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Moisture in Hay

posted by Horse Owner Today    |   July 15, 2011 09:11

by:  Andre Bonneau, BSA, P. Ag., Forage Management Specialist,

Saskatchewan Ministry of Agriculture.

All hay is put up with some moisture: it’s unavoidable and necessary to prevent leaf shatter and leaf loss.  However, there can be more moisture in the hay than necessary.  Too much moisture obviously makes the bale heavier and can eventually heat and spoil the hay. 

The safe amount of moisture in hay depends on the density of the bale and the ability of the bale to dissipate heat.  When forage is first baled up, the bale begins to heat almost immediately.  This is normal.  The problem begins when the heat builds up without dissipating and the forage breaks down.  Think of it in terms of surface area and volume relative to the weight of the bale: a small loose bale can dissipate heat better than a large heavy bale. 

For example, a small square bale weighing less than 75 lbs can be normally stored at 20% moisture with good ventilation.  Meanwhile, a heavy and dense large square bale should have less than 15% moisture.  

Note that bales should not be stacked until they’ve finished heating.  Stacking the bales too soon reduces the ability of the hay to dissipate heat and excessive heating can occur.

What’s happening?

Heat is generated by decomposition and microbial activity.  At a microscopic level, hay begins to decompose as soon as its cut.  In the swath, microbial activity will slow down as moisture levels decrease.  Once the hay is baled, the small amount of microbial activity is still generating heat but the heat isn’t dissipated as easily as it was in the swath.  Heating will continue as oxygen and moisture is used up and microbial activity is minimized.

Inoculants and hay preservatives

In general, hay preservatives make moisture unavailable to the microbes that may spoil the feed.  Hay preservatives do not increase the quality of the forage.  Adding any preservative to poor forage will only preserve poor forage. 

There are two common types of hay preservatives: organic acids and inoculants.


ORGANIC ACIDS, like propionic acid and citric acid, is the most common and effective hay preservative available.  The acid drops the pH of the moisture in the forage and makes it unavailable to microbes.  The amount of acid needed depends on the concentration of the acid and the amount of moisture in the forage.  Commercially available propionic acid will have instructions on the label.  Organic acids will help preserve hay at up to 30% moisture. 

INOCULANTS introduce lactic acid-producing bacteria (LAB) into the hay.   LAB is the same bacteria used in silage production.  These bacteria will form colonies in the hay to produce lactic acid and reduce the pH in the bale.  Lower pH reduces microbial activity and preserves the hay.   Inoculants are generally in a powdered form and are registered to preserve forage up to 23% moisture.

OTHER options are anhydrous ammonia and salt. 

Anhydrous ammonia ties up moisture and makes it unavailable to microbes.  However, the amount of anhydrous ammonia necessary makes it cost-prohibitive.  The difficulty and danger of using anhydrous ammonia also makes it an unpopular choice.

Salt used as a preservative works by making the moisture unavailable for microbial use.  The amount of salt needed to preserve hay is often impractical and expensive.  Some research suggests that the amount of salt needed can also make the forage unpalatable for livestock. 

Pay attention to moisture levels.

Some moisture in forage is unavoidable and acceptable.  It is impractical to put up feed absolutely dry.  Get a good hay moisture tester, learn how to calibrate and use it.  Monitor your hay as you bale and periodically in the hay yard. 

For more information, go to the Saskatchewan Ministry of Agriculture website at www.agriculture.sk.ca and search “Hay Preservatives – FAQs” or contact the Moose Jaw Regional Office at 1 866 457-2377.