Grazing is a full-time job for horses. Given their preference, they would graze for 12 hours or more every day. Horses’ broad, flat teeth and sideways chewing motions make short work of the tough, stemmy grasses and weeds they favor. Given that horses ferment ingested fiber in their hind gut, they can make a living on lower quality forage than a cow can. Horses don’t need to reduce the fiber as much as cows. You can see it by comparing horse fecals to cow fecals. Horse fecals have significantly larger forage particles. The goal of the horse digestive tract is to get available, rapidly digestible nutrients from feedstuffs and move it through. They make up what the gut doesn’t use by volume of intake. Horses get a significant portion of their energy from the fermentation acids in the hind gut, but the proportion of their energy need met by these organic acids will be lower than that seen in cattle. The usual number given is that the fermentation of fiber to organic acids in the hind gut provides 30 to 70 percent of the animal’s energy.
It’s common to feed grains and fat to horses (for example, MFA Eazykeeper or Legends feed) to provide additional energy the animal may need. But, it is important to remember that fiber is an essential and important part of any equine diet— with the possible exception of very young foals. Dietary fiber provides energy horses need for everyday maintenance metabolism. Without adequate fiber, the horse’s digestive system doesn’t function properly and puts the horse at increased risk of metabolic diseases.
In terms of horse nutrition, when we discuss fiber, we are talking about the cell walls of plants. Plants have a rather substantial cell wall and it will be variable in digestibility. The material on the inside of the cell tends to be very digestible. The way to determine the amount of fiber (cell wall) is to sample the forage and process it in a small machine that is sort of like a washing machine. We use detergent to break up the cells and put starch/protein into solution. It is then spun down. What’s left is the fiber-cell walls. Feed labs will call this neutral detergent fiber (NDF). This is principally cellulose, hemicellulose and lignin. If acid is added to the washing\ machine, it chews through some of the fiber, principally the hemicellulose, to give us the acid detergent fiber (ADF). The lignin is not digestible, the cellulose and the hemicellulose will vary in digestibility. In legumes, about 75 percent of the NDF is ADF. In grasses, the ADF is about 66 percent. Grasses will tend to have more fiber than legumes, and the lignin in grasses tends to do a better job of protecting fiber from fermentation.
When a forage sample is sent off to the lab, the energy values that are determined are calculations from the ADF number. Forages with lower ADF numbers give higher energy values. Young forages have higher values than more mature forages. Legumes have higher energy values than grasses at the same stage of maturity. Cool-season grasses have higher energy values than warm-season grasses at the same maturity.
The rule of thumb is that fiber should be, at the least, half of a horse’s daily diet. Plenty of horses survive on more of a 100-percent-fiber diet. But that kind of feeding is giving the animals too much forage as a percent of the diet. While a forage base may very well meet the animal’s energy and protein needs, it will not meet the animal’s mineral needs. It will certainly be short of sodium, and in much of the Midwest, it will lack iodine, selenium, zinc and vitamin E. Given those caveats, an easy keeper will often make a living on principally a forage diet. To get what’s missing from the forage diet, a complete horse feed is an option. Some horses might require a concentrate to maintain animal performance. Either way, while the feed fraction of the diet may be optional, the forage component is not.
Remember, too, that not all fiber is created equally. Depending on where it came from and how old the plants were, forage-based fiber widely varies in quality, digestibility and palatability.