When bio-ethanol is produced from corn, wheat or other grains, yeast microbes convert the grain starch into glucose, and ferment the glucose into ethanol. The non-starch components of the grain are not converted to ethanol, and end up in distillers dried grains with solubles (DDGS) and other byproducts. Fibre, protein, fat, and mineral levels are about three times more concentrated in DDGS than they were in the original grain.
High feed grain prices and increased availability of DDGS have increased the use of these byproducts in North American feedlot rations. Cattle can use the fibre, protein and fat found in DDGS for energy, but the increased protein (and mineral) levels may pose a problem. If cattle consume more protein than they need, the excess protein can be metabolized for energy. But protein metabolism also releases nitrogen (N), which is converted to urea in the kidneys and excreted (primarily in urine). Similarly, if cattle consume more phosphorus (P) than they can absorb, much of the excess P will be excreted in the manure. Provincial nutrient-management regulations are increasingly focused on levels of N, P, or both. As a result, it is important to know how incorporating DDGS into beef cattle finishing diets may impact manure nutrient levels. Several recent National Check-Off funded Beef Cattle Research Council projects have examined this issue.
What they did
In three metabolic studies, feeder steers or heifers averaging 925 to 1050 lb. were fed finishing diets containing varying levels of DDGS. A different diet was fed to each animal (or pair of animals) for a three-week period. After two weeks of adaptation to the diets, nutrient digestibility was evaluated from a series of intensive feed intake, rumen, urine and fecal measurements. The animals then were rotated to another diet, and this was repeated until all animals had been fed all of the diets. Two university studies examined whether replacing grain with DDGS affected N and P excretion. Kendall Swanson (at the University of Guelph before moving to North Dakota State University) compared an control diet (80 per cent dry whole corn, 10 per cent haylage, 10 per cent supplement) to three diets that replaced some of the corn grain with 17, 33 or 50 per cent corn DDGS. At the University of Saskatchewan, John McKinnon and Lee-Anne Walter compared a control diet (89 per cent rolled barley, six per cent barley silage, five per cent supplement) to four diets that replaced some of the barley grain with 20 or 40 per cent corn DDGS, or 20 or 40 per cent wheat DDGS. In the third study at AAFC s Lethbridge Research Station, Wenzhu Yang and Tim McAllister compared a control diet (83 per cent temper-rolled barley, 15 per cent barley silage, two per cent supplement) to three diets that replaced 20 per cent of the barley and five per cent, 10 per cent or all of the silage with 25 per cent, 30 per cent or 35 per cent wheat DDGS.
What they learned
In the two studies that replaced grain with DDGS, dietary crude protein and P levels rose as the amount of DDGS in the diet increased. Feed intake did not change a great deal, so this meant that the animals consumed more N and P as more and more grain was replaced with DDGS. Manure and/or urine production also tended to increase as the DDGS content of the diet increased. Greater amounts of N and P were excreted as DDGS inclusion rates rose. The University of Saskatchewan study also reported that the manure had higher N and P values when cattle were fed wheat DDGS than when they were fed corn DDGS.
When silage was replaced with DDGS, dietary crude protein and P levels also rose as the amount of DDGS in the diet increased. But (as noted in last month s column), feed intakes dropped as more and more silage was replaced with DDGS. So even though the dietary N and P concentration increased as silage was replaced with DDGS. This was balanced out by the drop in feed intake. As a result, cattle ate about the same amount of N and P, and excretion rates for both nutrients were fairly stable as dietary DDGS level increased from 20 to 30 to 35 per cent DDGS. But if manure output decreased (due to reduced feed intake), the concentration of N and P in the manure may have increased.
What it means
Incorporating DDGS into feedlot finishing diets can increase the amount of N and P excreted by beef cattle and could increase the land base needed for spreading. When DDGS replaces increasing amounts of grain in the diet, manure N and P levels rise steadily. When DDGS replaces silage in the diet, manure N and P concentrations increase, but this may be partly offset by lower manure production. Under commercial conditions, the N and P content of feedlot manure will also be influenced by bedding, weather conditions and other factors that affect nutrient runoff and ammonia volatilization. Ongoing BCRC research is examining nutrient losses in composted vs. non-composted manure from DDGS-fed cattle, as well as impacts on N and P release in different soil types. This will help develop more appropriate recommendations for valuing and applying beef manure.