I was asked recently on our FeedXL forums by a member whether FeedXL takes into account the differing bioavailability of minerals from various feed sources when calculating whether a horses requirements are met for a certain mineral. It is an important question and unfortunately one confounded by a whole lot of marketing and very little science. Anyway, thought I would share the answer here:
Basically there is an ENORMOUS amount we don’t know about mineral bioavailability in horses, even just in standard, everyday feedstuffs there appears to be huge differences in how much of certain minerals a horse can absorb.
Lets take calcium for example … the NRC base their requirements on the assumption that a horse absorbs about 50% of the calcium it is fed, but they note that younger horses may absorb up to 75% AND that different feed sources have different calcium digestibilities. For example lucerne/alfalfa was shown to have a calcium digestibility of 72 percent while couch/bermudagrass a digestibility of just 40%. And the amount a horse absorbs will also change with the amount in the diet, with more being absorbed when diet levels are low and vice versa. And other minerals in the diet can affect absorption. Again in the case of calcium, if you feed too much phosphorus the absorption of calcium will drop while feeding magnesium and salt can improve calcium absorption. AND there are other things like oxalates in forages that can reduce the absorption of minerals like calcium. And other far stranger things like how warm a barn is for example can apparently influence mineral bioavailability (in the case of phosphorus anyway).
The problem is, even a reasonably recent and very comprehensive resource like the NRC gives no solid (or in some cases no information at all) estimates for the bioavailability of minerals from various mineral sources. So for example, is limestone and better source of calcium than dicalcium phosphate, dolomite or an organic calcium??? There is decent information in other species but basically, equine requirements for minerals are calculated using a best guess, average bioavailability of minerals. So in the case of calcium, they assume bioavailability to be 50%, so if the horse NEEDS 10 grams of calcium per day, the actual requirement will be 20 grams per day. Or for magnesium they use 40%, phosphorus is about 25% etc.
Now if your horse gets most of its calcium from lucerne it is going to be getting more than it needs. If it gets most of its calcium from couch grass it may be getting less than it needs. But I guess the assumption is (and this is part of the reason why I ALWAYS recommend you use as much ingredient variety in your diets as you can) that your horse’s calcium will be coming in from many places and as such will likely average out to around 50% available.
So it is no simple matter of just assigning a bioavailability to a nutrient from a certain ingredient and taking that into account in daily intake calculations (for example those used by FeedXL.com) as it is way too multifactoral. There are some cases, for example the bioavailability of calcium in dolomite seems low, where I would recommend you avoid the ingredient all together, but it is a matter of knowing your ingredients, doing the research, asking us when you are not sure before you choose something to go into your horse’s diet.
There are quite a few companies who like to tell their customers that their products have superior bioavailability and therefore don’t need to meet NRC requirements. What I would do in this case is ask for the hard evidence that that is the case – either studies they have conducted themselves or studies that have shown the ingredients they are using have a higher bioavailability … there is very, very little evidence in non-ruminant species like pigs and horses that organic minerals for example are any more bioavailable than inorganic minerals.
A LOT of what you will read about bioavaialbility in horses is clever (or not so clever) marketing that attempts to have you believe a product is superior. My recommendation is take it all with a large pinch of salt and do some research into their claims.