This article is courtesy of Krishona Martinson, PhD, University of Minnesota.
Soaking hay in water is a common strategy used to manage the nutrition of some diseased horses. Current hay soaking recommendations include soaking hay for 30 minutes in warm or 60 minutes in cold water for removal of carbohydrates (Watts, 2003). Soaking hay is commonly done to manage horse diagnosed with laminitis, Polysaccaride Storage Myopathy (PSSM), hyperkalemic periodic paralysis (HYPP), and chronic obstructive pulmonary disease (COPD).
- Researchers have suggested that diets contain less than 12 and 10% nonstructural carbohydrates (NSC) for horses affected with laminitis (Frank, 2009) and PSSM (Borgia et al., 2009), respectively.
- Reynolds et al. (1997) determined that a diet less than 1% K is necessary for horses diagnosed with HYPP.
- Moore-Colyer (1996) determined that soaking hay for 30 minutes reduced respiratory problems for horses diagnosed with COPD or heaves.
However, how efficient is hay soaking, and are additional essential nutrients lost during the soaking process? Researchers at the University of Minnesota set out to determine the impact of water temperature and soaking duration on removal of NSC, crude protein (CP), minerals, and dry matter (DM) from alfalfa and orchardgrass hays.
Four hay types were soaked, including bud and flowering alfalfa, and vegetative and flowering orchardgrass. Individual flakes were submerged for 15, 30 and 60 minutes in 25 liters of cold (72°F) and warm (102°F) water, and for 12 hours in cold water. A control (non-soaked) sample was also evaluated. Water temperatures were determined by using the cold or warm only faucets, similar to practices implemented by horse owners and managers. Subsamples of entire flakes were submitted for nutrient analysis at a commercial laboratory.
- Prior to soaking, both alfalfa hays were below the 10 and 12% NSC threshold for horses diagnosed with PSSM and laminitis, respectively, and would not have required soaking. The orchardgrass hays were above these thresholds, however, after soaking for 15 to 30 minutes were at or below 10 to 12% NSC.
- Although soaking hay for longer durations did further reduce NSC content, it is not recommended. All horses, even diseased ones, require carbohydrates in their diet.
- The severely limited NSC content in hay soaked for greater than 1 hour, combined with increased fiber amounts (fiber components are not water soluble, thus they are concentrated in soaked hay), brings into question the palatability and availability of nutrients in hay soaked for longer periods of time.
- Crude protein leaching was variable in soaked hays, something other researchers have also observed (Moore-Colyer, 1996). More importantly, previous research looked at the nutrient availability and quality of rained-on hay fed to steers and suggested the nitrogen remaining in rained-on hay is more stable, water-insoluble (Rotz and Muck, 1994), and possibly less digestible by ruminants (Licitra et al, 1996). Additional research is needed to evaluate this concept when feeding soaked hay to horses.
- Calcium (Ca) is not as prone to leaching during soaking compared to other minerals, and appears to be dependent on hay maturity. As soaking duration increased, leaching of Ca increased in alfalfa bud and vegetative orchardgrass hays (immature hays). However, soaking had no effect on Ca leaching in the more mature hays.
- Conversely, magnesium (Mg) Mg and phosphorus (P) levels were reduced in all hay types as a result of soaking, with longer soaking durations leading to greater reductions. Because Ca is not as water soluble as P, high Ca:P ratios were observed in hays soaked for longer durations, specifically after 12 hours.
- Ideally, Ca:P ratios should range from 1:1 to 3:1 (up to 6:1) in horse diets (NRC, 2007). The high Ca:P ratios observed after longer soaking durations were exaggerated in alfalfa hays which had higher Ca:P ratios prior to soaking.
- After 12 hours of soaking, a deficiency in P was observed and ranged from a shortage of 1 to 8 grams for a 500 kg horse in light work (NRC, 2007), and Krook and Maylin  suggested that osteochondrosis may be associated with excess dietary Ca.
- Soaking both alfalfa and orchardgrass hay for 12 hours was necessary to sufficiently reduce K concentration to recommend levels prior to feeding horses diagnosed with HYPP (Reynolds et al, 1997). Although K levels can be reduced by soaking, neither alfalfa nor orchardgrass hay is an appropriate option for horses diagnosed with HYPP due to the naturally high levels of K.
Owners should rely on forage analysis as the primary method of determining the appropriate hay for horses, especially when feeding horses diagnosed with laminitis, PSSM, HYPP or COPD. Hay soaking for short durations (15 to 30 minutes in duration) is an acceptable management method, but should only be used if ideal hay is not available. Hay should not soak hay for greater than 1 hour. Soaking hay for long durations resulted in severely reduced NSC content, high Ca:P ratios, shortage of P in the diet and significant losses in DM.