Christine King BVSc, MANZCVS (equine), MVetClinStud
Fibre (lignin, ADF, NDF) Content
Fibre is the common name for the structural carbohydrates that give the leaves and stems of the plant their shape, internal structure, and strength. They are also called complex carbohydrates and slowly fermentable carbohydrates, but to avoid confusion with the potentially more problematic 'simple' carbohydrates, often referred to as "carbs," let's just call these ones fibre.
The fibre in forages such as grasses and legumes (e.g., lucerne) add substance or bulk to the contents of the horse's digestive tract, which is important for normal bowel motility.
It is said by equine nutritionists that "horses have an absolute need for roughage." Roughage refers to the high-fibre plant material that constitutes the majority of the horse's natural diet — grasses, legumes, and various other plants and plant parts such as bark.
In horses, the bare minimum roughage/forage intake for healthy gut function is 1% of body weight per day on a dry matter basis. For the average 450-kg horse, that's a minimum of 4.5 kg of dry matter per day. As hay is at least 90% dry matter, let's simplify the maths and say that 4.5 kg of hay per day is the bare minimum requirement for the average size horse who has little or no pasture access (or when there's little or no grass in the paddock).
Bearing in mind that fresh grass may be as much as 80% water at certain times of the year, a horse may graze all day and night and still not meet this minimum roughage/forage requirement when the grass is young. Horses on pasture full-time often benefit from supplemental hay in the spring, and perhaps in the autumn as well if there's been a lot of rain and the pasture is having a 'second spring'.
But forages are way more than 'filler'; they're food! When horses are fed a diet based on good quality forages, they may get more than 80% of their daily energy needs for maintenance (i.e., walking-around calories) just from the microbial breakdown of fibre in the large intestine.
So, skimping on hay because it's expensive or in short supply, or both, is short-changing the horse. Grasses and legumes are hands-down the best food for horses, because they are the most species-appropriate food for horses.
The green link below opens a diagram showing the different types of fibre that I'll be discussing:
(keep that tab open for reference as we go on)
The fibre content of a forage is broadly divided into three categories: neutral detergent fiber (NDF), acid detergent fiber (ADF), and lignin.
Why is lignin content important?
Let's dispense with lignin first, because it cannot be used by the horse. It can't be broken down by the horse's digestive enzymes and it is too slowly broken down by the gut microbes to contribute any significant nutrients to the horse.
The lignin and other fibres in forages such as grasses and legumes add bulk to the contents of the horse's digestive tract, which is important for normal bowel motility. However, too much lignin in a hay or pasture indicates a grass or legume that is too mature for horses, particularly for those with missing or damaged cheek teeth and for babies.
High-lignin hays are stemmy and coarse, have low palatability and digestibility for horses, and low nutrient content overall. These hays also increase the risk for impaction colic, which in most cases is caused by obstruction of the large intestine with a mass of firm, dry, fibrous feed material.
Good quality grass hays have a lignin content of 5% or less.
Good quality legume hays have a lignin content of 7% or less.
By the same token, forages with a very low lignin content are likely to be very immature (e.g., lush spring grass) and to have a high nonstructural carbohydrate (NSC) content, which is a concern in horses needing a calorie-controlled or low-carb diet, as I'll discuss on the next page about NSC.
Why is ADF and NDF content important?
Acid and neutral detergent refer to laboratory processes that don't concern us here. In short:
ADF consists of lignin + cellulose
NDF consists of lignin + cellulose + hemicellulose
Cellulose and hemicellulose are plant fibres that can be broken down by the microbes in the horse's large intestine. This microbial 'fermentation' generates small molecules (volatile fatty acids) the horse then absorbs and uses for energy. That is how a forage-based diet ably meets the daily energy (calorie) needs for maintenance in adult horses.
Cellulose and hemicellulose are both "cool calories" as plant carbohydrates go. Hemicellulose is more easily broken down by the gut microbes than cellulose, so we want forages that have substantially more NDF than ADF.
Good quality hays (grass or legume) have an ADF content of between 30% and 40%; mid-range is ideal.
Good quality grass hays have an NDF content of 50% to 60%; mid-range is ideal.
Good quality legume hays have an NDF content of 40% to 50%; mid-to upper range is ideal.
Healthy young adult horses can often contend with hays that have an NDF content of well over 60%, but the higher the NDF (and therefore the ADF and lignin), the lower the protein content, palatability, digestibility, and overall nutrient content — and the higher the colic risk, at least for impactions.
One study in horses showed that feeding nutritional yeast (Saccharomyces sp.) slightly improved the digestibility of very coarse hay — but it did not improve the digestibility of good quality hay. Rather than going this route, it is far better to source some good quality hay, whether baled or as haylage, chaff, hay cubes, or hay pellets. It is simply not possible to adequately improve poor quality hay such that it's worth feeding to horses. Veterinary care when the horse colics will eat up any money that might have been saved. Even then, it's the horse who ends up paying the most when things go wrong.
By the same token, the lower the NDF and ADF, the higher the content of nonfibre carbohydrates, including the potentially problematic nonstructural carbohydrates.
The 'sweet spot' for almost all horses, including seniors and babies, is an NDF content of between 50% and 60% for grasses and about 10% lower for legumes.