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.

'Roughage' and 'forage' are terms that are often used interchangeably. One could argue that roughage is the more appropriate term, as forage implies that the horse has to go searching for it (like one forages for wild mushrooms), which is not the case in most horses, whose food is brought to them by an obliging person. However, forage is the more widely used term for the high-fibre feeds in a horse's diet, and grazing horses do indeed forage, so it's the term I'll use most here.

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).

For horses with missing or damaged cheek teeth (premolars and molars) or with respiratory problems that are aggravated by hay, this daily hay ration may be fed as chaff, haylage, or as soaked hay cubes or hay pellets. However, there also appears to be a basic need for some 'long-stem' fibre in the diet, such as hay from a bale or mature pasture grasses. Even senior horses with few functional cheek teeth left can still benefit from having at least some 'long-stem' hay, although it really needs to be high quality hay (discussed later). Same goes for foals and weanlings, whose teeth and the rest of the digestive system are still developing.
The reason long-stem fibre is needed is jokingly referred to in the biz as the 'scratch factor' of the longer pieces in the intestine. Perhaps better called the 'bulk effect', these bulky foods help ensure good bowel motility by stimulating productive contractions of the bowel wall to mix the feed with digestive juices and propel it along the digestive tract.

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'.

Depending on the climate and on weather conditions that year, hay may also be needed in the summer or winter, when there's very little grass in the pasture or what's there is too fibrous. More on that later.

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.

Microbial fermentation of dietary fibre generates small organic acids, called volatile fatty acids (VFAs), primarily acetate and propionate. The VFAs are absorbed into the bloodstream; some are used right away as fuel, and the rest are converted into glucose or fats for storage and later use.

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.

Of course, the best food for young foals is their mother's milk. But by about 2 weeks of age, foals are eating whatever their mother is eating, in addition to their milk-based 'starter' diet.

The green link below opens a diagram showing the different types of fibre that I'll be discussing:

plant carbohydrates

(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.

A good analogy is the woody plant material in your compost pile (well, mine anyway) that takes years to break down. Wood is at least 25% lignin.

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.

As a point of comparison, wood is about 45% cellulose and 25% hemicellulose, so wood has an ADF of about 70% and an NDF of about 95%. Is it any wonder that horses fed insufficient roughage chew on trees, wooden fencing, stall doors, door frames, and any other wooden structure they can reach?!
The minimum roughage requirement (1% bwt/day) is simply not negotiable. In fact, most horses do best on twice that amount, which is a roughage intake of around 2% bwt/day; the range is between 1.5% and 3%, depending on the horse. For the average size horse (450 kg bwt), 2% is 9 kg of hay; 1.5% is just under 7 kg of hay, and 3% is 13.5 kg of hay per day.

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.

In general, low-NDF hays were cut when the grass or legume was quite immature. They tend to be soft, leafy, fragrant, delectable hays, but they may not store as well as more mature hays (in part because they're hard to dry well enough before baling), and they can be a serious problem for horses needing a calorie-controlled or low-carb diet.

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.