“What Good is Roughage?”
by Chris Reinhardt, feedlot specialist
Normally, we consider the energy value of roughage in the finishing diet to be, at best, very low. The energy value is obviously very low compared to corn, but it does vary by quality. The problem is that forages have one energy value in ruminants when it is the primary ingredient, and another, much lower energy value when it is fed in a high-concentrate, grain-based diet, especially if that grain is extensively processed.
Certain bacteria are more effective at fermenting the cellulose in forages vs. other bacteria, and those bacteria digest forage to a greater extent when the rumen pH is above 6.0, such as is common in the rumen of forage-fed cattle, than in the rumen of feedlot cattle, in which the rumen pH is more acidic, and closer to 5.0. Forages are not included in the finishing diet to provide energy; they are included to provide risk management.
Aside from the intrinsic energy content of forage as a nutrient source, forages provide 2 factors that we consider during finishing feedlot diet formulation: energy dilution and scratch factor. By including some percentage of forage in place of grain, that percentage of the diet provides a limit to the total volume of grain available within a single meal which is available for rapid fermentation, limiting acid accumulation within the rumen. Simple math tells us that the greater the percentage of forage that is included in the finishing diet, the lower the ceiling will be on the amount of acid that is produced and accumulated within the rumen, reducing the risk of acidosis. Because forage has a much lower energy value than grain in the finishing diet, cattle feeders typically try to get away with as little in the diet as is necessary. In fact, increasing forage percentage in the finishing diet results in a greater amount of feed intake and total energy intake daily. Trial and error and experience typically lead each cattle feeder to a level of forage that is effective at maintaining a balance of consistent intake, performance, and feed efficiency.
Scratch factor is a less well-understood factor. If we use the percentage of liver abscesses as a poorly digestible forages (cottonseed hulls, wheat straw vs. alfalfa hay or corn silage) in the finishing diet results in fewer liver abscesses than feeding more finely processed forages; therefore, the coarser forage provides a greater scratch factor and protection from severe acidosis.
So why then do we process the forage at all? Because excessively coarse forages, included at only 8-10% of the finishing diet allow cattle to sort the diet for whatever component, the forage or the grain, that they prefer, resulting in potential acidosis for those cattle which only got the grain, after the forage had been picked through and removed from the bunk first.
This is then where some of the science of feeding cattle becomes art: forages must be processed fine enough to prevent sorting within the bunk, but left sufficiently coarse to provide adequate scratch. If forages are excessively processed to ensure mix uniformity, then a greater percentage of forage should be included in the diet to avoid acidosis.
An alternative method has been proposed to simply increasing the amount of processed forage included in the finishing diet: pulse feeding forage. Coarsely ground or long-stemmed hay can be fed in the bunk, apart from the normal mixed finishing diet, on a once or twice weekly basis, at a rate of 2-3 lbs per head per week. This can accomplish 2 goals: (1) increase the amount of fiber mat within the rumen resulting in (2) a greater amount of scratch within the rumen to stimulate rumination and salivation, resulting in greater intake of the diet and greater energy intake throughout the finishing period.
