A: According to the USDA Food Composition Database, dried chia seeds contain 177 mg of calcium and 265 mg of phosphorus per 28 grams. So, indeed, they have what we call an inverted calcium phosphorus ratio (they contain more phosphorus than calcium). You are correct that this ratio is important when feeding horses. Ideally we want a horse’s ration to contain a ratio of between 1.5 to 2 times more calcium than phosphorus. Should the ration become inverted the National Research Council (2007) states that calcium absorption might be impaired. Even in the face of adequate calcium, excessive phosphorus intake can lead to a condition known as secondary hyperparathyroidism, as well as skeletal abnormalities.
Secondary hyperparathyroidism exists when circulating calcium drops due to low calcium absorption and parathyroid hormone is released resulting in the mobilization of calcium from bones. Chronic demineralization of the facial bones causes a malformation known as “big head.” Hyperparathyroidism was traditionally referred to as big head or bran disease (unfortified bran can have a calcium-phosphorus imbalance, so feeding it can cause the malformation).
Grains such as oats, corn, and barley, as well as grain by-products wheat bran and rice bran, are low in calcium and higher in phosphorus. Traditionally with the reliance on these feeds, bone abnormalities such as big-head were common. Today we feed far fewer grains, and they’re typically incorporated into commercial feeds that have added minerals to correct for such issues and insure a balanced diet. With the popularity of rice bran and flax and the general public’s concerns about inverted calcium phosphorus ratios, feed manufacturers have taken to adding calcium carbonate in order to create a 1:1 calcium-to-phosphorus ratio. Chia being a less well-known feed for horses does not typically have added calcium.
This being said if you are feeding approximately 1 cup of chia (which weighs about 160 grams), you’re only adding 1 gram of calcium and 1.5 grams of phosphorus. This is unlikely to make a dramatic impact on your horse’s diet as long as calcium and phosphorus requirements are met and the diet is well balanced with a good calcium-to-phosphorus ratio.
How much calcium your horse is getting from the pasture depends on a number of factors, including grass and soil types and whether the plants contain oxalic acid. Oxalic acid binds calcium to make calcium oxalate, which is insoluble and, thus, makes the calcium unavailable during digestion. While legume species such as alfalfa and clover tend to have high calcium levels some grass species, such as orchard grass and Kentucky bluegrass provide adequate calcium but might have close, or even inverted, calcium-to-phosphorus ratios. Because pasture or hay is consumed as the majority of the diet, this can result in a diet with a less than ideal calcium-to-phosphorus ratio. I have seen very close ratios in numerous grass hay samples. Grasses of tropical or sub-tropical nature might contain high levels of oxalic acid.
As calcium in grass pasture and hay can vary, I recommend feeding a ration balancing feed or supplement formulated to provide adequate levels of not only calcium and phosphorus but other minerals and key nutrients, as well. A mineralized salt block is not going to provide these important macro minerals. With this additional dietary fortification in place, I would not be concerned about adding a cup of chia.