The 2012 drought has stressed numerous crops and forages, and veterinarians should help their clients be on the lookout for problems caused by toxins in feed.

Steve Ensley, DVM, PhD, Iowa State University, says, “In a three- state area around Iowa we are seeing issues with green chop corn and nitrate toxicosis, corn in general and nitrate toxicosis, blue green algae toxicosis and concerns about aflatoxin in the corn that will be harvested this year. “

Ensley says there are weeds that accumulate nitrate in pastures and during drought they are more likely to become an issue because they will grow when grass won’t. “The general rule with potential toxic weeds are they like to grow in disturbed areas of the pasture so look out for areas around feeding areas etc.,” he advises.

If you want to green chop and feed forages immediately like corn or sudan, it is very important to test for nitrates first. If you are ensiling,then you need to test after 1 month of fermentation.

Clinical signs
Nitrate toxicosis interferes with the body’s ability to carry oxygen in the blood, Ensley says. The clinical signs observed are related to how anoxic the animal is. The clinical signs can range from lethargy, to increased resting respiration rate, inability to ambulate to acute death.

Nitrate toxicosis is a rapid occurring event. The dose of nitrate is directly related to how fast death can occur, and Ensley says the antidote for nitrate toxicosis is difficult to obtain.

The Iowa Beef Center’s fact sheet Nitrate Toxicity says a useful rule of thumb is that cattle and sheep can tolerate up to 0.5% nitrate on a dry matter basis. Total nitrate intake, including from drinking water, must be considered. Feeding non‑protein nitrogen such as urea does not affect susceptibility to nitrate toxicity. Intake of large amounts of nitrate at one feeding is more likely to produce toxicity than intake of the same levels spread out over several hours.

Livestock can adapt to higher levels of nitrate intake over a period of several days. Inclusion of grain in the diet speeds up the conversion of ammonia to protein and makes ruminants less susceptible to nitrate toxicity.

Nitrate Toxicity fact sheet summary:

  • Those who intend to feed drought stressed green‑chopped corn from high fertility soils should consider testing, especially if a short period of rapid growth has occurred just prior to harvest.
  • Cattle and sheep can tolerate up to 0.5% nitrate on a dry matter basis.
  • Cattle and sheep can tolerate more nitrates if feeding occurs over a period of several hours.
  • Nitrate tolerance is increased if grain is fed.
  • The nitrate levels in the feed and water sources are additive.
  • Drought stressed corn should be cut at 12 to 18 inches above the ground level, as the lower stock has the highest concentration of nitrate.
  • The ensiling process results in the loss of much of the nitrate and greatly reduces the risk of toxicity.
  • Gradually introduce cattle to suspect forages over a period of several days.

For information on how to sample and have drought-damaged corn tested for nitrates, click here.

Find out more at the Iowa Beef Center.