Managing Drought Stressed Corn for Silage

Drought-stressed corn that will likely produce poor quality grain or no grain at all may be salvaged as a silage crop. If corn has successfully pollinated and there is evidence of kernel growth (blisters are forming in the ear) despite drought conditions, the best alternative may be to wait for grain to develop and harvest what is available. If pollination is disrupted enough that plants will be barren, the crop may be harvested as soon as whole-plant moisture levels are appropriate for silage.

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Before harvesting, check with a crop insurance agent; you cannot harvest for forage or silage without first receiving permission from your crop insurance agent or you will forfeit indemnities. Also, check the labels of all herbicides and other pesticides applied to the field to determine if the minimum harvest interval has been met.

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Moisture. Harvesting drought-stressed corn too soon can result in silage with excess moisture, which can lead to poor fermentation and reduced feed value. The moisture level of drought-stressed corn can be much higher than it may appear upon field inspection. Even though the crop may appear quite dry with many or most of its leaves turning brown, it still may contain over 70% moisture. Check whole-plant moisture prior to harvest. The optimum moisture for harvesting corn for silage varies by method of storage (Table 1). Corn ensiled at moisture levels that are too low results in reduced fermentation activity and less breakdown of nitrate. If the moisture is greater than 75 to 80%, harvest should be delayed to avoid seepage and loss of silage quality, or windrow the corn and allow it to field wilt before chopping for silage. Drought-stressed corn should be chopped to 3/8 inch in length for harvesters without a grain processor and 3/4 inch for harvesters with a grain processor (0.08 to 0.12 mm roll clearance) to help it pack better for fermentation and storage.

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Table 1. Recommended moisture content for corn silage stored in various types of silos
Upright silos 60-65%
Upright oxygen-limiting silos50-60%
Horizontal silos65-70%
Bag silos60-70%
Source: Corn silage - Harvesting and storage. 2015. Corn Agronomy. University of Wisconsin Extension. http://corn.agronomy.wisc.edu/Silage/S004.aspx.
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Box 1. High Nitrates in Drought-Stressed Corn Nitrates taken up by plants are normally converted to ammonia for use in proteins. Nitrates can accumulate when there is a large amount of nitrate in the soil and normal plant growth is interrupted, such as during drought-stress. Nitrate accumulation is higher after a drought-ending rain or an irrigation. This occurs because nitrate is water soluble and highly mobile; the water helps move the nitrates in the soil into the root where they are taken up by the plant. It usually takes three to four days before these nitrates are converted by the plant into proteins. Therefore, harvesting corn silage following a drought-ending rain or irrigation should be delayed until nitrate levels in the plant recede.

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Nitrates. The main concern with drought-stressed corn is that nitrate levels may be elevated. Nitrates will accumulate in the stalks and leaves, with highest concentrations in the lower part of the plant and other conductive tissues (Table 2). High levels of nitrates in corn silage can be toxic when fed to animals and may produce a gas that can be lethal to humans and livestock. Obtain an analysis for nitrates in the crop prior to harvest. Cutting drought-stressed corn for silage is preferred over grazing or green-chopping because fermentation can reduce nitrate concentration by approximately 1/5 to 2/3.1 To help reduce nitrates at harvest, raise the cutter bar to 12 inches above the surface and avoid harvesting immediately following a drought-ending rain or irrigation as this can result in temporarily increased nitrate levels in plants (Figure 1). Allow silage to ferment at least one to two months prior to feeding. If high nitrate levels were found at harvest, test the product again before feeding to animals to ensure it is safe for consumption.

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Table 2. Nitrate nitrogen in 28 samples of drought-stressed corn
Plant part Nitrates in parts per million
Leaves 64
Ears17
Upper third of stalk 153
Middle third of stalk 803
Lower third of stalk 5,524
Whole plant 978
Source: Cassel, E.K. and Vough, L.R. 2002. Harvesting and feeding drought-stressed corn. 4017. South Dakota State University Extension.
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Drought-stressed corn yield can be estimated by assuming that one ton of 30% dry matter silage can be obtained for each five bushels of grain per acre. If no grain is expected, plant height can be used to estimate yield. For each foot of plant height (excluding the tassel) approximately one ton of 30% dry matter silage would be predicted. For example, if corn is six feet tall, five feet would be harvested, resulting in five tons/acre.2

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The feed value of silage from drought-stressed corn with minimal or no grain is typically between 65 to 100% of that of normal corn silage.3 It is generally lower in energy but higher in protein content than normal silage (Table 3).

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Table 3. Comparison of forage analyses for normal and drought-stressed corn silage
Silage type Dry matter Crude proteinAcid detergent fiberTotal digestible nutrients
Normal 35% 8.5%28%68%
Stressed25% 10%34%62%
Source: Cassel, E.K., Vough, L.R. and Fultz, S.W. 2006. Determining the value of drought-stressed corn.  4018. South Dakota State University Extension.
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A producer considering selling drought-stressed corn for silage should assess the quality of the feed, tonnage, moisture content and other factors based on current prices. Forage analysis is the best way to determine the feed value of corn silage and will aid in determining pricing.

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Several universities offer useful assessment tools to help with pricing. A few are listed below. This is not a complete list. Check with your university extension office for local recommendations.

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Corn harvested as silage can remove several times more nitrogen (N), phosphorus (P) and potassium (K) than a crop harvested for grain. Stalk removal accounts for the large differences in nutrient removal between silage and grain harvest. Drought conditions may limit nutrient uptake, but by the silking (R1) stage of development, corn will have accumulated more than 60% of the total N, P, and K requirement. Producers shouldn’t lose sight of the consequences of nutrient removal and plan to replace lost nutrients prior to seeding the next crop in the rotation, particularly for crops with high K requirements (alfalfa). The value of nutrients not returned to the soil and the loss of residue cover for soil moisture conservation are considerations to factor into crop rotation and fertility planning.