"Worth placing on a strict grass-free, non-glycaemic, low calorie, balanced diet to achieve weight loss and improve insulin sensitivity.
Typically an effective diet must be relatively severe and be strictly implemented for a few weeks to a few months depending on starting body condition. Recommended rates of weight loss are generally between 0.5 to 1% loss of body weight weekly. Total dietary intake should generally be restricted to be in the range of 1.2 to 1.7% of body weight daily (feed weighed as fed rather than dry matter). The staple of the diet should be hay which is weighed dry and then soaked for an hour pre-feeding. It is also important that a feed balancer is included to ensure adequate protein and mins/vits on the restricted dietary plan."
TLS agrees that weight loss should be no more than 0.5 - 1% of bodyweight per week - so 2.5 - 5 kg/wk for a 500 kg horse. Restricting calories excessively can lead to other health problems such as gastric ulcers and hyperlipaemia, and can increase insulin resistance - the opposite of what we want.
For weight loss, energy in (feed) needs to be less than energy out (metabolism/exercise). Equine Applied and Clinical Nutrition (2013) suggests that reducing energy intake to around 70% of maintenance requirements can produce weight loss of 0.5-0.7% of bodyweight/wk.
Interestingly, Gordon et al (2009) found that a decrease in energy of less than 3% led to an average of 32.5 kg weight loss over 12 weeks (a total loss of ~5.6% bw, being ~0.47% per wk) in horses that weren't exercised (diet was calculated according to % bw - horses were fed 1% bw hay and 0.5% bw feed but when the energy was calculated, the diet was estimated to provide 16.64 Mcal/day and maintenance requirements were estimated at 17.09 Mcal/day - so the diet was only just below NRC recommended energy requirements).
Horses on a slightly stricter diet (as above but the feed was reduced to 0.3% bw halfway) that had moderate exercise lost 52 kg over 12 weeks (a total loss of ~9% bw, being ~0.75% per wk) - their diet was estimated to provide 15.27 Mcal/day (which would be around 89% of their maintenance requirements) but the exercise increased their estimated energy requirements to 25.87 Mcal, therefore their diet was only providing around 59% of the NRC recommended energy requirements.
If actual energy fed cannot be calculated, then basing a diet on % bodyweight is reasonable. The ECIR group suggests feeding 1.5% of the horse's actual weight, or 2% of the horse's target weight. So a 500 kg horse needing to lose weight, if fed at 1.5% bw would need 7.5 kg DM feed (mostly hay) per day, this would be ~8.3 kg as fed (hay is usually around 10% water). Liphook suggest feeding 1.2% (so 6 kg for a 500 kg horse) to 1.7% (8.5 kg for a 500 kg horse) actual bodyweight as fed - so taking their top end recommendation, that works out much the same - around 8.3-8.5 kg of hay/feed as fed per day for an overweight 500 kg horse.
Research has shown that individual horses lose weight at different rates. It is usually recommended to start at the upper end of the suggested diet, and if weight loss isn't seen after a reasonable period, the diet may have to be restricted further. Significant dietary restriction should only be carried out under veterinary supervision.
Interestingly Liphook are suggesting soaking hay for an hour before feeding, (not the 12 hours that is so often suggested in the UK and which often causes hay to become slimy and unappetising) - this is in line with the ECIR group's recommendations. TLS tends to suggest soaking for around 1 - 6 hours, depending on the environmental temperature (sugar will be lost more slowly in colder temperatures) - what is important is the quantity of water used, as hay loses sugar by diffusion down a concentration gradient, so the more water used, the greater the gradient and the more sugar can be lost. If the sugar in the water equals the sugar in the hay, no more sugar can be lost from the hay, no matter how long it is soaked. Changing the water midway increases the concentration gradient and should lead to more sugar being soaked out.
Great that Liphook emphasise that a balancer must be fed to provide adequate mineral, vitamin and protein levels. TLS suggests using a balancer that will provide (close to) the NRC minimum amounts of copper (100 mg/500 kg horse), zinc (400 mg/500 kg horse) and selenium (1 mg/500 kg horse), as these minerals are usually low in UK forage. Good levels of vitamin E (~1000 IU/day) may also be beneficial for horses on a hay diet and/or with PPID. TLS would also recommend feeding micronised linseed to provide essential fatty acids, which are in grass but usually low in hay - linseed is one of the few sources of EFAs with higher omega 3 (anti-inflammatory) than omega 6 (inflammatory) levels. Most balancers/mineral supplements do not provide enough salt, so TLS would also recommend adding salt to the feed (use the NRC program to calculate how much sodium is needed - salt is ~40% sodium). A 500 kg horse at maintenance needs 10 g sodium, so 25 g salt - this is just over a tablespoon (allow for sodium/salt already present in the diet). Allow horses access to a pure salt block too.
TLS' recommended diet