Longland, Barfoot and Harris have conducted extensive research on non-structural carbohydrates (NSC) in forages and feeding management. Their most recent work is forthcoming in the Veterinary Record (doi:10.1136/vr.101820), titled Effect of period, water temperature and agitation on loss of water-soluble carbohydrates and protein from grass hay: implications for equine feeding management. The study examined four different types of hay, including both ryegrass and timothy hay, sampled in both the summer and winter, and soaked under different temperatures to represent the season (winter 8˚C and summer 16˚C; summer treatment included both an agitated treatment and non-agitated treatment where the hay was swirled in the tub) or with warm water (49˚C).
Hay samples were tested after 1, 3 or 16 hours of soaking. These authors found that none of the treatments affected crude protein (CP) content of the hay, in fact, when losses of NSC were taken into account, the CP% of the remaining hay increased slightly. The authors found that soaking hay for 16 hours was most effective in reducing water-soluble carbohydrates (WSC, the simple sugars such as glucose, fructose and sucrose, and fructans; though fructans and fructose were not reduced as effectively as glucose and sucrose were). While colder water (8˚C) resulted in fewer reductions in WSC, there was no difference between summer temperature water (16˚C) with or without agitation, or with warm water (49˚C). It was also reported that soaking for even 16 hrs did not reduce WSC to levels that are considered acceptable for horses that might have metabolic problems such as insulin resistance or polysaccharide storage myopathy and are sensitive to WSC in their diets (<10% dry matter).
Other studies have reported losses of some minerals and vitamins with soaking, and these would need to be replaced through a fortified feed or supplement. It is important to know what the nutritional content of your hay is (including CP, NSC, WSC and many minerals) before and after soaking, so that your horse’s diet can be adequately balanced to suit your his needs, and supplemented with other nutrients as needed.