PG . 44
HORSE HEALTH NEWS
for milk synthesis exceeds immediate replenishment from the digestive tract and from bone stores . Most reported cases occurred around 10 days after foaling or a few days after weaning but it can occur anytime from before birth through to weaning .
Oxalate pastures Mares grazing grasses such as kikuyu , buffel and setaria have a much greater need for calcium supplementation . High levels of soluble and insoluble oxalates inhibit calcium absorption from the gut . Insoluble calcium oxalate doesn ’ t dissolve in the gut and passes with the calcium attached into the manure – which means any calcium in the grass is lost to the horse . But soluble oxalates are equally important . The soluble oxalates ( mainly sodium and potassium ) are dissolved , releasing the sodium and potassium , but the dissolved free oxalate will bind tightly to any other calcium in the diet – and can be absorbed into the blood and deposited in the kidneys leading to kidney failure .
The calcium : oxalate ratio of the diet is important as it determines how much calcium the horse can actually extract from the diet – on paper a diet might appear to provide enough calcium but in reality much of the calcium is not available . A ratio of 0.5 or less is unsafe .
To increase the ratio , we need to increase the calcium and / or decrease the oxalate .
To reduce oxalate intake we need to reduce pasture intake and this can be done by feeding some hay – preferably lucerne which provides additional calcium as well as protein and energy . This will decrease the amount of pasture they need to eat . Oxalate levels are highest in leaves of young growing grass and hay is a good way to reduce grass intake .
Some horses need 120g of limestone which will bind to soluble oxalate and prevent absorption and kidney damage . The soluble oxalate is a big problem because it binds to calcium in the gut AND it ’ s absorbed into the blood . Setaria has 35 – 40g of oxalate per kg . A horse eating ½ - 1kg of grass an hour will consume about 10kg grass / day . It will also take in 350 to 400 g of oxalate . Depending on the soil sodium , potassium levels the amount of soluble oxalate will vary . Generally around 50 – 80 % of the oxalate will be soluble – hence the importance of inorganic calcium to bind it ie lime .
WHEN THE DIET IS CALCIUM-DEFICIENT , THERE ARE 5 ISSUES TO CONSIDER : 1 . meet the daily calcium requirements 2 . provide extra calcium to remineralise the bones - for horses on oxalate pasture or low calcium intake for 2 - 3 months this can take up to 12 months
3 . mop up oxalates in the gut before they are absorbed into the blood and lead to kidney failure
4 . block dissolved soluble oxalates ( magnesium , potassium and sodium ) which then bind to any free calcium . This is why some products recommend removing the horse from pasture for a couple of hours before and after feeding their supplement
5 . balance the calcium : oxalate ratio for horses on oxalate grasses
IT ' S IMPORTANT TO KEEP IN MIND : i . any product that contains only organic / chelated calcium can ’ t prevent oxalate absorption into the blood and can ’ t bind free oxalate in the gut
ii . any product that doesn ’ t include an organic form of calcium will be exposed to oxalate attack in the gut .
On oxalate pastures , the diet is calcium-deficient and the horse moves calcium out of the skeleton in an effort to maintain blood levels . Sometimes ( in 15 % of affected horses ) the bones of the head are affected , hence the term ‘ bighead ’. But in over 80 % of horses there is no head swelling – but there is generalised skeletal osteoporosis with a cascade of symptoms - including lameness , fractures , tendon and ligament problems , joint pain , growth retardation , retained placenta and poor body condition .
Unfortunately , due to the wide range of oxalates in the grasses ( Table 1 .), how long the horse has been grazing the pasture and the individual horse , it is difficult to predict the calcium deficit in the diet and your veterinarian can advise . Pregnancy is an additional risk factor for triggering the disease .
TABLE 1 . RANGE OF OXALATE LEVELS IN SOME GRASSES ( g / kg ) |
Setaria |
Setaria Sphacelata |
12 - 70 |
Kikuyu |
Pennisetum Clandestinum |
4 - 25 |
Buffel |
Cenchrus Ciliaris |
12 - 50 |
Napier |
Pennisetum Purpureum |
26 - 38 |
Guinea |
Panicum Maximum |
11- 23 |
A general tendency of indiscriminately administrating mineral supplements to pregnant and lactating mares has developed in the last few years . Risks associated with using multiple supplements to meet individual nutrient requirements include selenium and iodine toxicity – and recently , high zinc intakes which have recently been found to be toxic to the beneficial bacteria in the gut .
During pregnancy and lactation the mare has to cope with remarkable mineral requirements , particularly in relation to calcium and magnesium and supplementation is generally indicated .
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