Corn silage harvest has already begun in a few fields in Iowa. "Harvest starts first in some of the droughty areas with light soils," points out Brian Lang, Iowa State University Extension field agronomist in northeast Iowa. "But be careful with this. You cannot eye-ball harvest moisture on corn shutting down prematurely from stress. It's often wetter than it looks." He offers the following guidelines and information to help farmers deal with the question of when to start harvesting corn silage.

Related: Keys to Higher Corn Forage Yields

SUCCESS WITH SILAGE: Corn silage harvest is beginning to get underway in Iowa. ISU specialists say the keys to making good quality corn silage are to harvest at proper moisture content, fill the silo rapidly and pack it well.

Good corn silage is an excellent feed for cattle, both beef and dairy cattle, as it contains plenty of protein (avg. 8.1% on dry matter basis) and energy (mostly starch). It often is one of the most economical feedstuffs in terms of cost per unit of protein or energy. It forms the basis for most lactating dairy cow rations and can provide supplemental forage or roughage for beef cattle. The keys to making good quality corn silage are to harvest at the proper moisture content, fill rapidly and pack well.

Harvest corn silage at optimum moisture content
Harvesting a crop that is too wet often results in a poor, undesirable fermentation and, in the case of upright silos, extensive nutrient loss in seepage. Material that is ensiled too dry is difficult to pack in bags, bunkers or piles, and the resulting oxygen within the silage mass will cause extensive nutrient loss and is prone to spoilage.

The optimum moisture content for harvesting corn for silage depends on the storage structure utilized. The values below are for the whole plant, not just the ear or kernel.

Related: Max Corn Silage Yield & Quality

Moisture determination is important for silage timing
Here is a short article from the University of Wisconsin on whole plant moisture variability in the field, desired moisture for various storage structures, and procedures for moisture determination of corn silage. You can read the article at midwestforage.org/pdf/592.pdf.pdf.

The most accurate means of checking silage moisture is with the Koster Tester kostercroptester.net/ or a similar product, notes Lang. This is not an endorsement of companies, individuals or their services, nor is criticism implied of similar companies, individuals or their services not mentioned. Directions for using a microwave oven to measure silage moisture are included in the following publication.

Harvesting high quality corn silage, here's how to do it
The following article from the University of Minnesota summarizes many aspects of corn silage harvest. The author is Jeff Coulter, a Minnesota Extension corn agronomist.

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What are the steps involved in harvesting high quality silage? Proper harvest management is critical for high-quality corn silage, and it starts with harvest timing. This ensures that the harvested crop is at the optimum moisture for packing and fermentation. Silage that is too wet may not ferment properly and can lose nutrients through seepage. If silage is too dry when harvested, it has lower digestibility because of harder kernels and more lignified stover. In addition, dry silage does not pack as well, thus increasing the potential for air pockets and mold.

Optimum silage moisture at harvest ranges from 50% to 60% for upright oxygen-limiting silos, 60% to 65% for upright stave silos, 60% to 70% for bags, and 65% to 70% for bunkers. Due to variability among hybrids and growing conditions, it is necessary to measure silage moisture using a commercial forage moisture tester or microwave oven rather than simply estimating it from the kernel milkline.

Instead, kernel milkline should be an indicator of when to collect the first silage samples for moisture testing. A general guideline is to begin moisture testing when the milkline is 25% of the way down the kernel for horizontal silos, and 40% of the way down the kernel for vertical silos. Then, assume a constant dry-down rate of about 0.6% per day, and measure moisture again prior to harvest.

Length of cut and crop processing are also important
Length of cut and crop processing are also important for obtaining high-quality corn silage. This is because breakage of cobs and kernels increases surface area; which improves digestibility, reduces cob sorting, and results in higher density silage that packs better. Although crop processors are expensive, the higher-quality silage produced can increase milk production by 300 pounds per cow per year.

Related: Corn Silage Management

The benefit of crop processors is greatest when there are harder kernels resulting from delayed harvest or drought. When using a crop processor, chopper cut length can be increased to reduce horsepower requirements while maintaining optimum particle size. For unprocessed corn, ideal chop length is 0.375 inch theoretical length of cut. For processed corn, recommended settings are a 0.75 inch theoretical length of cut with 0.08 to 0.12 inch roll clearance.

Be careful with drought-stressed corn, avoid nitrate problems
A 4 inch to 6 inch cutting height is generally recommended for corn silage, as it maximizes silage yield and milk per acre. However, drought-stressed corn can accumulate nitrate in the lower part of the stalk, thus increasing the potential for nitrate poisoning, particularly in older livestock on lower-energy rations. The potential for high nitrate silage can be even worse if drought-stressed silage is harvested within 10 days of rainfall, since rainfall increases crop uptake of soil N. 

Silage with high nitrate levels can be managed by dilution with other feeds or by increasing the cutting height to 12 inches. Silage cut at this greater height has been shown to have 8% less silage yield and 2% less milk per acre. This same study found that a cutting height of 18 inches resulted in 15% less silage yield, 12% greater milk per ton, and 4% less milk per acre when compared to a 6-inch cutting height. 

Increased silage quality with high cutting is due to a higher ratio of grain to stover. However, corn stalks are a good source of fiber and the lower tonnage with high-chop silage typically makes it difficult to justify in the absence of high nitrate levels. When harvest begins, fill silos rapidly to reduce exposure of silage to oxygen and to reduce fungal growth. For bunker silos, pack silage as tightly as possible in progressive wedges in depths of 6 inches or less.