Fiber in Hay: What's the Magic Number?

Horses evolved to eat a lot of fiber, spending up to 17 hours a day grazing various forage plants. But not all fiber is created equal, especially when it comes to hay.

Hay carries a few challenges compared to living forages. One, compared to fresh forage, dry hay lacks the moisture needed to move fiber along the digestive tract. Unlimited access to fresh clean water is essential when feeding a lot of hay, as impaction colic can result if the hay is too high in fiber, says Kathleen Crandell, PhD, an equine nutritionist with Virginia Tech's Middleburg Agricultural Research and Extension Center. She also says if bulky hay is too high in fiber (as in very mature hays), horses will fill up on the hay, but in doing so might not consume enough calories to maintain body condition if they are hard keepers or hard-working horses.

So what's the right amount of fiber? There are two measures of fiber in forages that can give the owner an idea of fiber content and forage quality. Horses need a combination of fiber types to maintain digestive tract health.

  • Neutral detergent fiber (NDF) is a measure of plant cell wall carbohydrates. This measure includes fiber the horse can digest (hemicellulose and cellulose) and fiber he can't (lignin).
  • Acid detergent fiber (ADF) is a measure of cellulose, which is less digestible in horses, and lignin, which horses cannot digest. The difference between NDF and ADF is the fiber hemicellulose that the horse can readily digest.

Both fiber measures help indicate forage quality; as they increase, quality decreases. Crandell says high-quality forages tend to have NDF around 35-55% and ADF of 25-35% (as-fed). Lower-quality forages will have NDF of 55-70% and ADF of 35-45%. Hay with the lowest ADF for a comparable amount of NDF is better-quality hay.

Caution should be taken when feeding hay that's over 65% NDF, because it increases the risk of impaction colic, especially if horses aren't drinking a lot of water, warns Crandell. On the flip side, if horses don't get enough fiber, they might try to find it on their own by chewing on fences, stall walls, or even eating their own manure. (These behaviors can occur for other reasons as well, such as boredom).

Crandell says you can get a rough idea of fiber content by the hay's appearance: "You can almost look at the hay and tell how much digestible fiber it has by seeing how stemmy it is and estimating its leaf-to-stem ratio. Good-quality hay will have considerably more leaf than stem. Very leafy hay will be a lot lower overall in ADF and NDF, and it will be more digestible than something with few leaves, many seed heads, and lots of thick stems. High-quality forages are great for horses with high energy demands, and lower-quality ones may be more suitable to horses with slower metabolisms."

You can also take core samples from several bales and send them off for analysis to find out exactly how much fiber and other nutrients your horse will get from that hay. For more information on sampling hay and pasture, see "Analyzing Forages."

So what are the magic fiber numbers for your horse? Keep NDF under 65%, but beyond that the ideal amounts to maintain weight and health will depend on his individual metabolism, work level, and whatever else he's eating.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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