Distillers dried grains with solubles has become a popular livestock feed source, but it has some significant differences than feed grain.
Fibre, protein, fat, and mineral levels are about three times more concentrated in DDGS than they were in the original grain. Although cattle can use the fibre, protein and fat from DDGS for energy, the increased protein and mineral levels in it may be problematic.
When 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 the 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.
In two studies, finishing diets containing varying levels of DDGS were fed to heifers weighing 925 to 1,000 pounds. 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.
The University of Saskatchewan study 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.
The Lethbridge Research Station study 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.
When grain was replaced 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 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 feed intakes dropped as more and more silage was replaced with DDGS. So the increase in dietary N and P concentration that occurred as silage was replaced with DDGS 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 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.
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 run-off and ammonia volatilization.