Another potential problem is founder, especially in dairy cows, when livestock are fed drought-damaged corn silage high in nitrates. Founder, or laminitis, is a condition that adversely affects the cow’s feet causing lameness and off-feed problems. This condition may dramatically decreases milk production and result in the culling of affected cows.
Factors increasing the likelihood of nitrate accumulation: It is nearly impossible to predict the level of excess nitrates that drought-damaged corn silage may accumulate. However, here is a list of factors that contribute to increased nitrate accumulation:
- Nitrogen availability: The more nitrogen that is available to the crop increases the likelihood of excess nitrate accumulation. If the crop received application(s) of commercial fertilizer and/or manure, it is much more likely to have excess nitrate levels. However, the only way to be certain is to have samples of the silage tested to be certain of the exact levels of nitrate accumulation. Representative, fully ensiled samples of the suspect corn silage should be tested by a certified commercial laboratory to determine the level of nitrates before feeding to livestock. Corn silage samples may also be tested for nitrates at the Michigan State University Veterinary School’s Diagnostic Center for Population and Animal Health.
- Type of drought: Nitrate accumulation in corn plants tends to be higher when the drought is short and intense as compared with a longer and more sustained drought. Nitrate levels can also be high immediately after a drought-ending rain which results in rapid nitrate uptake if corn plants are still alive. In such cases, delaying harvest 3-4 days after the rain event allows enough time to pass for the living plant to convert excess nitrate into proteins.
- Cloudy weather: The enzyme that converts nitrate into proteins in the corn plant is less active in cloudy weather. Therefore, harvest should be delayed for several days following a period of cloudy weather.
- Nutrient deficiencies: If the corn plant is deficient in such nutrients as phosphorus, potassium and manganese the level of nitrate may be elevated. Such nutrient deficiencies result in higher levels of nitrate not because affected plants still take up nitrates but because of the nutrient(s) deficiency, growth and incorporation of nitrates into proteins is reduced.
- Plant age and plant part: The lower third of the stalk is the oldest portion of the plant and accumulates the most nitrates. Some experts recommend leaving 6-12 inches of stubble to reduce nitrate levels in the harvested corn silage. However, such a strategy lowers overall yield by 5-10 percent and is not necessary if management strategies are employed to dilute total ration nitrate level to < 1,000 ppm. Thus, Michigan State University does not recommend chopping corn at 12-16 inches above ground level.
- Frost damage: All corn, including drought-damaged corn, may have elevated levels of nitrate-nitrogen if harvested 2-6 days after a heavy frost. Such silage should be tested and treated with the same precautions as drought-damaged corn silage.
- Harvesting and utilization considerations: Here are some harvest and utilization guidelines producers should evaluate when considering harvesting and feeding drought-damaged corn as silage:
- Harvest moisture: Generally, in drought-damaged corn, moisture levels are much higher than they appear to be upon field inspection. Even though drought-damaged corn may appear “fired up” and quite dry, it still may contain over 70 percent moisture. Thus, as long as plants are growing allow them to continue to grow and accumulate biomass. Corn plants that do not have grain will generally still yield one ton of 30 percent dry matter per foot of plant height. Therefore, even if plants are devoid of grain and continue growing, delay harvest until the crop reaches the recommended harvest moisture level consistent with the type of storage structure. Proper moisture levels for harvest are: 68-70 percent for bunkers, 60-68 percent for upright silos (sealed and non-sealed), 60-68 percent for silage bags. Visual inspection and/or rules of thumb are very inaccurate in predicting whole plant moisture levels, therefore, the only way to accurately determine moisture level is to take a representative sample and use a microwave or Koster Tester to determine dry matter. Do not take samples from headlands as they usually are not representative of the entire field. Ensiling corn silage reduces nitrate levels 25-50 percent, but all silage should be specifically tested for nitrate levels. Once drought-stressed corn plants quit growing, dry down occurs very rapidly, especially if the weather is hot and dry with strong winds.
- Harvest considerations: Try to harvest on bright sunny days when nitrate conversion to protein is high. Do not harvest for at least 3-7 days after soaking rain that follows an extended period of dry weather since nitrate accumulation will tend to be higher and the crop needs time to convert these absorbed nitrates into protein.
- Ration formulation: Make sure the total ration does not exceed 1,000 ppm nitrate-nitrogen. Silages with nitrate-nitrogen levels up to 4,400 ppm can be fed if diluted to obtain the overall total ration nitrate-nitrogen of no more than 1,000 ppm nitrate-nitrogen. Testing for nitrates should not be performed until the silage has completed the fermentation process (~4 weeks). All laboratories do not express total nitrate-nitrogen in the same manner. Table 1 contains multiplication factors to convert various nitrogen compounds to nitrate-nitrogen.