INTRODUCTION
Current feeding practices on Pennsylvania dairy farms consist of providing a substantial portion of dietary dry matter as corn grain. Given the growing demand for renewable fuel production and the current reliance on corn for meeting this demand, the cost of corn for Pennsylvania dairy farm feed may become prohibitive. The recent expansion of fuel ethanol production capacity has resulted in an increased availability of ethanol byproducts for dairy cattle feed. Availability of corn distillers grains (DG), often called distillers grains, has increased substantially and, consequently, the interest in using these feeds in dairy cattle diets has also increased. New ethanol plants in the eastern part of the US including Pennsylvania, eastern Ohio and western New York state will increase the availability and potential cost effectiveness of wet and dried distillers grains in our typical Pennsylvania dairy rations. While byproducts of corn fermentation to ethanol have been used for ruminant diets for many years, interest and research in feeding distillers grains to dairy cows has increased in parallel with overall fuel ethanol production.
Along with most of the feeds that can be utilized to provide nutrients to the dairy cow, distillers grains present the dairy farmer and nutritionist with unique opportunities and challenges that must be evaluated on an individual farm basis. The current paper will review data related to feeding distillers grains to lactating dairy cows and the challenges that may arise due to their inclusion.
PRODUCTION AND STORAGE OF DISTILLERS GRAINS
Traditionally, distillers grains (DG) have been produced in conjunction with distilling alcohol for human consumption. While the procedures involved in traditional and modern DG production are generally similar, the advancement of technology, difference of ethanol consumer, and the recognition of livestock producers as important byproduct consumers have resulted in a new generation of DG. As a result, it is important to realize that the old analyses for DG as found in the NRC (2001) are not accurate due to the more recent systems being used to make ethanol.
Ethanol can be fermented from any high carbohydrate substrate such as sugarcane, corn, and switchgrass with varying degrees of efficiency. Currently in the United States, the primary carbohydrate used for this fermentation is corn starch. The primary method for corn ethanol production is dry grinding (Kalscheur et al., 2008). With this process, corn grain is ground to a medium- fine particle size and water and enzymes are added to degrade the starch into glucose. This mixture is cooked and sterilized to kill bacteria. The sterilized mash is then cooled and inoculated with yeast that will ferment glucose to ethanol and carbon dioxide. Ethanol is then distilled and the remaining solids and water are processed into wet distillers grain (WDG) and distillers solubles. The solubles are typically recombined with the WDG to result in WDG with solubles (WDGS). The WDGS can be dried to yield dried distillers grains with solubles (DDGS). Each of these byproducts has been feed to dairy cattle.
The most easily transported and stored byproduct of ethanol production is DDGS. Since this form is dry, it can be shipped to any location from any location in the country and can be stored under reasonable conditions indefinitely. Drying DG requires energy and time and costs money; as a result DDGS are more expensive than WDGS on a dry matter basis. However, WDGS are less efficient to ship due to the requirement to ship water and more difficult to store due to more favorable conditions for spoilage. As a consequence, WDGS are usually only available for feed on farms in close proximity to an ethanol plant and with the ability to feed each delivery of the byproduct rapidly. Spoilage may occur within 5 to 7 days after production but this depends on the ambient temperature. Facilities for storing WDGS can also be problematic due to its high moisture content (typically 35% DM). Storage losses can be very high with WDGS. The shelf-life of WDGS may be extended by limiting oxygen during storage, although storing in combination with other feeds such as soybean hulls or with the inclusion of a preservative may also extend the duration of storage. Research in optimal storage conditions for WDGS is still a very active area; with some looking at adding forages or other dry feeds to the WDGS prior to ensiling or bagging to improve packing abilty. However at this time few recommendations exist.
NUTRIENT COMPOSITION OF DISTILLERS GRAINS
Since fuel ethanol in the United States is produced by removal of starch from corn, the composition of DG is, for the most part, predictable from the composition of corn. As a general conversion rule, with the exception of starch (which is primarily removed), the nutrient composition of DG will be roughly 3 times as concentrated as it is in corn grain. This is because the weight of starch is approximately 2/3 the weight of corn. If the conversion of the nutrient composition of corn to DG was always true, the nutrient composition of DG would have low variability since the nutrient composition of corn has low variability. This has not generally been the case however, both within and between ethanol plants. Reasons for differences in nutrient composition can be related to small differences in any of the processes within plants over time and processes between plants. Additionally, solubles are added back to DG in variable proportions, increasing the chemical variation in both WDGS and DDGS. These factors can create notable differences in DDGS coming from each individual ethanol manufacturing plant.
Chemical composition and variation of DDGS from different ethanol plants are shown in Table 1. Distillers grains is primarily considered to be a supplemental source of crude protein (CP), especially rumen undegradable protein (RUP). From Table 1 it can be seen that, along with CP, DG contain an equally high level of neutral detergent fiber (NDF) and a relatively high level of ether extract (EE) and phosphorus (P). Additionally, concentration of sulfur (S) may exceed 1% of DM—the concentration of this nutrient is higher than expected based on its concentration in corn and is quite variable due to the addition of sulfuric acid during the production process.
The NDF in DG has a rapid rate and a high extent of digestion. By contrast, the CP of DG is slowly degraded in the rumen due to being comprised primarily of the protein zein. Consistent with the absence of lysine in zein, the amino acid composition of CP is similar to that of corn, in that lysine is often a limiting amino acid for milk production.
It is apparent that the variation between the maximum and minimum nutrient composition of DG is quite large for many of the components. This is especially true for crude protein and the protein fraction RUP, which is a fundamental reason to put DDGS in the dairy ration. This observation leads to the recommendation of securing a good supplier of DG that produces a consistent product over time as well as the need for frequent wet chemistry feed analyses of the product for use in ration balancing. In most cases the specific plant that the DDGS originates from will have a specific analysis due to that plant’s specific ethanol manufacturing process. Influential characteristics of the process include plant design, processing or particle size of the grain, extent of fermentation and drying temperatures. In addition, field experiences have shown a large amount of variability in DDGS analysis can occur from batch to batch and year to year due to differences in the corn (source and quality) that is used. Some commercial DDGS products have been developed to minimize this variation problem and create a more uniform DDGS product for delivery to the feed mill or farm.
