Get a grip on forage nitrates

Are you tired of the weather extremes that seem more common with each passing year? Your forage crops could use a break, too.

Cool, cloudy weather patterns, such as those we experienced last week across much of the northern U.S., can set up a situation for toxic nitrate levels in forage crops used for hay and grazing.

Ohio State University (OSU) Extension Forage Specialist Mark Sulc notes that plant stresses contributing to toxic levels of nitrate include frost, extended cold weather, cloudy conditions, hail damage, and drought. “All of these conditions slow down plant photosynthesis,” writes Sulc in a recent issue of OSU’s C.O.R.N Newsletter.

During favorable conditions, a plant’s metabolism converts nitrates into a series of substances to ultimately produce amino acids and plant proteins. When a plant’s photosynthesis and metabolism rate slows down, it continues to absorb nitrates from the soil, which can build up and become toxic to animals consuming the forage.

Similar to plants, an animal’s metabolism converts nitrate into a series of other compounds, starting with nitrite and ending with proteins comprised of amino acids. Sulc explains that if an animal experiences an excess of nitrite in its rumen, it will be absorbed by the bloodstream, prevent oxygen transport, and possibly lead to death.

Like many other illnesses in livestock, older or sick animals are generally more susceptible to nitrate toxicity than young, healthy animals. Sulc adds, “The fetus of a pregnant animal is very sensitive to high nitrates ingested in the diet.”

Cereal forages are especially vulnerable to accumulating high concentrations of nitrates, but most grass crops, alfalfa, and brassicas can also be subject to toxic nitrate levels.

Sulc offers these considerations to keep in mind as they relate to possible forage nitrate issues during the spring and early summer:

• Nitrogen fertilizer or manure applications made this spring increase the risk of nitrate build up.

• Nitrate accumulation is highest in the lower third of the plant.

• Making dry hay from affected crops does not reduce the amount of nitrates in the forage.

• Risk of nitrate toxicity is highest when animals are grazing.

• Harvest timing can make a difference. Letting plants mature and harvesting later in the day can result in plants with lower nitrate levels.

• Successfully fermenting silage can reduce nitrate levels by as much as 10% to 60%. Beware, however, as the nitrates are lost in the form of deadly silo gas. Crops with extremely high nitrate levels can still be toxic after fermentation.

• Several weed species such as lambsquarters, pigweed, dock, and many others are known to be nitrate accumulators. High populations of these weeds can increase a forage crop’s overall nitrate levels.

“The bottom line is that if you suspect the forage could be high in nitrate levels, the safest thing to do is sample the forage and have it tested before harvest,” Sulc notes.

The forage specialist also recommends contacting your forage lab for instructions on collecting and shipping samples for testing. Generally, waiting for warmer weather and for the forage to advance in maturity will reduce the level of nitrates in the crop.

C.J. Weddle

C.J. Weddle is serving as the 2020 Hay & Forage Grower editorial intern. She currently attends Mississippi State University, majoring in agricultural education, leadership, and communications. She grew up on a farm in Vardaman, Miss., where her family raises sweet potatoes and soybeans.