Nitrate toxicity is a topic that resurfaces every summer, especially when weather becomes unpredictable. Producers often ask whether they should be worried about nitrates in their hay or pasture, and the answer is usually the classic university extension response, “It depends.” Recent research is helping us better understand those relationships between weather conditions, fertilizer rates and forage management.
Drought is one of the most common situations associated with nitrate accumulation. When moisture becomes limiting, plant growth slows, but roots may continue taking up nitrate from the soil, particularly in fields that receive nitrogen fertilizer. Recent research conducted at the University of Tennessee over two very different weather years found exactly that. In a relatively dry growing season, sorghum-sudangrass produced greater forage yields and crude protein, but nitrate concentrations increased substantially at higher nitrogen fertilizer rates. In contrast, an unusually wet growing season resulted in lower forage production and lower crude protein, but nitrate concentrations remained much lower across nearly all fertilizer rates.
While rainfall allows plants to resume growth, grazing immediately after a drought-breaking rain can actually be risky. Plants often begin taking up soil nitrogen before they have produced enough new growth to convert accumulated nitrate into plant protein. Waiting for several sunny days before grazing allows actively growing plants time to metabolize much of that nitrate.
Fortunately, producers may have another management option if nitrate levels are elevated. Unlike dry hay, the fermentation process that occurs during baleage production can reduce nitrate concentrations. The aforementioned research trial found that ensiling sorghum-sudangrass as baleage generally lowered nitrate concentrations, with the greatest reductions occurring in forages that started with the highest nitrate levels. In some cases, forage containing moderately toxic nitrate concentrations was reduced to safe feeding levels after a six-week ensiling period. However, baleage is not a guarantee. Extremely high nitrate concentrations did not always decline to safe levels, so forage testing is still essential before feeding.
The takeaway is simple: Nitrate risk depends on more than fertilizer rate alone. Weather patterns, particularly drought followed by rainfall, strongly influence accumulation. Before grazing or harvesting stressed warm-season annuals, consider the recent weather, nitrogen applications and stage of growth. When conditions suggest elevated risk, a forage test is inexpensive insurance. If nitrate concentrations are high, harvesting the crop as baleage may reduce the risk, but testing the finished forage remains the best way to ensure it is safe for your cattle.
