okla. — Summer typically brings periods of heat and drought that impact forages grown for hay and grazing for beef cattle.

When these plants are stressed, forages such as Johnson grass and forage sorghums are prone to accumulate dangerous concentrations of nitrates or prussic acid, both toxic to the cattle that ingest them.

High-nitrate plants, either standing in the field or fed as hay later in winter, can cause abortion in pregnant cattle or potentially death to all cattle if consumed in great enough quantities. Prussic acid, while a hazard to grazing cattle, is volatile and dissipates if plants are cut for hay and poses no problem when the hay is fed later.

There have been reports of grazing cattle being poisoned by prussic acid in the last few weeks, and producers should be aware.

Producers should test hay fields for nitrates and prussic acid before they cut them for hay or turn grazing cattle to them. Testing the forage gives the producer an additional option of waiting and allowing for the nitrate or prussic acid concentrations to drop before grazing or harvesting the hay.

The primary sources of forage nitrate toxicity and prussic acid in Oklahoma will be summer annual sorghum type plants, including sudan hybrids, sorgo-sudans, sorghum-sudans, millets and Johnson grass. Also, weeds such as pigweed, kochia, mustard, nightshade and lamb’s quarters accumulate nitrates, too.   

These typically undergo periods of stress during the summer growing season. If they become drought-stressed or are heavily fertilized, they are at greater risk to accumulate these toxins.

Nitrates normally are taken up by the plant roots to be used in the production of plant protein. When the plant is stressed, however, that process is impeded and byproducts are accumulated that inhibit the ability of the animals’ blood hemoglobin to carry oxygen.

Prussic acid, also known as hydrocyanic acid, quickly blocks cellular respiration, the conversion of nutrients to energy.

Cattle producers should contact the Oklahoma Cooperative Extension Service office for information on testing, feeding and haying potentially dangerous forages. Forages can be tested at the office with some samples requiring more extensive lab analysis before being harvested or grazed.

Nitrate and prussic acid risks cannot be eliminated, but they can be reduced. It’s recommended the crop be tested before harvest, and raising the height of the mower when cutting, since nitrates are in greater concentrations in the lower stem. Tonnage may be less, but high-nitrate forage has little to no value. Conversely, though, prussic acid concentrations are highest in the leaf.

Different classes of cattle; i.e., pregnant cows, open cows or stocker steers, have different susceptibility to nitrates, so knowing the nitrate concentration can provide options as to how it can be fed, whether as-is or diluted with other feed and hay.

Regardless, cattle should be allowed to become adapted to forage with elevated nitrate levels.

Testing and an Oklahoma State University Fact Sheet on nitrates and prussic acid are available at the Extension office.  

Nelson is Oklahoma Cooperative Extension Service ag educator Garfield County.