Making Quality Corn Silage in 2002
Livestock Update, September 2002
Mark Wahlberg, Extension Animal Scientist, 4-H Livestock, VA Tech
Corn silage can be a great feed, especially for growing cattle and lactating cows and ewes. It can also be a feed with various quality problems. The difference is when it is cut and how it is stored.
Storage: Corn silage must be stored in air-tight conditions to minimize spoilage. Anywhere that air contacts the surface of silage it deteriorates. Two critical steps in the process accomplish air-tight storage. One of those is to store silage in a structure that excludes as much air as possible. Upright silos must be free of cracks or other leaks. They should have a roof on top. Horizontal silos, often called bunker silos, are going to have a lot of exposure to the air just by their design. The main way to reduce this is to have the silo in a long, narrow shape with a narrow face exposed to the air. Silos that are very wide expose a lot more silage to the air. Silage removal should be planned so that a minimum of 6 inches of material is removed from the exposed surface every day during the feeding season.
The second critical step in excluding air from the silage is proper packing. With upright silos this is accomplished simply by the weight of the material itself. Packed corn silage typically weighs around 40 pounds per cubic foot, so a lot of weight is applied to the silage in a silo that is 40 to 70 feet tall. The weight of material in a horizontal silo is not adequate to pack the pile tightly. This is accomplished by driving over the pile with a tractor to compress the material as it is being layered into the silo. Wheel tractors exert more weight per square foot than do crawler tractors, so they do a better job of packing. Packing a horizontal silo can be dangerous, due to the risk of rollover of the tractor. Be very cautious and do not allow inexperienced drivers to do this task. Corn silage can also be stored in silage bags, which do a great job of excluding air and preserving the silage well.
Stage of Harvest: The proper stage to harvest corn for silage is somewhat of a compromise. The proper stage to harvest is determined mostly by the stage of maturity of the corn plant. Highest dry matter digestibility occurs when the silage is from 30 to 40% dry matter. Silage that is wetter than this has excessive runoff and seepage, which carries important nutrients with it. Silage over 40% dry matter does not pack well, thus more likely to have air pockets, resulting in mold growth and heating.
The best guide to plant moisture content is the stage of development of the grain. When you cut through a kernel of corn, from top to bottom, you will see a line between where the liquid, or milk, is still visible and where the dry starchy material has formed. The location of this milk line in the kernel is a good guide to plant maturity. The desired stage is having the milk line about 1/2 of the way down the kernel. This corresponds to about 35% dry matter in the whole-plant silage. If the milk line is closer to the top of the kernel (less starch visible), the plant is wetter. As more starch and less milk is visible in the grain, the milk line moves closer to the tip of the kernel (where it attaches to the cob). When there is no milk line visible the dry matter content is about 40%. Once a black line is visible at the tip of the kernel, no more starch will develop and the plant simply gets drier as the leaves die.
Drought Stricken Corn: Drought and heat can seriously impair growth of the corn plant. Often the development of a good ear is diminished, or even nonexistent. The only way to salvage anything useable from this kind of crop is to harvest it as silage and feed it to livestock. Surprisingly, the feed value of silage from this kind of corn is quite high. The energy content of drought-affected corn with little or no grain content is only slightly less than that of normal corn, and the protein content is often just a little higher. The big difference is the reduced yield. So don't consider silage from drought stricken corn to be inferior. It has high feed value, and should be harvested, stored, and fed with the same care you would give to a normal corn crop.
Nitrate Risk: Corn is normally fertilized with a fairly high amount of nitrogen. When corn has adequate moisture and proper temperature, it uses the nitrogen for rapid growth and development. However, when affected by drought, it may still take up some nitrogen (in the form of nitrate), but not use that nitrate for plant tissue growth, especially grain development. Thus, the nitrate accumulates in the plant. If livestock consume a feed with high nitrate, it can cause a toxicity that results in reduced feed intake, abortions, and death. Nitrate level is highest in corn just after a rain following a drought. It takes a few days for the plant to metabolize the nitrate it takes up following the rain.
If conditions exist where you expect nitrate to be a possible problem, then sample the feed and get it analyzed for nitrate content. Many feed labs, including the lab at Virginia Tech, will do this analysis and promptly provide the results. Contact your Extension office for information on taking and submitting samples.
Nitrate in the feed at up to 0.5% of the dry matter is considered safe to feed. At more than 1.5% of the dry matter it can be fatal. During the ensiling process anywhere from 30-50% of the nitrate is converted to other nitrogen compounds, thus reducing the nitrate content towards a safer level. If corn with an elevated nitrate level is chopped and put in the silo, then a follow-up sample should be taken several weeks afterwards to check for the nitrate level in the fully fermented silage.
It is fairly easy to make good corn silage. Once the crop is planted and growing, the key decision is timing of the harvest. Cutting corn for silage when it is around 35% dry matter, corresponding to the 1/2 milk line stage of maturity, results in a highly digestible feed material. After proper packing and storage, and developing a balanced ration using corn silage as a key ingredient, high animal performance should result.