Phosphorus, the other side of Calcium
When it comes to an equine diet and the minerals involved, most are aware of the importance of Calcium. Most are also aware that Calcium (Ca) and Phosphorus (P) have an important relationship, but the focus always remains with feeding calcium…..What about Phosphorus?
The calcium to phosphorus ratio is important and is best in a ratio of 2:1. When the calcium intake is inadequate the ratio becomes unbalanced because there is more phosphorus than calcium. When horses graze pastures that contain oxalates, the oxalates bind to calcium making it unavailable and the diet becomes calcium-deficient. Calsorb Forte and Bone Formula Forte contain chelated calcium which is protected from oxalates.
Phosphorus deficiency is extremely rare in horses and most feeds and pastures contain plenty of phosphorus. Roughages and grain are more uniform regarding phosphorus content when compared to calcium.
Let’s look at an example.
The phosphorus requirement for a 400kg horse is 14-38g per day: 14g for 4—12month old, 23g for a pregnant mare, 29g for a 400kg horse in hard work and 38g for a lactating mare. Horses grazing pasture would be getting twice this amount. Most commercial feeds also contain added phosphorus. Grain typically contains around 3–3.5 g/ kg as dry matter. As phosphorus is located in the outer layer of the kernel, by-products such as bran are up to threefold higher in phosphorus in comparison to the unprocessed grain. The defatted by-products of soy or rape seed and other high fat seeds contain P at 4–10 g/kg DM.
Table 1 shows the content of phosphorus in a list of feeds and pastures commonly seen in diets. If we use this to continue our example above – Our 400kg horse is on Setaria, an oxalate containing pasture. So whilst the pasture contains calcium, it is locked up by the oxalates and therefore no calcium is available. Not only is there no calcium but the Phosphorus level of this pasture would mean our horse is getting approximately 48g/day. More than the recommended daily intake, remembering this diet is pasture only.
And, while it’s important to meet the phosphorus requirements of horses, it is equally important not to feed more phosphorus than necessary. Excess phosphorus is excreted mainly in manure and in run-off, it contributes to eutrophication – where excessive fertilizers and manure run into lakes and rivers. This encourages the growth of algae (algal bloom) and other aquatic plants. In Sweden, studies have shown that there is a high proportion of phosphorus in manure, which indicates that phosphorus overfeeding of horses is potentially harmful to the environment.
EQUINE CLINICAL NUTRITION
Dr Jennifer Stewart
