Dry Matter (DM) Content

The dry matter content is an index of the water content, as DM is everything in the forage minus water. For our purposes, DM content is most useful when reported as a percentage, because DM + water = 100% of the forage content.

Particularly with hay, the DM content is an important indicator of storage quality or storability.

A good quality hay has a DM content of at least 90%.
That is, the hay has a water content of 10% or less.

Of course, that's not the only thing that defines a good quality hay. A DM content of less than 90% (water content greater than 10%) does, however, disqualify a hay from being considered good quality, in my experience. Particularly in wet or humid climates/seasons, such hays do not store well.

The DM content of fresh pasture varies tremendously with the stage of plant growth (i.e., the season) and with weather conditions. In the spring, when the grass is lush and growing rapidly, the DM content may be less than 20% (i.e., the grass may be more than 80% water). The same thing may happen again in the autumn in areas that have autumn rain and a second 'flush' of grass after a dry summer.

In the late summer, the DM content may be 70% or more (i.e., the mature grass may be only 30% water, or less), depending on rainfall. The average DM content of pasture over the course of the year is about 45% (i.e., about 55% water), but that average doesn't mean very much when a single pasture may swing from a DM content of less than 20% in the spring to over 70% in the late summer.

Because of this tremendous variability in water content, the nutrient contents of pasture samples in particular are usually interpreted and compared on a 100% DM basisi.e., water is taken out of the equation completely. But when calculating the amount of nutrients the horse is getting from a forage, I find it most useful to make my calculations using the 'as fed' or 'as sampled' figures — because that is what the horse is actually eating.

Why is DM content important?

Hays with a DM content below 90% are prone to mould and mildew, especially during the warmer months. They may also overheat enough to degrade some of their more sensitive nutrients, particularly vitamins.

It’s best not to buy hay with a DM content much below 90%, because there’s no way of adequately drying it out once it’s baled. Even in the winter months, the temperature inside a stack of hay can get high enough to encourage fungal growth if there’s enough moisture in the hay. Add a humid climate or a particularly wet rainy season, and that hay will most likely spoil.

Mouldy hay can lead to respiratory problems (for horses and humans!), ranging from irritation and swelling of the nasal passages — not to mention irritated eyes — to severe asthma. Equine asthma, or ‘heaves’, was once called COPD (chronic obstructive pulmonary disease), but as it’s more like asthma than COPD in humans, it’s been renamed equine asthma — and fungal spores in hay are one of the biggest triggers.

If you think inflammation of the nasal passages doesn't sound all that bad, then consider that the nasal passages are normally the narrowest part of the upper airway in horses, and the diameter of the nasal passages is one of the main factors limiting airflow during exercise in horses. Also note that horses are 'obligate nose-breathers' — horses cannot breathe through their mouth unless they displace their soft palate, which then causes an airflow obstruction! Any narrowing of the nasal passages is significant in performance horses, and any other horse who needs good oxygen intake (which is all of them).

But the problems don’t end there. Several different fungal toxins in forages have been linked to colic, poor appetite, and liver disease in horses. The horse's intake of the hay may be decreased because the hay tastes bad, but affected horses also appear dull and lethargic, so it’s a good bet that eating mouldy hay decreases their appetite because it makes them feel crummy.

I suspect that some fungal toxins in mouldy hay can even cause bone marrow suppression. I once had a case in which no other explanation could be found for the horse’s low blood count, which included both the red cells (which carry oxygen to the tissues) and the white cells (immune cells). The horse slowly recovered after he was changed to a good quality hay. I also treated him with zeolite powder and chlorella, but changing the hay was the single most important thing we did.

So, take the DM content of hay seriously. In my experience, reputable haygrowers rarely bale hay with a water content above 10%, so spoilage is seldom an issue with commercially produced hays that are properly stored. However, it can be a problem when people cut and bale their own hay, especially in unusually wet seasons.