Forage Testing: Nonstructural Carbohydrate (NSC) Content

Nonstructural Carbohydrate (NSC) Content

Now we come to one of the most important reasons for forage testing: the potentially problematic nonstructural carbs. They include starch, simple sugars, and the rapidly fermentable carbohydrates such as fructans.

A brief note before we get into it: When I say horse I mean all types of horses, including ponies, miniature horses, and draft breeds. Despite their differences in size, shape, heritage, and occupation, they're all horses, inside and out.

Here's the link to the plant carbohydrate diagram again, in case you don't already have it open:

plant carbohydrates

(keep that tab open for reference as we go on)

This subject could fill a book, but to get right down to what interests us most — the horse — here is a list of some (some!) of the conditions that are prevented or more easily managed with a low-NSC diet:

equine metabolic syndrome (EMS) — most horses with EMS are overweight or obese, and if they don't already have laminitis, they are at high risk on a high-NSC diet
equine Cushing's disease (ECD), also known as pituitary pars intermedia dysfunction (PPID) — this condition is most common in senior horses; although many horses with ECD/PPID are underweight, they generally do best on a low-NSC diet, supplemented with dietary fat and/or protein (minus the fat if they don't need to gain weight)
pasture-associated laminitis (PAL) — laminitis in horses on pasture is most common in those with EMS and/or ECD, but not always; there are other, less common, factors that increase a horse's risk for PAL
equine polysaccharide storage myopathy (EPSM or PSSM) — this muscle disorder is one of the causes of 'tying up' (exercise-induced muscle damage); it is most common in Quarter Horses, Warmbloods, and draft breeds
recurrent exertional rhabdomyolysis (RER) — this muscle disorder also causes 'tying up', but via a different mechanism from EPSM; it is most common in Thoroughbreds, Arabians, and Standardbreds
equine systemic proteoglycan accumulation (ESPA) — this systemic connective tissue disorder is best known for causing degenerative suspensory ligament desmitis (DSLD) but it actually has body-wide effects
developmental orthopaedic disease (DOD), such as physitis (formerly 'epiphysitis'); angular limb deformities in foals ('crooked legs'); flexural limb deformities in foals, weanlings, and yearlings ('contracted tendons'); and...
osteochondrosis — just like the other DODs, this abnormality of joint cartilage and bone development in the limbs and spine is responsive to NSC content and to trace minerals such as copper and zinc, but only while the horse is growing
colic — because NSCs are rapidly fermented by gut microbes, high-NSC diets carry an increased risk for several different types of colic, including gas colic, spasmodic colic, large colon displacements, anterior enteritis (duodenitis-proximal jejunitis), and probably several other types of colic as well
equine gastric ulcer syndrome (EGUS) — high-NSC diets, particularly high-starch diets such as those with a lot of grain, are a risk factor for EGUS
anxiety and other stress-related behaviours

Note that a low-NSC diet doesn't have to be a low-calorie diet. Performance horses, pregnant or lactating mares, and growing horses are all categories that often need more calories and other nutrients than forages in any amount can provide. Adding a source of fat (oily seeds, nuts, vegetable oil, rice bran, etc.) provides 'safe' extra calories for horses needing a low-NSC, high-calorie diet.

I'm not a fan of 'fermentable fibres' such as beet pulp and the various seed hulls (oat, soyabean, etc.). They provide a few more calories per kg than grasses, but nowhere near as much as fats. Also, they are not always low in NSC. In my view, these byproducts of the human food industry represent a triumph of marketing rather than an advancement in equine nutrition. They may have their place in equine nutrition, but in my considered opinion that place is very small. These byproducts are way overused; and when they are used in place of available forages, they are being misused. Horses are designed to run on grasses primarily, with lesser amounts of legumes and various other plants. Whenever we stray too far from this species-appropriate diet, trouble ensues.

*****

As discussed on the fibre page, horses on forage-based diets may get more than 80% of their daily energy needs from the microbial fermentation of fibre in the large intestine. The nonstructural carbs in forages contribute much less in a caloric sense, but their contributions to obesity, laminitis, colic, exercise-related muscle disorders, etc. can be far greater.

When it comes to laminitis risk, starch and simple sugars are a potential problem because they are rapidly broken down by the horse's digestive enzymes, starting in the mouth with amylase in the horse's saliva. This process is called hydrolysis, so these carbs are called 'hydrolysable'. That term doesn't interest us here. What's important about these particular carbs is that they cause a rapid rise in blood glucose ("blood sugar") after a meal, which stimulates the pancreas to release insulin into the bloodstream.

In small amounts, that's generally a good thing, because glucose cannot enter most cells to be used for energy production unless insulin is present to 'unlock the door' for it. However, large spikes in insulin or persistently high insulin levels in the bloodstream increase the horse's risk for laminitis. The higher the insulin, the greater the laminitis risk.

Fructans increase the horse's laminitis risk in a different way. These polysaccharides cannot be hydrolysed, so they do not cause large spikes in blood glucose, and thus insulin. However, they are rapidly fermented by the microbes in the large intestine. This process generates gas and lactic acid, which in large enough quantity causes a massive die-off of the acid-sensitive bacteria and absorption of microbial byproducts into the bloodstream through 'leaky' junctions between the cells lining the intestine (low-grade colitis, or 'leaky gut'). One of the manifestations of the systemic inflammatory response that ensues may be laminitis.

This is a common mechanism of pasture-associated laminitis, but it can occur in horses fed high-fructan hay or chaff as well. High-fructan pastures and hay can also cause colic and either loose manure or diarrhoea.

For this reason, we pay these 'hydrolysable' and 'rapidly fermentable' carbohydrates a great deal of attention, particularly when selecting hays for horses needing a calorie-controlled or low-NSC diet.

Of the various NSC components in forages, I'm most interested in water-soluble carbohydrates (WSC).

Why is WSC content important?

The water-soluble carbs include the simple sugars (mono- and disaccharides), oligosaccharides (including fructo-oligosaccharides, or FOS*), and fructan polysaccharides (usually just called fructans). They don't include starch because starch is not water-soluble (ever tried to dissolve flour in water...?). However, starch is low in most forages (generally less than 3% in grass hays, and often less than 1%), so it's the WSC content that causes the most problems in grass hays and pastures.

* Fructo-oligosaccharides (FOS), also known as oligofructose, have been appearing in processed horse feeds for several years now. The most common type of FOS is inulin (not to be confused with insulin), derived from plants such as Jerusalem artichoke, chicory, agave, and even bananas. They're promoted as "prebiotics" (substances fed to support the gut microbes), based on widespread use in human supplements.

In small quantities, they're probably not an issue (and probably not of any use, either), but bear in mind that they are rapidly fermentable carbs, in the same category as fructans. In large quantities, they have the potential to cause digestive upsets (e.g., gas colic, loose manure/diarrhoea) and potentially even laminitis. As a grisly example, oligofructose is commonly used in laminitis research because in sufficient quantity it reliably causes laminitis. Don't be swayed by the addition of FOS to horse feeds. It's a marketing gimmick at best; and at worst it's a potential problem. It also increases the price of the feed.

For horses needing a low-NSC diet, the WSC content of the forage should be less than 11% on a DM basis.

With hays, that translates to a WSC content of less than 10% as-fed (as-sampled).

The as-sampled or as-fed value includes the "diluting" effect of whatever water is in the forage sample. Good quality hays are typically only about 10% water, but fresh pasture may be over 80% water when the WSC content is likely to be at or near its highest. Particularly with fresh pasture samples, it's best to use the 'DM' cutoff (less than 11% WSC) because it's taken the water out of the equation entirely. With hay, it's generally OK to look at the as-sampled or as-fed cutoff (less than 10% WSC).

You'll often see recommendations for at-risk horses of less than 10% WSC (Dry Matter), which works out to less than 9% WSC as-sampled. This 1% here-or-there may seem like splitting hairs, and perhaps it is; but many of these overweight, laminitis-prone horses are living on a knife-edge, and such details can mean the difference between developing laminitis and not. That said, I can tell you that I've seen no problems with using an upper limit of 10% WSC as-sampled; and as I'll discuss in a bit, there are distinct advantages to gracing your horse with a less mature hay than one with a WSC content of less than 10% DM.

Cereal hays, such as oat ('oaten'), wheat ('wheaten'), barley, and triticale, are commonly used to produce chaff for horse feeds. However, the WSC content of these cereal hays/chaffs can be very high. For example, FeedTest in Victoria reports the following values for cereal hays tested during the 2018–2019 season:

barley hay — average 24.9% WSC (range, 1.28% to 36.8%) DM
oaten hay — average 24.1% (range, 0.5% to 42.6%) DM
wheaten hay — average 23.4% (range, 0.2% to 41%) DM

Not only is the upper end of the range scary-high for all of these cereal hays, but even the average WSC values are simply too high for horses needing a reliably low-NSC diet.

Cereal hays with WSC values at the low end of the range are safe, but...

(a) they are likely to have very high NDF values (up to 79% NDF for oat and wheat hay, and 83% NDF for barley hay in the 2018–19 season), with all the associated problems; and

(b) you won't know if the hay/chaff is low in WSC unless you forage test it.

These cereal hays are attractive because they are often more widely available and less expensive than grass hays. However, they're just not worth the risk in horses needing a calorie-controlled or low-NSC diet.

Why is NSC content important?

Nonstructural carbohydrates = starch + WSC. With grasses (pasture or hay), the starch content is fairly low, so the NSC content generally doesn't provide much more information than the WSC content.

Cereal hays, however, may contain enough starch to be problematic. For example, Equi-Analytical reports that while the average starch content of cereal hays is in the 3–5% (DM) range, oat and wheat hays may be as much as 8% starch, and barley hay may be as high as 10.5% starch. Add that to their WSC content, and these cereal hays/chaffs can be high-risk feeds.

Many nutritionists and veterinarians talk about the NSC content when discussing how to feed at-risk horses. A common recommendation for overweight, laminitis-prone horses is to feed a hay that has a NSC content of less than 10% (Dry Matter). That translates to less than 9% NSC as-fed.

I have some problems with that recommendation:

1. Poor palalatability — that's a selling point for this 'cardboard' diet, but in my experience it greatly increases hunger, discontent, and the incidence of potentially disastrous escapes onto fresh grass — i.e., real food!

2. Poor digestibility — such a low NSC content typically goes hand-in-hand with a high ADF content. Not only does that increase the risk for impaction colic (not even the gut microbes can do much with this hay!), it practically guarantees malnutrition, and although it may not be as severe as frank starvation, it greatly impedes recovery.

3. Inadequate crude protein — often the protein content is too low to meet the horse's maintenance needs when healthy, let alone when recovering from laminitis or dealing with metabolic syndrome (which I find is very responsive to careful protein supplementation).

4. Inadequate vitamin and mineral content — these overly mature hays are often low in phosphorus (and sometimes calcium as well) and trace minerals; all hays contain negligible amounts of vitamin E, and these ones may also be low in vitamins A and D.

Managing 'carb-sensitive' horses is a topic for another time. Here is my bottom line for hays (use the DM values for fresh pasture):

The NSC content for healthy horses with no special needs should be no more than about 18% as-fed (20% DM).

I use this cutoff so that these horses don't become at-risk horses...

The NSC content for at-risk horses should be between 10% and 12% as-fed (11–13% DM), but no lower.

I've been using this recommendation in my practice for many years without any problems. In my experience, nudging up the NSC cutoff by a percentage point or two represents a 'happy medium' between medically safe and nutritionally adequate — which includes palatable and satisfying.

back