What any animal (including humans) eats contains energy measured in megajoules/MJ. The animal initially uses this energy to fulfil day to day maintenance requirements. Any energy not required gets stored in the form of fat just in case a day comes along when the animal won't be able to eat enough food with enough energy in to fulfil its maintenance requirements (In the case of our domestic horses and ponies this day rarely occurs). In short, daily energy intake has to be equal to daily energy output otherwise they gain weight.
Grass is the largest contributor to excessive calorie intake in most horse's diets and a horse on pasture will commonly consume many times its own daily maintenance energy requirements. As such the initial step should be restricting grass intake and increasing exercise. It should though be noted that horses and ponies are more intelligent than we often give them credit for and they will soon learn that they are being turned out for a shorter period of time and will ensure they eat a full days worth of feed in this reduced time period.
However, if your ponies are still not losing weight then their energy intake is obviously still too high. In these cases turnout should be provided in a ménage or alternatively a bare paddock where the grass has been sprayed off, this enable you to completely control their energy intake and know precisely how much they are getting. Young grass shoots contain a surprisingly high amount of energy. Suitable low calorie forage should be provided to replace the grass, the amount fed needs to be carefully weighed and controlled.
A horse would like to eat about 2% of it's body weight a day (400kg x 2% =8kg) however to get a pony to lose weight this can be reduced to 1.25% (400kg x 1.25% = 5kg). This should be fed in the form of poor quality forage. Commercial feeds will provide far too much energy (see below). However feeding forage alone will not provide a balanced diet, as such we would recommend feeding a balancer, just ensure it isn't one that is high in energy/carbohydrates, this value should be stated on the label, although some feed manufactures can be cagey about giving the value in an easily understandable form...
If you have any worries about your horse/pony/donkey's weight then please ring the practice to discuss their specific situation with one of the vets. In extreme cases there are scientifically formulated, prescription type weight loss feeds available (similar to those offered by small animal vets for your dogs and cats). Below is a simplified method of how to calculate your ponies feed requirements and a few weight loss tips. I hope this helps.
A horse's energy requirements in megajoules are approximately one eighth of their ideal weight e.g. A 400kg pony = 400/8 = 50 MJ. However the same horse would also like to eat roughly 2% of its body weight 400 x 2% = 8kg therefore on average the dietary intake should be no more than 6.25MJ per kg (50MJ/8kg). Commercial feeds will state how much energy they contain on the bag e.g. Speedibeet contains 12.4 MJ/kg which means that 1kg of speedibeet will contain almost twice the daily maintenance energy requirements of a 400kg pony.
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