Be aware that adding Mg to the diet will only help if your horse has a deficiency. That can change with the seasons. I think you'd be better off trying to find the cause through other means first, like tack fit, body pain, etc.
That being said, here's some very good information about magnesium and vitamin d3 (also important this time of year). Pay attention to the type of magnesium you're feeding. Bioavailability of different forms varies greatly and makes a big difference in the amount fed and the body's ability to utilize it effectively.
When it comes to adding Mg to your horse's diet, you first need to look at a rough estimate of what they're already taking in. Sometimes you can find rough estimates online for certain types of hay in your area, you may not be able to find it for complete feeds, and you should be able to find it for any supplements you're already giving. That being said, the specific type of Mg being fed can be harder to find. SmartPak, for instance, doesn't list the specific type and if you call to ask, they don't know. You want to look for organic anions, so Mg malate or Mg citrate are really good examples of this. As far as bioavailability goes, these are the best.
Mg oxide is harder for the body to absorb and *can* cause digestive upset in horses with sensitive tummies because of the way it breaks down. From what I understand the non-organic forms (I believe that typically means they come from rocks or something similar, hence "non-organic") are not easily digested and are not as utilized. What does that mean? It means you end up putting something into your horse that it won't get much out of and will end up passing much of the Mg right out the other end. So, you can have a horse that is getting the recommended amount but still has a mild deficiency because it can't utilize what it's being given.
I took a quick look at some of the popular brands and MagRestore was the only one I saw mentioned that specifically listed using Mg malate. That one would get my thumbs up as long as you're okay with the fillers. The Quiessence and the Remission both specifically listed Mg oxide, so I would pass on those personally. The SmartCalm doesn't have a specific type (my trainer went through this already and couldn't get an answer from them), so they wouldn't be a first pick either. Now, the MagRestore is offered in a pellet and a powder. Some horses really won't eat the powder, so the pellets are great! If you have a horse that doesn't care, I recommend the powder. For 45 servings, it's $15.99 (not including taxes, shipping, etc) from the website that was already listed on this thread. That's ~$0.36/serving.
I wanted to add Mg to my horse's diet, too. I went to a Vitamin Shoppe near me because they sell Mg malate tablets - no fillers - for $15.99 for 180 pills (they also have a 360 count for $28.99) and I figured that would be enough for me to see if it makes a difference. I did the math on my horse's needs and determined that he needed 2 pills/days. That's 90 servings for half the cost of the MagRestore, but I do have to crush the pills and put them in containers to be added by the BO in the morning. It's a little more work, but I just cut that cost in half compared to the powder from the horsey company and no extras in the supplement that I don't want. If I bought the 360 count, it drops to $0.16/serving, so I would save another $0.30/month (that may not necessarily mean much, but it's still a savings).
Speaking of knowing what a horse needs, there aren't many conclusive studies out there like there are for calcium (Ca) and phosphorus (P), but the studies available have estimated daily requirements at 15-30mg/kg of body weight. According to 3 different studies conducted, the average horse (maintenance to moderate riding - no racing, upper level jumping/dressage/etc, reining, etc) weighing ~500kg (1100lb horse) needs anywhere from 4-15.5g/day of Mg. From what I've also read and from my discussion with my trainer, I've concluded that my horse needs ~8g/day (800lb horse, 22mg/kg of body weight). I allow a generous amount for all the grass and hay he eats throughout the day and I took into account what is already listed (type of Mg unknown) with his supplements. Then, I used a moderately conservative number of ~3g additional per day. The pills are 1,250mg, so I give him 2 pills per day.
At the same time, I started adding Vitamin D3 to his diet in a similar fashion. Horses getting less than 4 hours of sunlight per day at a much higher chance of being deficient in Vitamin D3. These horses need 500IU/kg of body weight - again, I'm referring to adults horses with a light to moderate workload. With the days getting shorter, sunlight being much weaker, coat being more dense and being blanketed in the cold, I know he isn't getting nearly as much from natural sources as he does in the spring/summer. Plus, with nasty weather he isn't even going outside (so rain, snow storms, ice, sleet, etc) in the first place. Looking into what would already be provided in his current diet, I give him an additional 12,000IU/day to help supplement the loss of sunlight and natural D3 sources. Instead of finding yet another horse supplement, I went back to Vitamin Shoppe and bought liquid Vitamin D3 in the 2,000IU dose size bottle. This was, like the Mg, to give me an idea of how he'll do and decide if I want to continue. The bottle I got has 295 servings (6 drops/day) and is $19.99. So that comes out to a little under $0.06/serving.
Vitamin D3 is very necessary for the body and has a couple of similar deficiency symptoms as Mg. The simple fact that D3 is hard to come by this time of year (for SW Ohio, anyway) means it's all the more important to add it to my horse's diet. The same as it's important to add for most people for all the same reasons. A few of the symptoms of deficiency are: lameness, swollen joints, decreased immune system function, insulin resistance with blood sugar issues, tying up disease, poor reaction to stress resulting from travel, training and competing to name a few. D3 also aids in the body's absorption, transport, and deposition of Ca and P.
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