Summer annual forages have many potential benefits for Kansas livestock producers. There are two potential concerns with these crops producers must keep in mind, as well: nitrate toxicity and prussic acid toxicity. This week I'll share information on nitrate toxicity, followed by the prussic acid topic next week.
For a variety of reasons, summer annual forages are often accumulators of excessive nitrates. It is important for forages to be tested by an accredited plant analysis laboratory to determine nitrate (and prussic acid) levels in forages, and also to determine feed quality.
Nitrate toxicity is really nitrite toxicity, as nitrate is converted to nitrite in the animal. Nitrites interfere with the ability of red blood cells to transport oxygen and animals can asphyxiate. Cattle and horses are much more susceptible to nitrate poisoning than sheep and swine. Animals under stress (lactating, pregnant, hungry, sick) are more susceptible than stress-free animals.
Factors affecting nitrate levels in forages include:
* Plant factors
Species: All typical summer annual forages are likely nitrate accumulators, as are weeds such as pigweed, sunflower, lambsquarter, kochia and, under certain circumstances, Johnsongrass.
Growth stage: Most plant nitrate levels are highest in young plants or regrowth, and decrease as the plant matures. Sorghums and sudangrasses are exceptions to this rule and may maintain high nitrate levels at any growth stage. Stress at any growth stage may result in increased nitrate levels.
Plant part: Nitrate levels are highest in stems and conductive tissues. Nitrate levels are also normally highest in the lower one-third of the plant. Grain and leaves typically have low nitrate levels.
Time of day: Nitrate levels are typically higher in the morning throughout the plant and decrease throughout the day.
* Environmental factors
Drought/drought-ending rainfall: Nitrates accumulate under conditions of moderate drought as roots continue to absorb nitrate but heat and drought stress inhibit nitrate conversion in the above-ground parts of the plant. Under severe drought conditions, nitrate uptake is inhibited and levels remain low. However, after a "drought-ending" rain, nitrates are rapidly taken up by plant roots much more rapidly than they can be converted. It is necessary to wait a minimum of one week, preferably two, under good environmental conditions before harvesting forages so that nitrates can be converted to plant compounds such as amino acids, and nitrates can drop to safe levels.
Sunlight: Forages need sunlight to convert nitrates into plant compounds in young developing leaves. Therefore, any significant shading can increase nitrate levels. Plants next to tree rows, exposed to extended cloudy conditions and, most significantly, plants receiving significant rainfall after an extended period of drought followed by cloudy conditions may be extremely high in nitrates.
Stresses (frost, hail, diseases): Factors that damage leaves and reduce leaf area necessary for photosynthesis decrease energy available for the plant to convert nitrates and results in nitrate accumulation.
Air temperature: Temperatures below 55 degrees reduce photosynthesis and allow for nitrate accumulation. Extreme high temperatures also increase nitrate concentrations.
* Management factors
Fertility: Large amounts of nitrogen fertilizer applied all at once increase nitrate accumulation. This is why split applications are preferable. Remember, however, that nitrogen is necessary to optimize both yield and forage quality. Proper nitrogen fertilization will not result in excessive nitrate accumulation except under stress conditions. Producers also need to remember that for nitrogen to be efficiently used, and to minimize the potential for nitrate accumulation, it is necessary to have adequate levels of other plant nutrients in the soil, especially phosphorus and potassium.
Weed control: As mentioned earlier, certain weed species common in Kansas during the summer are typically high in nitrates. Therefore, controlling weeds is an important factor in minimizing nitrate levels.
Harvest method: In hay, nitrate levels remain constant. To minimize nitrates in hay, raise the cutter bar and take the previously discussed environmental factors into account when determining when to cut. Nitrate levels are reduced by 40 to 60 percent during fermentation when forages are cut for silage. However, this may not reduce nitrate concentrations to safe levels, so a laboratory analysis is recommended. When green chopping, feed forages immediately as nitrates can be rapidly converted to nitrites as the green-chopped forage heats up. Nitrites are the real culprit in nitrate toxicity, so forages with low nitrate levels can become toxic if green-chopped forages are allowed to heat up too long. Remember that nitrites are approximately 10 times more toxic than nitrates.
The chart below describes ranges of nitrate levels and their potential effect on cattle. Those wishing to use summer annual forages for horses should remember that horses are much more sensitive to nitrate toxicity than cattle. If at all uncertain of nitrate levels in a forage source, have the forage tested and indicate its use to the laboratory.
Nitrate levels in forage on a dry weight basis and potential harmful effects on cattle:
Nitrate level of zero to 3,000 ppm, essentially safe to healthy animals; 3,000 to 6,000, moderately safe under most circumstances (limit to 50 percent of total ration for stressed animals); 6,000 to 9,000, potentially toxic to cattle under certain circumstances (should not be the only feed source); 9,000 and above, dangerous to cattle and will often cause death.