Well yes. Mesh size does affect how fast the horse can eat and for how long the hay will last, BUT as long as the horse has access to the feeder 24/7 mesh size doesn’t affect how much hay the horse can eat. If he wants more he will just use a little more time eating. Mesh size does, however, affect how much a horse want to eat. We do not want our horses to have to eat continuously to get what they need since that would be completely unnatural and create unnecessary stress. We only want them to stop eating too fast and too much. This will come naturally as soon as they have become content and they have forgotten that there ever can be an end to the supply of hay. To become content the horse must chew a certain number of times during a couple of hours. Chewing too much means that the horse is stressed and will most likely eat himself fat. Not being able to chew enough will make him frustrated since he never becomes content. Extend the eating time just enough to have him chew enough to be content without getting too many calories into the system is what we should aim for.
Too many water-soluble calories (carbohydrates) always make both horses and humans overweight. For water-soluble substances to pass through the body they must be transferred into the bloodstream, pass through the whole system and finally, through the kidneys leave the body as urine. For non-water-soluble calories to leave the body they just need to stay in the intestines and let the body decide if it wants them or not.
The perfect mesh size depends on hay quality and feeder design (not the horse). Soft hay is easier to pull through the meshes and therefore works the same as coarse hay does with larger mesh sizes. Coarse hay can block the openings completely if the meshes are too small and therefore stop the horse from getting anything to eat creating an enormous frustration or making him give up completely. A net laying loose on top of the hay is more effective in slowing down the eating compared to hay-filled hanging nets. Personally, I would never consider a mesh size smaller than 3 cm for hanging nets and 3.5 cm for nets laying loose on top of the hay. Mesh sizes larger than 4 cm have no “slow feeding effect” but only works as waste reduction nets.
Like all technical modern applications also nets are specified in the metric system.
A = 3 cm = a little more than 1 inch and a little less than 1 1/4". B = 3 1/2 cm = a little less that 1 1/2". C = 4 cm = a little more than 1 1/2" but still less than 1 3/4". D = 5 cm = very little less than 2". E = 6 cm = a little more than 2 1/4" but less than 2 1/2" or exact 2 23/64" (happy now?) Have you heard that both The UK and The US are going metric..... inch by inch? :-)
Mesh size A is only recommended for soft hay in hanging nets.
Mesh size B is standard for hanging nets and works well for all horses regardless of breed, age, sex, and activity level as long as they have access to hay 24/7.
Mesh size C is recommended for coarse hay, straw, and when laying loose on top of hay.
Mesh size D is only for waste reduction and has no slow feeding effect.
Mesh size E might reduce waste but nothing else.