As corn prices rise a common question among cattle feeders is, How much can I feed?. The desire to feed higher levels has been tempered by concurrent increases in the prices of corn coproducts. However as of early 2007, wet CGF, modified DG and wet DG are priced at 75-85% of the price of corn, adjusted for moisture. DDG and dry CGF were approximately 100 and 90% of the corn price respectively. Any time the net cost of distillers grains in the feedbunk, adjusted for moisture, is less than the cost of corn, then the incentive is to feed levels beyond meeting the protein requirement.
The exact level depends on the first limiting factor which will vary from product to product. Beyond the economic factors just mentioned, the factors which might limit the inclusion of distillers grains in a feedlot ration include the level of fat in the total diet and the total sulfur intake of the animals. Moisture levels could reduce feed consumption at high levels of very high moisture coproducts, and the high fiber content or lack of starch has been theorized as a limitation for feedlot diets. Moisture and fiber appear to be secondary to the fat and sulfur issues. For feedlots with limited land resources, the concentration of P and N in the manure could become an issue.
Sulfur is likely the first factor to limit the amount of corn coproducts that can be fed in many situations. Sulfur levels of most corn coproducts can range from .4 to .9% S on a dry matter basis. Some liquid coproducts have been tested as high as 1.5 to 2% S. Sulfur is added during both the wet and dry corn milling process, so the coproducts contain additional levels above that concentrated from the original corn. Although it is based on limited research in cattle, the NRC recommends a maximum tolerable level of .4% of the ration dry matter for sulfur in the ration. Using that recommendation as a guide the maximum level of corn coproducts would range from 30% of the dry matter at high sulfur levels (.9%) to over 70% at low levels, based strictly on the sulfur content.
As far as fat is concerned, previous research with high oil feeds such as whole soybeans or cottonseed meal suggest that feed intake in feedlot cattle starts to back off when greater than 5% of the ration dry matter in the form of fat is added. Since distillers grains are 9-12% oil, fat would restrict their use to around 50% of the ration. This would give a total fat content of the ration of around 8%. In rations where fat is added, typically no more than 3-4% is added in the form of tallow or yellow grease. Several experiments have been conducted where excellent performance was achieved at levels in the 40-50% range.
Studies where higher levels have been fed are fewer, but less successful. It appears that the practical limit for feeding distillers grains to beef cattle is approximately 50% of the dry matter. Changes in milling technology that reduce oil and/or sulfur content could dramatically increase this level.