Distillers Grains in Cattle Diets: Energy by default?

Corn distillers dried grains (DDGS) are valued by ruminant nutritionists because of their high nutrient concentration. During ethanol production most of the starch in corn is fermented which leads to a nearly three-fold concentration of the rest of its nutrients. As a result, DDGS contain almost 30 percent crude protein, an energy density similar or even higher than corn grain, are a good source of phosphorus, and on top of it all, they do not require grinding. Research has shown dairy and beef cattle perform adequately in diets where DDGS constitute almost 1/5 to 1/3 of all feedstuffs in the total ration. Regardless if it is milk or weight gain cattle fed to maximize performance, require balanced, high energy diets. In the US this high energy density is obtained by increasing the concentration of cereal grains, particularly corn. Cattle challenged with higher starch concentrations however can be prone to suffer metabolic problems, acidosis being the most common. To avoid these problems nutritionists either regularly include buffers (i.e. sodium bicarbonate) in the ration or offer them free choice. In fact, to perform at their genetic potential dairy and beef cattle are oftentimes in the verge of subclinical acidosis. This low pH shifts the rumen microbial population with proliferation of non-structural carbohydrate-digesting bacteria in detriment of those that degrade structural carbohydrates. This results in a slow-down of fiber digestion which can usually be verified by the presence of undigested forage particles in the manure. As a result, even in the absence of clinical acidosis, the energy benefit of the inclusion of high energy feedstuffs can be offset by the fact that less energy is obtained from fiber fermentation.

Effects of low rumen pH

Subacute ruminal acidosis is not only a function of low pH but also for how long does this condition remain. Figure 1 shows the rumen pH of a cow which in spite of bouts of subclinical acidosis only developed the clinical form when pH was below 5.5 for almost 24 hours. The time elapsed between days 3 and 5 is likely required for microbial changes to occur. A Gram stain of ruminal fluid will reveal a change from the normal predominantly gram-negative bacteria to more gram-positive bacteria. Krause and Oetzel (2006) found that during the first 140 days of lactation, 12 to 40% of the cows had ruminal pH below 5.5. This threshold is more often used in beef cattle as subacute acidosis causes a reduction in feed intake and animal health issues, without negatively impacting fiber digestion (Beauchemin and Mcallister. 2016). Furthermore the authors suggested the threshold of pH 5.8 for dairy cows below which there was a negative effect on milk production. Sub-clinical acidosis is a common problem in feedlots and dairies across the US. As expressed above it happens because of the need to challenge animals with energy dense diets. Cattle fed mostly roughage have rumen pH that ranges between 6 and 7 whereas that in high concentrate diets have 5.5 to 6.

Figure 1. Sub-clinical acidosis occurred below pH 5.7; its clinical form below pH 5.2. Clinical acidosis developed on day 6 and was preceded by a prolonged period below of sub-clinical acidosis (Beauchemin; unpublished data).

“Functional effects” of certain feedstuffs

Putting a price tag on the effects of feedstuffs which could be labeled as “functional” is difficult. One example is the “effective fiber effect” (EFE) since there are at least two ways to assess it. One is the capacity to increase chewing, rumination, and as a result, elicit changes in the patterns of volatile fatty acid production in the rumen (more acetate and less propionate). The other are the changes in fermentation patterns (again more acetate and less propionate) because of the intrinsic higher digestibility of the fiber. Examples of such differences are the EFE of soy hulls compared to that of cottonseeds. With soy hulls the effects are mostly due to changes in ruminal fermentation patterns of volatile fatty acids (more acetate rather than propionate). With whole cottonseeds however, the effects are compounded between increased chewing and rumen buffering effect by saliva, plus changes in volatile fatty acid ratios in the rumen because of the high digestibility of cellulose in cottonseeds. Although different in nature, the end-results are the same, which are the changes in ruminal fermentation patterns.

With DDGS however the situation is different; they are high in digestible NDF, however, this fiber is ineffective (as measured by the increased chewing activity) due to its small particle size. The difference though, is that in spite of supplying energy in the form of fermentable fiber, protein, and fat, they supply little to no fermentable starch. In spite of the absence of starch their net energy value is high because of the higher concentration in oil, fermentable fiber, and protein. In the past DDGS contained almost 10% more energy than corn grain precisely because of this high oil concentration. Modern corn grain varieties contain between 3-4% oil and when starch is fermented to ethanol, oil is concentrated to 9-12%. Ethanol plants however that remove nearly half of the oil through centrifugation obtain as a result reduced-oil DDGS with 5-6% oil. Since oil has more than twice (2.25) the energy than carbohydrates or protein the removal of 50% of the fat results in reduced fat DDGS with very similar energy value compared to corn grain. Distillers’ grains have other beneficial effects that are derived precisely from what they do not add to the diet.

The importance of what DDGS do not supply

Nutritionists tend to assign values to feeds mostly depending on the nutrients they supply. In fact, least cost ration balancing programs compare feeds based on their nutrient concentration and their price per ton.

Most of the plant-derived feeds fed to cattle are relatively high in rumen degradable protein (RDP) with values close to 65-70%, which is fermented to ammonia in the rumen. If the rate of ammonia production exceeds the microbes’ ability to utilize it to form amino acids, there is an accumulation of rumen ammonia. Ammonia absorbed from the rumen into the systemic circulation is normally detoxified by the liver via the urea cycle a process that requires additional energy expenditure. This detoxification system however can also be overloaded resulting in elevated blood ammonia levels. One of the advantages of DDGS is that they usually contain between 50 to 35% RDP posing a less “taxing” effect on the ammonia detoxification process. As a result they leave room for other feeds with higher RDP (i.e. alfalfa hay, soybean meal, etc.) which are important sources of energy of fermentation, carbon chains, and lysine for both the rumen microbes and the ruminant.

In addition, little attention is usually paid to the fact that because DDGS have no fermentable starch left, they do not lower pH as much and as a result do not interfere with forage fiber fermentation. The strategic use of DDGS replacing part of the corn in the diet decreases the starch load. In spite of this, energy supplied by corn protein, fat, and structural carbohydrates not only offsets this removal but it does so while reducing the acidity of the rumen fluid. This higher pH improves fiber utilization allowing for greater energy obtained from roughage. The greater feed efficiency consistently observed in cattle diets at higher DDGS inclusions, is likely the combination of DDGS higher nutrient density, with a reduction on the negative effects on the digestibility of other feedstuffs in the daily ration.

The pH Paradigm

Feedlot rations that contain 85-90% concentrate are not unusual and the performance expected from a beef steer fed that level of concentrate are weight gains bordering 4 pounds per day. Figure 2 shows a beef feedlot cattle ration formulated using the NRC’s Nutrient Requirements of beef cattle. This ration has 90% concentrate inclusion and expected weight gains of almost 4 pounds per day.

Steers on ration 1 would be expected to eat almost 22 pounds of dry matter per day of which 19.8 pounds or (22 pounds as fed) will be concentrate. The forage fraction at 2.2 kg of dry matter (6.3 kg as fed) is supplied by corn silage (with 50% grain). With this ration both the metabolizable protein (MP) as well as the Degradable Intake Protein (DIP) are adequately covered. In fact, the first limitation in this ration would not be MP but energy. It supplies 12.5% CP of which 59% is DIP. The potential problem however is apparent in the second screen shot. The effective NDF (effective fiber or eNDF) supplied is 1.27 pounds per day whereas the animal requires 1.76. As a result the pH is predicted to be right at 5.67, borderline with subclinical acidosis as mentioned above. In the presence of any management constraints such as feed sorting/selection and/or not enough bunk space, it is likely the animals could develop clinical acidosis. In addition the utilization of the 2.2 pounds of corn silage would not be optimized since the acid conditions would not allow fiber-degrading enzymes to function at their ideal pH (6 and above).

One limitation though is that this balancing program uses eNDF to predict the diet’s ruminal pH. The predicted pH is used by the model to calculate microbial efficiency, which impacts the DIP requirement and MP supply. As a result, when corn is replaced by DDGS, eNDF remains the same (DDGS do not supply eNDF) and thus ruminal pH does not change in the model. In reality, the substitution of corn with DDGS removes fermentable starch and results in higher ruminal pH, in spite of having the same or even lower eNDF. This allows for rations to be formulated with lower eNDF while still maintaining a ruminal pH that is not likely to induce acidosis. With reduced fat DDGS having an energy content similar to that of ground shelled corn and no fermentable starch, substituting corn with DDGS will reduce subclinical and clinical acidosis episodes. This allows cattle producers to increase the energy density of the diets while maintaining or enhancing cattle health and animal performance.

Figure 2. Ration 1 Dry matter basis: ground shelled corn 86.4%, corn silage (33% DM) 10%, soybean meal (45%) 1.4%, minerals and vitamins 1.30%, urea 0.90%.