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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

Treating Corn Silage to Enhance its Protein Content

Livestock Update, September 2004

Mark L. Wahlberg, Extension Animal Scientist, VA Tech

Corn silage is great cattle feed. When harvested and stored correctly it has a lot of digestible fiber, plenty of starch from the grain to provide lots of energy, and is well-liked by cattle. However, like most feeds from the corn plant, it comes up short on Calcium and Protein. While producers can provide these needed nutrients as supplements when the silage is fed, another option is to mix these into the silage as it goes into storage. This provides the producer with a "ready-to-feed" material coming out of storage, thus simplifying the feeding process.

Harvesting Corn Silage - Good corn silage is a function of timing. The corn plant needs to be mature enough to have maximized the development of grain, but not so mature that the crop is too dry to properly ferment in storage. The goal with corn silage is in most cases to harvest a crop that contains from 30 to 40% dry matter. The wetter silage is preferred with horizontal silos, and the drier material will pack well in upright silos and bag-style storage. For oxygen-limiting upright silos the recommendation is for 40 to 50% dry matter. The wetter silage corresponds with the early dent stage of development of the grain, with progressively drier silage corresponding with the milk-line appearing or of the way towards the cob end of the grain kernel. The milk line disappears at about 40% dry matter.

Proper moisture level is important to help in the process of excluding air from the mass as the structure is filled. Getting the air out is important to start the fermentation process. Also helpful is to chop the material to the right length. Sharp blades and shear bars adjusted to provide a cut length of to inch is desired. Recutter screens and kernel processors are not necessary to make good corn silage.

Planning for Treatments - Protein is the first critical nutrient needed to meet animal requirements when feeding corn silage. Untreated corn silage has from 7 to 8% protein on a dry matter basis, and the protein requirements of beef cattle and sheep range from 10 to 14%, depending on the stage of production and desired rate of gain. High production dairy cows have even higher requirements.

The most concentrated sources of protein for cattle are actually not true protein feeds, but non-protein nitrogen (NPN) sources, such as anhydrous ammonia and urea. When added to silage, the bacteria living in the rumen of the cattle are able to make protein from these nitrogen sources, plus the other nutrients contained in the corn silage.

While anhydrous ammonia contains a high nitrogen content (81%), treating silage with ammonia requires special equipment. It is a pressurized gas, but must be converted to a liquid to apply to the silage. It is highly caustic and can cause burns. Because of the high nitrogen content a fairly small amount is needed to treat silage, approximately 6 to 8 pounds per ton. In addition to raising protein level, ammonia also reduces heating and spoilage, and extends the bunk life of silage after it is fed.

Urea, the other non protein nitrogen source, contains 45% nitrogen. When the rumen microbes use the nitrogen in urea to make protein, approximately 2.8 pounds of protein can be manufactured from the nitrogen in just 1 pound of urea. Consequently, the crude protein equivalent of urea is 281%. It is a dry, granular product that is easy and safe to handle. It must be kept dry prior to application, though, or it can clump together and be hard to handle. Because of ease of handling and safety all but the largest producers use urea as opposed to ammonia when they treat silage with NPN.

These NPN products bind with the moisture in silage. If adequate moisture is present, then minimal loss of protein content occurs. Silage wetter than 30% DM will have lots of seepage, which will take some of the dissolved nitrogen with it. Silage drier than 40% has too little water to which the urea or ammonia can bind. Good results will occur if the silage is between 30 and 40% DM.

Some minerals may be added to silage as it goes into storage. The critical minerals would be calcium and sulfur. See section below about adding Sulfur.

Calculating the Right Amount - Because of the variable dry matter content of silage, most all calculations are done using the dry matter values, and then converted to the as fed basis using the proper dry matter content for the silage in question. The simplest process to use to calculate treatment amounts is called the Pearson Square.

For silage that contains 8% protein on a dry matter basis, and a desired final blend of 12% protein, here is the calculation:

Feed Percent Protein Parts Percent of DM
Urea 281 4 1.5%

Corn silage 8 269 98.5% (DM basis)
Total 273 100 lb of DM

Using the Pearson Square process, you set up as shown above. Then, you subtract on a diagonal the number in the middle (your desired final protein content) from the percent protein of each feed, and put it on the right side of the square. This gives you parts of the total mixture. By dividing each part by the total, you get percent in the final mixture. Thus, to raise silage from 8 to 12% protein on a dry matter basis, you wind up with a final mixture that is 1.5% urea and 98.5% corn silage. However, this is dry matter, not fresh silage with the moisture included.

The amount of urea needed to treat a ton of silage depends on the final desired protein percentage, and also on the dry matter content of the silage. The table below shows the amount of urea needed to treat a ton of silage to produce either a 12% or 13% final protein level in the dry matter.

Pounds of Urea Needed to Treat a ton of Corn Silage at Various Dry Matter Levels and Final Protein Levels

Final Silage Dry Matter Percentage
Protein 30 33 36 40
Percent For Silage With 8% Protein (DM Basis)
12 9 10 11 12
13 11 12 13 14
  For Silage With 7% Protein (DM Basis)
12 11 12 13 14
13 13 14 16 18

To use the table, find the section with the appropriate beginning protein content of the silage. Then, select the final protein level desired. Finally look for the dry matter content of the silage and follow that column down to find the proper amount of urea to treat a ton of silage. For example, if your cutting corn that contains 8% protein and is 36% dry matter, and you want 13% protein in the final mixture, then you need to add 13 pounds of urea to each ton of silage.

Sulfur must be Added to urea-treated silage. The production of protein by the rumen microbes will not occur properly when sulfur is deficient. Thus, for every ton of silage treated with ammonia or urea, about 3 pounds of a sulfur-containing compound should also be added. Sources such as Calcium Sulfate, Potassium Sulfate, or Sodium Sulfate should be used. TOXICITY WARNING - Do not use Copper Sulfate, as this amount of added copper could kill livestock.

Applying Urea and Minerals to Silage - The amount of urea needed to treat a ton of silage is quite small, typically ranging from 10 to 15 pounds. The best results occur when the urea is mixed fairly uniformly into the silage. Methods to get this done depend on the method of harvesting, transport, and storing of the silage. Here are a few methods which work pretty well. However, each method requires that you have a pretty good idea of how much silage is contained in the truck or wagon which is being unloaded, since the treatment amount is per ton of silage.

Self-Unloading Silage Wagon. - This one is easy, since the process of unloading the wagon will mix the contents pretty thoroughly. Simply scatter the proper amount of urea on the top of the material in the wagon before unloading. SAFETY WARNING - Make sure the PTO shaft of the wagon is not connected to the tractor when you get in the wagon to apply the urea.

Dump Wagon or Truck going into a Blower - The blower will blend the material fairly uniformly. Urea can be scattered onto the silage as it comes from the truck, or as it is transferred to the blower itself. Trial and error will get the right rate of application.

Horizontal Silo - Once a load is dumped on the top of the pile, spread it in a layer with the tractor/loader used to pack the pile. Once the layer is made, you can scatter the urea across the top of this new material. A hand-crank type of seeder or fertilizer spreader used for lawns will work well for this. After application of the urea, the tractor operator can go back to packing the silage before the next load arrives.

The cost to treat silage with urea is quite reasonable. The material itself is fairly expensive, but the amount used per ton is very low. In some markets feed-grade urea is around $300 per ton. At a rate of 12 pounds per ton of silage, the cost of urea would be just $1.80.

Summary - Corn silage does not have enough protein to meet animal requirements. Treating silage as it goes into storage is a fairly easy process, especially if feed-grade urea is used. Sulfur should also be added if urea is applied to silage. It takes from 10 to 15 pounds of urea to treat a ton of silage properly, with a cost of just a few dollars per ton.

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