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Forage Testing for Nitrates and Prussic Acid
Wednesday, July 1, 2009 • Posted June 30, 2009

Texas AgriLife Press Release -

The current drought has many producers attempting to salvage failed sorghums, sudans and similar crops by grazing or baling the stover for later use as animal feed. The Texas AgriLife Extension Service Soil, Water and Forage Testing Laboratory (SWFTL) has received numerous corn and sorghum samples containing highly elevated nitrates, often at levels significantly high enough to prevent safe use of the forage. Agricultural producers can follow several steps to minimize nitrates in the baled forages, including raising the cutter height to leave the high nitrate lower stalk in the field.

Since nitrate accumulation and prussic acid formation occur in different parts of the plant, no one sample will adequately address both potential threats. Producers should sample multiple plants and segregate the lower stalks into ground-8” and 8-16” samples. The cutting heights are only suggestions and should be based on the producers equipment and ability to bale the hay in standing stalks. The more aggressive sampling protocol will allow a producer to better understand the concentrations of nitrates in the lower stalk and then raise or lower the cutter bar to maximize forage baled or minimize the nitrates in the baled hay. While stalk nitrate levels are highest in these lower stalks, a producer may elect to measure the nitrate concentrations in the remaining plant to provide added assurance that it is safe for grazing, in the event the lower 16” of stalk is highly elevated with regard to nitrates.

Prussic acid accumulation only occurs in a select number of crops and weeds. In general, concern is mostly directed to sudangrass, sorghum, johnsongrass, shattercane and any crosses or hybrids of these species. A more complete listing is available in E-543, Nitrates and Prussic Acids in Forages at the AgriLIFE bookstore or contact the Extension office. For these species, prussic acid can form in the newest leaves or recently damaged leaves. Samples collected for prussic acid analysis should be comprised of the newest leaves and damaged leaves. Sampling instructions for prussic acid is described in this publication or producers can contact the Texas Veterinary Medical Diagnostic Laboratory (TVMDL) for further sampling and testing information. Unlike nitrates, prussic acid will dissipate after cutting; however, tight bales may require more than 9 months for prussic acid levels to decline below levels of concern. Nitrates levels will remain constant unless significant water leaches through the bale, a factor often reducing the feeding value of the hay to near zero. If the crop is still green and good rain is in the near-term forecast, delaying cutting may allow for nitrates in the plant to be converted to protein and other nitrogen containing compounds and structures. Generally, 3-5 days is required, following adequate rainfall, for significant nitrate reductions.

A number of sources can be located both on the web and through university publications citing defining safe nitrate levels. The cited values will vary considerably and are often significantly more conservative of the 1% nitrate value historically recognized by the Texas Veterinary Medical Diagnostic Laboratory (TVMDL). Producers should be cautioned that the TVMDL 1% level assumes that cattle are healthy, have good conditioning and have an overall high energy level in their diets. Prior to feeding forages with nitrate levels in the 0.5-1% range which are going to be fed to weak, lactating and animals with reduced body condition scores, producers should consult their veterinarian. Producers should also avoid using average nitrate values within the forage, as more timid or later feeding cattle will likely be exposed to a higher percentage of lower stalk material, thus resulting in the consumption of unsafe levels of nitrates. The use of a bale grinder can be used to reduce selective feeding. Grinding nitrate tainted forages with clean forages, thus lowering the nitrate levels to less than 1%, is another management option available to some producers.

Both the SWFTL and TVMDL routinely analyze forages for nitrates, while the TVMDL is the sole prussic analyzing laboratory within the Texas A&M University System. Both laboratories prioritize these samples during times of drought and attempt to provide next business day results.

Texas AgriLIFE bookstore website:

Texas Veterinary Medical Diagnostic Laboratory (TVMDL) website: Phone# (888)646-5623 toll free

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