Soil Management

Late-Season Potential Forage Toxicities

Cristie Preston, Ph.D.

Cristie Preston

Cristie Preston, Ph.D.


Senior Agronomist

Agriculture has always been an integral part of Dr. Cristie Preston’s life. She grew up in southwest Virginia and had interest in crop and animal agriculture since an early age. Once she began college, she initially chose to study animal science but switched to soil science. Dr. Preston attributes her decision partly to an influential professor who told her, “You can’t understand animals until you understand what they eat.” She received a Bachelor of Science degree in animal science and a Master of Science degree in crop and soil environmental science from Virginia Tech. Dr. Preston holds a Ph.D. in agronomy, focusing on soil fertility from Kansas State University. While completing her advanced degrees, Dr. Preston conducted more than six years of field and lab research. Dr. Preston has experience in laboratory research measuring volatility loss from urea-based fertilizers. Her field research has focused on phosphorus availability and the interactions with tillage and placement. She also has extensive experience in working with large data sets and analysis. Her main priority is helping growers to identify yield-limiting factors and fix those issues as cost efficiently as possible.

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Following drought stress and even early frost, forages can be susceptible to toxic levels of nitrate accumulation. Levels of toxicity vary based on species, but farmers need to be aware of the potential issues if they plan on grazing this fall. Specifically, accumulation of nitrates can occur in corn, oats, sudangrass, sorghum sudangrass, Johnson grass and alfalfa, to name a few.

In general, when plant growth is normal due to adequate moisture and normal temperatures, nitrates taken up from soil are converted to amino acids and proteins. When growing conditions are not ideal, the uptake of nitrate is faster than the conversion within the plant, and therefore, accumulation occurs.

When it comes to cattle, sheep and goats grazing forages with high nitrates, animals convert nitrate to nitrite faster than nitrite can be converted to ammonia. When nitrates in forages are high, there is an excess converted to nitrite, leading to the accumulation of nitrite in the rumen due to the lack of conversion to ammonia.

Being able to predict nitrate toxicity in forages can be difficult, but some analytical labs do offer nitrate testing of plant material. A good rule of thumb is for farmers to watch grazing crops that had less than normal growth or excessive nitrogen applications in regards to growth/yield.