Nip forage nitrates in the bud

This spring, I seeded some container varieties of lettuce in planter boxes near a shaded area of my makeshift home garden. My harvest was far from fruitful and only afforded enough leaves to put on a sandwich or garnish a burger for most of the summer. After allowing the lettuce to grow out for a few weeks, however, I finally had enough foliage to make a single salad. To my chagrin, it was bitter.

Any master gardener will tell you bitter lettuce is the result of “bolting,” or when hot temperatures signal the cool-season vegetable to use energy from photosynthesis for flowering stalk and seed development instead of leafy growth. It’s just one example of how certain weather conditions can induce or interrupt physiological processes in plants. Nitrate toxicity is another one.

Nitrates accumulate in forage when environmental stress causes photosynthesis to slow down. Under normal conditions, plants take up nitrate from the soil and convert it to nitrite, then ammonia, and then amino acids, which are used for plant proteins. When that conversion process is paused due to drought, frost, cloudy weather, or hail damage, though, nitrates build up quicker than they can be metabolized.

Similarly, when ruminants take in surplus nitrate, rumen microbes convert it to nitrite faster than it can be used for protein synthesis. The excess nitrite is then absorbed into the bloodstream, preventing oxygen transport, which can lead to livestock death.

Emphasis on drought

Jason Hartschuh with The Ohio State University notes drought has been a defining characteristic of the growing season in his state this year. These dry conditions have significantly slowed plant growth in pastures and hayfields, amplifying the incidence of high-nitrate forage.

In addition to forages, some weeds are also heavy nitrate accumulators, which can spell trouble when these less desirable plants are grazed or incorporated into hay. Lambsquarter, pigweed, horsenettle, quackgrass, and some mustards are among the species that are most likely to cause livestock nitrate consumption to spike.

The bottom line, Hartschuh says, is to test forage nitrate levels prior to harvest. If levels are high, the simplest solution is to delay harvest until nitrate concentrations dissipate. Other mitigation strategies include raising the cut height, ensiling forage instead of making dry hay, and adjusting your feeding practices.

Delay harvest. Younger plant growth is generally higher in nitrates compared to more mature forage, so waiting to graze plants or delaying harvest until flowering or heading stages can reduce nitrate levels.

The time of day at which forage is cut can also affect nitrate levels to an extent. Hartschuh notes nitrate concentrations can be high in the morning before plants start photosynthesizing, so mowing hay in late afternoon or evening, especially on a sunny day, offers more time for those processes to play out.

Raise the cut height. Nitrate concentrations tend to be greater in the bottom one-third of forage stems. Raising the cutter bar to avoid lower plant parts can help offset toxic levels.

Ensile forage. Nitrates don’t dissipate during the drying process, but they can under the right ensiling conditions. With that said, the drier, more loosely packed forage is, the longer it will take nitrate levels to decline. Take baleage, for example.

“Baleage is often harvested on the drier side, and even when it is harvested at the ideal moisture range, reductions will not be as much as well-packed silage,” Hartschuh explains. “Baleage densities are much lower than properly packed silage, so the additional oxygen slows fermentation. If nitrate levels are reduced in silage in three weeks, it will take six or more weeks for levels to be reduced in baleage,” he adds.

Tweak your feeding practices. If nitrate levels are still concerning at feedout, mix high-nitrate forage with low-nitrate forage to dilute the toxicity before feeding. Nitrate concentrations below 3,000 parts per million (ppm) on a dry matter basis are considered safe for all cattle, but dairy cattle rations should stay below 2,500 ppm.

If it’s not possible to mix two silages thoroughly, Hartschuh recommends feeding low-nitrate forage first and then offering cattle high-nitrate forage. This ensures animals fill up on safe feed before consuming too much of the riskier material. Alternate forages in this way for every feeding.