NORTH PLATTE, Neb. — Grid sampling and application of dry fertilizer is common in the fall, but is it safe to graze corn residue fields that have had the fertilizer applied?
The answer comes down to how much risk in animal wellbeing we are willing to take. The risk potential is different for different fertilizer components. Potassium, zinc, nitrogen, and sulfur could all be toxic, but that risk is all about consumption rate and the amount animals can physically consume. To reduce risk, wait to graze until after precipitation whether that be rain or snow melt. Waiting for rain is more important if the application stuck to damp or wet residue.
Calculations can tell if what was applied would pose a risk.
Nitrogen fertilizers can cause toxicity in different ways and differing amounts so any spill available for consumption is a higher risk. There are other fertilizer components in a grid sampled applications that may or may not be a problem due to the how much is in the mix. Here are some other components and elemental levels that would be toxic. Potassium would be toxic at 30,000 mg/kg of intake or 3% of the diet which is very unlikely. A very high potassium intake could cause a magnesium deficiency, so making a high magnesium mineral available might be considered. High sulfur intake could cause polio encephalomalacia or PEM, but that is also unlikely. A lot of downed corn could also increase the risk of PEM. Cattle can tolerate 0.5% sulfur in the diet, but corn residue only contains close to 0.1%. Zinc consumption is fine up to 1000 mg/kg without problems and phosphorus can be tolerated up to 1% of the diet.
Bottom line: The safest approach for grazing corn stalks is too wait until a rain or graze before fertilizer application, but that may not be realistic. Assessing the fertilizer amount will give a good idea of risk level. If the fertilizer can easily be seen on the residue, be more cautious when grazing.
— Brad Schick, Nebraska Extension
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