Test First, Don't Gamble: Managing Nitrate and Prussic Acid Risks in Forages

Sandy Johnson, extension beef specialist, Colby

Many Kansas cattle operations rely on some type of harvested feed to use in the winter months and common among those sources is forage sorghum, sorghum-sudangrass, and sudan. Forages in the sorghum family are prone to two different problems for feeding cattle, nitrate poisoning and prussic acid (hydrocyanic acid, HCN) poisoning. They are easy to confuse because both result in a lack of oxygen availability to the animal and are more likely to occur when the plant is stressed (e.g., by fertility, hail, or drought).

Table 1. Key characteristics of nitrate and prussic acid poisoning.

Item	Nitrate	Prussic Acid
Plant parts most affected	Base of plant	Young or new growth
Types of plants	Many, especially sorghum family, pigweed (palmer amaranth, redroot, waterhemp), kochia, oat hay	Many (> 3,000 plant species), sorghums including Johnson grass, white clover, birdsfoot trefoil, Indiangrass, Cyndon spp. (Bermuda)
Grazing problems	Consumption of lower plant, rarely a problem unless extremely high levels or forced to eat entire stem	Consumption of newest growth
Hay	Not impacted by drying	Not impacted by drying
Time of death	Several hours after consumption	Within minutes of consumption, treatment can rarely occur fast enough to save the animal
Blood/Oxygen	Chocolate brown colored blood, hemoglobin converted to methemoglobin, and unable to carry oxygen	Blood is bright cherry red, hemoglobin contains oxygen, but not available to cells

 

In dry areas of the state, cattle may be removed from pasture early. Bringing hungry cattle into pens with weeds can be very dangerous as the nitrate concentration may be elevated throughout the plant and animal intake high. Manure in corrals can contribute to elevated nitrate levels in weeds. Elevated nitrates may not result in death, but could cause abortions.

Because nitrates are highest at the base of the plant, raising the cutter bar can reduce the overall nitrate content in the harvested forage. If drought stressed forage receives enough moisture to resume growth, nitrates in the plant can be even higher for at least 2 weeks, depending on conditions. In this case, testing before harvesting is especially important. As temperatures cool in the fall (less than 55^◦ F), photosynthesis of warm-season plants is retarded and could also favor nitrate accumulation.

Our current KSU forage fact sheet on prussic acid poisoning indicates that prussic acid potential dissipates as the forage dries. It goes on to say that hay or silage that likely contained high cyanide concentrations at harvest should be analyzed before it is fed. We tend to forget the second statement and assume that when the plant dries, all the cells rupture and any HCN is released. To confirm this, we measured dhurrin content in sorghum hay. The dhurrin content was stable from 1 to 10 weeks of dry storage and in standing forage from October through January. In the plant, dhurrin (the precursor to HCN in sorghum species) and the enzyme that converts it to cyanide, are stored in separate compartments within the cell. When the plant is eaten, the compartments are ruptured, and the cyanide formed and released. Measurement of dhurrin directly is used in some research studies. However, commercial labs do not typically measure dhurrin directly. Rather, the procedure includes something to stimulate cyanide release. If hay is made from forages in the sorghum family or other susceptible species, testing for prussic acid in forage that has suffered from drought, hail, or fertility issues is advised. Testing is cheap compared to the cost of losing even one animal.

Management recommendations common to both prussic acid and nitrates include:

• Test first, don’t gamble. Keep in mind that different labs use different tests that have different scales for nitrates.
• Feed animals with a known safe feedstuff(s) before introduction to potentially problematic feeds. Don’t turn in hungry.
• Ensiling will reduce concentrations of either by about half in well-made silage, but silage put up under less-than-optimal conditions could still contain very high levels. If extremely high before ensiling, a 50% reduction may not be enough to result in safe feed.

If testing before grazing, samples should reflect what the animals are expected to consume, generally leaves and upper portion of the plant. Sample a minimum of 15 sites across a given field. One method is to sample from each corner and the center by walking diagonal lines and sample plants every 50-100 steps or as appropriate for field size.

We expect nitrate and prussic acid levels to vary across a field, so more samples are better than fewer. A rule of thumb is to sample 10 to 20 % of the bales per field or cutting as a minimum. Be aware of areas of the field that exhibited more plant stress than others. If the areas are large enough, you may want to sample them separately. Your acreage size and feeding methods likely factor into this decision. Use a forage probe that cuts across all plant parts in a bale rather than a grab sample from individual bales or windrows. Most county extension offices can help with sampling procedures and equipment.

For more complete information on these problems, see these publications : Nitrate Toxicity and Prussic Acid Poisoning.