It’s a good question: should we continue to feed vitamins (and minerals) to our horses during summer when they are out at pasture for some, or most of their time? The answer is – as is the case with many nutrition questions – ‘it depends’!
Horses require a daily intake of vitamin A (or betacarotenes, which are converted to vitamin A in the body), vitamin E and vitamin D. Healthy horses manufacture their own supplies of vitamin C, K and the B-complex vitamins.
Plenty of healthy, green, growing grass supplies more than adequate vitamin A and vitamin E for most horses, and also supplies vitamin K. Grass doesn’t actually contain vitamin A, but instead betacarotenes, which are called ‘provitamin A’ because they are converted to retinol (vitamin A) in the horse’s body. Vitamin D may be short in fresh pasture, but if the horse is exposed to sunlight for at least part of the day, he will obtain enough from conversion in his skin. Horses covered muzzle to knees with light-blocking fly or light turnout rugs may need extra vitamin D.
However! you need to look at the whole diet of the horse when assessing its balance, and not just the pasture. If your horse is healthy and his entire diet is healthy, green, growing grass and he lives out without rugs, then he won’t need supplementary vitamins. If your horse is on restricted grass, the grass is overgrazed, he has just a few hours turnout and is fed preserved forage such as hay or haylage when he’s off the pasture, he is turned out with sunlight-blocking fly rugs or turnout rugs, he is old and can’t eat much grass due to dental problems, or he is ill, then he may well still need supplementary vitamins.
The vitamins in feeds and multi-vitamin and mineral supplements are formulated in such a way as to be safe when fed with plenty of healthy pasture, so it won’t do any harm to add them. The betacarotenes in pasture are not harmful if ingested in large quantities, or along with extra vitamin A in supplements or feeds. Higher levels than actual requirements of vitamin E, from pasture and supplements or feeds is not a problem. And vitamin D from sunlight along with that in supplements or feeds is also not a problem.
The situation for minerals is different from vitamins, however. UK grass – no matter how well it is managed – does not usually supply enough copper, zinc or selenium for any horse, working or not, so in theory, you do need to supplement all year round. For horses who do not need vitamin and mineral-fortified compound feed (because they’re maintaining condition on the grass), a supply of essential minerals is important. Bespoke pasture-balancing mineral mixes are hard to come by, and most owners find that feeding a well-formulated multi-vitamin and mineral product (supplement or balancer) works well for their pasture-kept horse. Some find that by feeding such a product during winter, spring and autumn, they can stop feeding it for 3-4 months in the summer without any problems for healthy horses in little work who are not breeding. Ideally offer a mineralised salt lick for the months where you are not adding a multi-vitamin and mineral product.