Fiber digestibility. The term has been around a long time, but it’s not an easy concept to grasp. It’s complicated. Dr. Gonzalo Ferreira, associate professor at Virginia Tech, breaks it down for us.
To understand fiber digestibility, it’s important to first define fiber, and there are two definitions. One is agronomic, the other is nutritional. From an agronomic perspective, fiber is a structural cell component that gives support to the plant. It’s considered a positive attribute – more fiber equals a stronger, healthier plant. From a nutritional standpoint, fiber has components that are slowly digested in the gastrointestinal (GI) tract. In this case, fiber typically has a negative connotation because it can require more of the cow’s energy due to its slow digestibility.
“Starch and cellulose (a fiber component) chemically have the same composition, so we need to understand why starch is highly digestible and cellulose isn’t,” said Ferreira. “The difference is due to the way the molecules of glucose are attached. For starch, the molecules are attached loosely, but for cellulose, they are tightly packed and aligned to each other, making it harder to break down the molecule. But for starch, there’s less structure, so it’s easier to break apart and digest.”
How is fiber digested?
Digestibility means the hydrolysis or breakdown of a substrate and the nutrient release in the GI tract. Digestion of fiber starts with carbohydrates being released from the fiber, then the bacteria take the carbohydrates and they ferment it, producing volatile fatty acids: propionic acid, butyric acid and acetic acid. The cow absorbs those volatile fatty acids and converts them in different organs. For example, propionate typically is captured by the liver and converted into glucose. Acetic acid is taken by the mammary gland and converted into milk fat, or it can be taken by the animal’s tissue and goes into body reserves and body fat.
“Fiber is composed mainly of hemicellulose, cellulose, lignin and pectin. Pectin usually ferments very rapidly and behaves like a sugar. Hemicellulose can ferment rapidly, depending on the rest of the fiber,” Ferreira explained. “So we look at digestion rate and passage rate. The digestion rate is how fast the cellulose and hemicellulose can be digested, which depends on bacteria availability and giving the bacteria time to ferment the components. The time to ferment is the rate of passage. If the feed passes too fast, then the bacteria have no time to ferment the fiber.”
Having a fibrous layer within the rumen, known as a rumen mat, helps to retain feed in the rumen and give it time to digest.
“The key is that the rumen mat should not be highly undigestible. Why? Think of it like a sink. If you block the sink, nothing’s going to get out and you can’t put anything in,” Ferreira noted. “If nothing is passing through the rumen, then dry matter intake is limited. So, we need to find the right equilibrium between digestion rate and passage rate.”
High and low fiber digestibility
The main determinant of high or low fiber digestibility is going to be the concentration of lignin in the cell wall. Lignin is considered undigestible or minimally digestible. Another important characteristic of lignin is that it’s hydrophobic, meaning it repels water. It doesn’t allow enzymes that hydrolyze the other carbohydrates, cellulose and hemicellulose to get in and do their job.
“Lignin is like a shield and is not good from a nutritional perspective,” he said. “When we feed the whole corn plant, we need to look at the ratio between the leaves and the stems because fiber digestibility depends on the composition of the fiber. Typically, the fiber in the leaves has a very low concentration of lignin, and the stems, because they give support to the plant, have a high concentration of lignin. Therefore, the leaves are more digestible than the stems.”
Early feedout this season
Given the wet spring much of the Midwest experienced, many producers may be harvesting later than usual, which could require an early feedout. Ferreira said the first step is to conduct a forage inventory and a forage balance to determine how much silage you have right now. Next, estimate when you’re going to be harvesting. Based on your balance, it’s possible to start making decisions.
“Many producers typically have up to six months’ carryover, but if that’s not available, have a conversation with your nutritionist to determine your best option,” he explained. “One choice is to stretch what you have, which means lowering the forage in the diet. So instead of feeding out a 60% forage diet, you’d bring it down to 50%, allowing you to stretch your inventory. The other approach would be to buy more forage, which is not cheap, but it’s another option.
“The fermentation of silage stabilizes between 14 and 21 days. Data suggests a longer storage can help starch digestibility, but that doesn’t mean it is going to help fiber digestibility,” Ferreira said. “The fiber digestibility of your silage is going to be the same as the digestibility of the fiber of the crop. The fibrous components of your silage will never be better than the composition of the standing crop.”
While not ideal, it is possible to open a silo at 14 to 21 days, which is likely earlier than you normally would during a typical growing season. Use of an inoculant can also kick-start the process and ensure a good, quick fermentation.
Feature photo courtesy of the University of Wisconsin
Dairy Herd Management