This year, with the severe drought taking hold in many areas, producers grazing sorghums, sudangrasses and millet varieties were cautious about nitrate and prussic acid toxicity. In these species, the risk of both nitrate and prussic acid toxicity increases with drought stress.
Nitrate toxicity occurs when the nitrates (NO3-) in forages are converted to nitrite (NO2-) in the rumen by microorganisms. As cattle ruminate, they inhale the nitrite, which then binds to the hemoglobin in the blood, preventing the binding of oxygen to the blood cells. This can result in spontaneous abortion in pregnant animals and respiratory distress or even death at high levels. Stressful environmental conditions such as drought cause plants to accumulate nitrates due to the inability to convert them into plant proteins.
Prussic acid is also known as hydrogen cyanide (HCN). The compound is present in the leaves of the plants in a compound called dhurrin. Under normal conditions, plant membranes separate dhurrin from the enzyme responsible for hydrolyzing HCN from dhurrin. Ruminants such as cattle, sheep and goats are susceptible to prussic acid poisoning due to the chewing of their cud. As those animals ruminate, the cell membranes are damaged. This allows the enzyme access to dhurrin, thereby releasing HCN into the rumen. The HCN is then absorbed directly into the bloodstream where it binds hemoglobin. The bound hemoglobin cannot transfer oxygen to individual cells, and death by asphyxiation is the result. An added risk for prussic acid poisoning is posed by stressed and damaged plants. Drought-stressed plants accumulate more unbound HCN in their leaves.
A common question I continued to receive this year was: Is nitrate and prussic toxicity related? Therefore, without having a good answer to this question, I took a set of reports where producers had properly sampled to analyze both prussic acid and nitrate toxicity (n = 79).
Table 1 shows the breakdown of risk of prussic acid and nitrate toxicity poisoning in 79 samples submitted by producers throughout the grazing season in 2022. Most producers cited the drought and dry conditions as their reason for testing.
As shown on Table 1, 61% of the samples were safe for nitrates, and of those samples, 47% were marginal or toxic for prussic acid. So a safe nitrate result does not imply low risk for prussic acid toxicity. Further, 46% of samples tested for prussic acid were safe. Of those safe prussic acid samples, 28% were marginal, toxic or very toxic for nitrates. Therefore, a safe prussic acid result also does not imply a safe nitrate result. Based on this breakdown, it is wise to test forages for both nitrates and prussic acid to evaluate grazing risk during drought conditions.
To further evaluate if nitrates and prussic acid are correlated when under the stress of dry conditions, Figure 1 shows a scatter plot of the actual laboratory data. It is clear, there is no trend in the accumulation of nitrates with the presence of prussic acid. In fact, some of the most toxic prussic acid samples were completely safe for nitrate accumulation.
This brief analysis shows that producers should test both nitrate and prussic acid in accumulating forages including sorghum, sudan and millet varieties to evaluate toxicity risk prior to grazing. If forages are low in nitrate but marginal to toxic in prussic acid, producers can wait until a hard freeze, then five days later they can safely turn out cattle. Alternatively, they might consider cutting the forage and allowing it to sit for five days before windrow grazing or haying. If forages are high in nitrate, depending how far along in the season, ensiling might be a possibility. Otherwise, sampling the leaves and top part of the plant separate from the nitrate-accumulating bottom part can allow producers the choice to graze and pull cattle off the field early.
