Soaking hay before feeding it to horses has become a popular management technique for a variety of health issues. Some managers soak hay in water to make it easier to chew for older horses or to reduce dust. For horses at risk of obesity or metabolic issues (like some broodmares), it's also touted as a way to reduce weight. Hay soaking has long been thought to remove carbohydrates before the horse gets its forage.
Researchers at the University of Surrey and University of Liverpool set out to quantify how much weight a horse could lose as a result of hay soaking, and whether other nutrients would be lost in dangerous quantities, too. In a study reported at the most recent American Association of Equine Practitioners annual convention, 12 overweight horses and ponies were fed soaked hay in amounts depending on their body mass.
Horses did lose weight, but researchers found that soaking the hay actually took away a bigger portion of the available digestible energy than intended. Although a .5 to one percent loss in body mass per week is considered acceptable, horses in the study lost roughly .1 percent to two percent of their body mass. However, so many other critical nutrients were lost through soaking that horses eating soaked hay were only getting about 64 percent of their daily energy requirements from the hay, meaning there was a 23.5 percent higher magnitude of energy restriction than would have occurred had the hay been fed fresh.
Not only did soaking drastically alter the horses' energy intake, it created an imbalance of the remaining nutrients in the hay. Water-soluble carbohydrates were reduced 33 percent from soaking, but ash, the measure of minerals like calcium and phosphorous, were also down 27 percent. Perhaps counter-intuitively, the amount of crude protein accessible in this batch of soaked hay was increased from fresh.
Ultimately, experts say: if you're going to soak hay to help horses lose weight, be prepared to work with a veterinarian or nutritionist to ensure the rest of their diet will balance out the energy and minerals you lose in the water.
An abstract of the study is available from The Veterinary Journal's September 2015 edition here.
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