So, thankfully we got some much-needed rain, but with that rain has come some calls into my office with concerns of poisoning in livestock. Two things to be cautious of occurring with rain after drought are Prussic Acid Poisoning and Nitrate Poisoning.
Prussic Acid Poisoning primarily occurs in sorghums commonly less than a foot and half tall (johnsongrass, grain sorghum, sorghum-sudan), wild cherry, and occasionally on white clover and birdsfoot trefoil. It does not occur in pearl millet or corn.
Poisoning is associated with consumption of plant parts with high levels of prussic acid (HCN), by a cyanogenic compound that is highly poisoning. Prussic acid is associated with rapidly growing plants. It occurs in young plant tissue that is damaged or stressed (for example: after a frost or drought, after heavy N fertilization), or mechanically injured (after 4 wheelers or ATVs are run over a field). Under these conditions the cells are ruptured mixing their enzyme content with that of Dhurrin (a cyanogenic glyceride in sorghums), breaking down Dhurria to prussic acid.
HCN causes acute respiratory inhibition by inhibiting the enzyme cytochrome oxidase. This causes labored breathing which may occur minutes (10-15 min) after consumption of feed.
If plants have been injured, defer grazing until they are recovered from injury. After a hard freeze, or severe drought, avoid grazing for approximately 1 week. After a rain or irrigation on drought stressed fields wait at least 2 weeks after plants begin to grow before grazing. Due to volatilization of CN compounds, hay can be fed; however, you still need to monitor forage by taking samples and have them tested to confirm ‘safety’ in feeding the hay. HCN forage samples can be sent to the Texas Veterinary Diagnostic Medical Laboratory (TVMDL) to determine toxic levels.
So, if that doesn’t scare you, you need to also be aware of Nitrate Poisoning (or Nitrate NO3 accumulation). Drought (moisture stress) or cloudy (low-light) conditions prevents normal plant growth. Under these conditions, the plant accumulates nitrates (NO3) mainly in stems and lower leaves instead of converting the nitrate to protein. This is especially true for those feeding sorghums, millets, corn, oats, wheat, rye, and pigweed.
The term “nitrate toxicity” is commonly used but the toxic principle is actually nitrite. Nitrate is converted to nitrite in the rumen. Nitrite is absorbed from the rumen converting blood hemoglobin to methemoglobin. Methemoglobin cannot transport oxygen to body tissues, so animals die from oxygen insufficiency. A few ways to prevent is to not graze during periods of stress, monitor nitrate levels to determine levels in forage are safe; don’t graze too short (nitrate accumulates mainly in stems and older leaves) and don’t feed high nitrate forage free choice.
Nitrate does not dissipate from hay like HCN (prussic acid). Once high nitrates levels are reached they stay high. It must be diluted by feeding it mixed with hay that is nitrate free or should be discarded. Horses and hogs are less tolerant than ruminants. Plant samples can be sent to the Texas Veterinary Diagnostic Medical Laboratory (TVMDL) or the Texas A&M AgriLife Extension Soil, Water, and Forage Testing Laboratory.
For more information please contact Chelsea Dorward at the Texas A&M AgriLife Extension office at 254-435-2331