Corn distillers grains are becoming more readily available, due primarily to the rapid growth in the number of ethanol plants. Wet milling of corn is designed to produce pure starch and capture the maximum value from each kernel of corn. Each bushel of corn yields on average 31.5 pounds of starch, 12.5 pounds of gluten feed, 2.5 pounds of gluten meal and 1.6 pounds of oil. Four major types of livestock feedstuffs are produced:
•corn steep liquor (25 percent crude protein [CP] on a 50 percent solids basis)
•corn germ meal (20 percent CP, 2 percent fat and 9.5 percent fiber)
•corn gluten feed (21 percent CP, 2.5 percent fat and 8 percent fiber)
•corn gluten meal (60 percent CP, 2.5 percent fat and 1 percent fiber)
Dry milling of corn is designed primarily to produce either ethyl alcohol for beverages or ethanol for addition to gasoline. The dry milling and fermentation of a bushel of corn grain produces, on average, 2.7 gallons of ethanol, 18 pounds of dried distillers grains with solubles and 18 pounds of carbon dioxide (CO2). Since only the fermentable starch is removed during the production of ethanol, the remaining products contain approximately three times more nutrients than corn. Typical nutrient composition values for corn and distillers grains are in Table 1*.
By incorporating different enzymes in the fermentation process, plant operators have been able to increase the yield of ethanol from the traditional 2.65 gallons per bushel of corn to at least 2.8 gallons per bushel. However, there can be considerable variation in the distillers grains produced within a plant on different days and between plants. Dairy producers need to be aware of this potential variation and monitor supplies closely to avoid dramatic changes in animal performance caused by poor quality.
Distillers grains are a good source of rumen undegradable protein (around 57 percent of CP), although the value is slightly less for wet than for the dried product.
Distillers grains that are heated too much during the drying process will be darker in color and more of the protein will be in the unavailable form. As with other corn products, the amino acid profile is quite good, with lysine being the first limiting amino acid. Removal of the starch in the corn kernel results in the other nutrients becoming more concentrated, with the higher levels of fat and phosphorus being of greatest concern in ration formulation. Consequently, producers should monitor levels of these nutrients closely, especially when including other sources of supplemental fat.
Several studies have been conducted to quantify the production response of lactating cows fed distillers grains. Most were designed to look at distillers grains primarily as a protein source, but some have considered it as an energy source as well. Nearly all research shows cows produce as much or more milk when distillers grains are in the ration compared to when soybean meal is the protein supplement. However, milk production did not always increase when ruminally- protected lysine and methionine were added to diets containing distillers grains.
For example, one trial compared soybean meal with corn distillers grains, both with and without ruminally-protected lysine. Both dry matter intake (DMI) and milk yield were increased significantly by feeding corn distillers grains compared to soybean meal. Supplementing the diets with rumen-protected lysine and methionine did not enhance intake or milk yield with either protein source.
In a subsequent trial, distillers grains and a blend of other protein sources (fish meal and soybean meal) with distillers grains were fed, with and without ruminally- protected lysine and methionine. Again, there was no response to the added amino acids, and in this trial there was no difference in response to distillers grains versus the protein blend.
Historically, most nutritionists have recommended feeding up to 20 percent of the diet dry matter (DM) as distillers grains. This would equate to 10 to 13 pounds per day of dry distillers grains and 30 to 40 pounds per day of the wet product. Few studies have been conducted to evaluate the effects of feeding much higher levels.
Hippen et al. fed wet distillers grains at 10, 20, 30 and 40 percent of the diet DM, then in a related study fed dry distillers grains at 0, 13, 26 and 40 percent of diet DM. They found DMI and milk yield decreased significantly when wet distillers grains comprised more than 20 percent of the diet. Similar results were observed when dry distillers grains comprised more than 13 percent of diet DM.
Schingoethe et al. fed wet distillers grains at 31.2 percent of the diet DM and saw a significant reduction in DMI but not in milk yield when wet distillers grains were included at this level. Neither milk fat percentage nor milk protein percentage changed as a result of feeding increasing amounts of wet distillers grains, although the yield of milk fat decreased. However, when feeding dry distillers grains, milk fat percentage decreased when more than 13 percent of the product was included in the diet.
When feeding diets based on equal proportions of corn silage and alfalfa, distillers grains can usually replace most if not all of the protein supplement. However, when corn silage is the only or predominant forage, the diet could contain more than 20 percent distillers grains. In this case, an additional source of supplemental protein may be needed, as well as additional lysine. Finally, when feeding diets consisting of mostly alfalfa, levels of distillers grains will need to be limited due to the dietary protein needs being met more easily.
As mentioned previously, one of the major concerns regarding distillers grains is the variation that can occur between distillers grains coming from different plants and products coming from the same plant that have been produced at different times or on different days. Producers will need to monitor supplies closely and adjust their diets to reflect changes in quality.
The variation in fat and phosphorus content is the greatest concern to dairy nutritionists and producers. Distillers grains are considered a moderate fat source, running around 10 percent ether extract on a DM basis. The general limit is to feed not more than 1.5 pounds of free fat, and if other sources of free fats, especially whole oilseeds such as soybeans or cottonseeds, are included in the diet, the maximal amount of distillers grains that should be included will be reduced.
Phosphorus supplementation can be reduced or eliminated when feeding distillers grains. Again, producers should monitor these levels carefully, especially since phosphorus is the basis for most nutrient management plans.
Other concerns relate to the storage life (usually five to seven days, longer in colder times of the year) of the wet product. Wet distillers grains can be ensiled in silage bags or a preservative can be added to lengthen storage life. A third option would be to mix the wet distillers grains with another feedstuff, such as soyhulls, corn silage, corn stalks or beet pulp, then ensile the mixture in a bag. PD
References omitted due to space but are available upon request.
—From 2005 4-State Dairy Management Seminar Proceedings
Table omitted but is available upon request to firstname.lastname@example.org.
Lee Kilmer, Associate Professor and Extension Dairy Specialist, Iowa State University