Along with, 'How can I build topline?' and its close cousin, 'How can I get my horse to put on weight?' this question is among the most commonly asked.
There are seven macro minerals and seven micro minerals essential to a horse's diet. These minerals in excess or deficiency can affect the biological function and therefore, overall health of you horse. Deficiencies or excesses can occur even with an adequate diet and it is important to consider that as well as these minerals being present, they must also be balanced.
To be 100% confident your horse needs a supplement you would need to perform several tests - pasture, hay, blood, urine and so on, but if this is out of your reach (like most of us!) then you'll have to determine the answer by asking the following three questions:
1. What else am I feeding - and what minerals and vitamins does it contain?
If you're feeding a good quality, pelleted 'complete' feed at the recommended daily amount then, depending the answers to the below questions there is a chance your horse does not need a supplement. They keywords here are 'good quality' and 'at the recommended dose'. If you're feeding pellets that cost you $20 a bag with vague ingredients then I can almost guarantee your horse is not getting what it needs.
Additionally, if you have a 500kg horse in moderate work being fed only 1kg per day, and the bag recommends you feed 4kg then your horse is also not getting all the minerals and vitamins it needs.
If you can feed that 4kgs of pelleted feed over two+ feeds a day (your horse's stomach is not big enough to cope with 4kgs all at once), and 4kgs of that feed doesn't make your horse fat or hot then it probably doesn't need a supplement.
2. What pasture/soil does my horse have access to?
If you're lucky enough to have plentiful pasture full of native, low starch grasses with few high oxalate plants and weeds and your horse is in light to moderate work then it probably only needs a very basic supplement which contains the four minerals almost all Australian pasture/hay is lacking.
These four minerals are copper, zinc, selenium and iodine. These minerals are required for myriad functions including growth and metabolism, bone development, skin integrity, connective tissue, wound healing, thyroid function and so on.
Selenium needs to be provided at approximately 1mg daily and iodine between 2mg and 4mg daily. Copper and zinc must be provided in the correct ratio to iron. Vet and leading equine nutritionist, Dr. Eleanor Kellon, recommends an iron: copper: zinc ratio somewhere between 10:1:3 to 4:1:3, with the latter ratio being recommended for insulin resistant horses. Excess iron can prevent the absorption of copper and zinc, so if you know how much iron your horse is ingesting you can know how much copper and zinc to provide. If you don't know how much iron your horse is ingesting then providing somewhere around 300mg of copper and 900mg of zinc to a large horse in work is about right.
Horses in light to moderate work who have 24/7 access to good quality native hays, but don't have access to grass will need the above, but will also need a vitamin E supplement (around 1,000IU per day) and an omega 3 supplement as they will be lacking the omega 3 naturally supplied in grass. Omega 3 should be supplied by providing flaxseed/flaxseed oil or chia seed/chia seed oil to your horses diet. Provision of vitamin E is especially important if your horse is laminitis prone.
If you can afford a pasture/soil/hay analysis then I would absolutely recommend it. It's doesn't cost much (especially in comparison to regular shoeing, saddle-fitting, bodywork ,etc) and gives you a far clearer picture of what your horse needs. Australian soil is typically high in iron so I would immediately discount any supplement containing iron, as horses can't easily excrete excess iron. As mentioned above, our soil is low in minerals such as copper, zinc and selenium. If however, paddocks have been over fertilised or had a fungicide applied at some stage, the soil may not be deficient in these minerals and may in fact contain an excess of minerals such as copper and zinc. Therefore, if you suspect your soil may be unusual I would recommend getting a pasture analysis. Feel free to get in touch with me to find out how you can easily organise it yourself.
Keep in mind that minerals such as selenium can be toxic if over supplemented. There is a fine line between your horse not having enough, and having too much selenium in their diet, so knowing what your horse needs and what else it's ingesting is important. You cannot simply just provide a supplement and be confident that your horse is getting all it requires.
If your horse is in work it also needs to have salt added to its diet. Sodium and chloride are required for things such as nerve function, intestinal movement and adequate hydration. Depending on its level of work, whether it's dehydrated and what the environmental conditions are like, your horse may need up to 70 or 80 grams of added salt each day! A safe level of supplementation would be 30-40 grams per day for a large horse. If your horse isn't in work then a salt lick MAY be enough, but providing salt in a handful of chaff is a better alternative.
3. What are the individual needs/symptoms of my horse? (And does my horse have any ailments such as tying up?)
People who don't eat meat may be deficient in iron. People who do eat meat may be deficient in iron. Are people who don't eat meat more likely to be deficient in iron? Probably, but this doesn't mean that other factors such as blood loss or an inability to absorb iron don't also play a role in an individual's iron deficiency. Is it prudent to supplement a small amount of iron to a vegetarian to 'be on the safe side'? The answer is really up to the individual and the presence or absence of clinical signs associated with iron deficiency.
Some deficiencies in horses are fairly easy to see - if your horse has a sun-bleached coat and cracked feet then chances are it's deficient in copper and zinc, but other deficiencies can be more difficult to decipher. Getting a professional to look at your horse, analyse its current diet and make suggestions for improvements may be something you wish to consider if you're having issues with weight, feet, coat, behaviour etc.
If your horse suffers from an ailment such as tying up you may want to consider additional supplementation of minerals and vitamins to assist with this ailment. For example, I would err on the side of caution when supplementing selenium to the majority of horses, however if a horse suffers from tying up there is evidence to suggest that providing some additional selenium, and its pair, vitamin E (along with sodium, potassium and chloride), may help to prevent/alleviate symptoms so I might recommend supplementing up to the maximum recommended daily rate for a horse suffering from this condition.
If you've read this far, you've realised that I cannot easily answer the question of whether your horse needs a supplement and which supplement it needs. Almost all horses need copper, zinc, selenium, iodine, sodium and chloride supplemented, but if you're feeding a 'complete' feed at the recommended rate then it MAY be getting enough.
Both my horses get a supplement, however they're different so get different ones. I don't always recommend the same supplement to my clients as their horses are different too. There some popular brands of supplements that I would never recommend. Some are barely worth the bag they're packaged in (popular, well known ones too). There are probably about four or five I find myself coming back to again and again and these are not always the most expensive on the market either.
Supplementing a mineral (or vitamin) without analysing the current diet is a slippery slope. At the very least I would recommend a diet analysis and preferably also a pasture or hay analysis to be more confident that your horse is getting what it needs to be happy and healthy and looking and performing to the best of its ability.