Calcium Supplementation – How this Relates to Oxalate Grasses

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Normal Pasture Conditions

Calcium, for a horse, is a very important mineral. Keeping a horse on good quality pasture, feeding lucerne chaff or fibre mix and not overfeeding grain means that you will be providing the daily calcium intake a resting horse needs in the correct ratio of other minerals. What is that intake? Well an average size horse in none or very light work needs 20 gms daily and the same horse in hard work needs up to 40 gms daily (ref National Research Council). The levels will naturally be increased for the horse in hard work simply because he will be given more feed, and provide with a correctly balanced mineral supplement.

Oxalate Grass Conditions

The most common oxalate grass in New Zealand, Kikuyu and rootIn New Zealand the most common oxalate grass is Kikuyu. It has been established that oxalate grasses bind calcium and so the level of uptake by the horse is severely reduced. This can lead to calcium deficiency and is especially of concern for broodmares when in the third trimester of pregnancy and when lactating with a foal at foot. The extra demand for calcium at this time, and the reduced available level in kikuyu grass, may result in the mare providing calcium to the foal by means of her bones demineralising. The effect of binding the calcium is higher in pastures in the Summer. There may be no choice with where the horse can be grazed but care should be taken not to suddenly transfer a horse from good normal pasture to a high intake of kikuyu, restrict the time allowed only gradually building up to full time if necessary.

How to Compensate Horses Grazing on Kikuyu

Horse grazing oxalate grass, kikuyuThere is no need to go overboard and start giving high quantities of calcium supplements. Overfeeding calcium is detrimental to the horse as it blows out the all-important calcium/phosphorus ratio and it reduces the uptake of magnesium. It is a bit like trying to level the legs of a table, tweak a bit on one leg and it rocks the other way and keep on tweaking until it becomes a coffee table. Unbalancing the ratio of minerals and trying to correct may have a see-saw effect on the horse’s physiology. So if your horse has to live on Kikuyu then make sure he receives a balanced additional hard feed with lucerne, normal to low grain and good quality sun dried hay from a quality grassed source. Even better if you keep the pasture intake reduced and provide more meadow hay. In addition a good quality mineral supplement is essential and will provide all the minerals needed for a correct ratio without having to risk giving too much calcium. The mix must have calcium higher than phosphorus at least 1.1 or 1.2:1 Ca to P and a horse should not have a ratio greater than 2.5:1 Ca to P. If he has to be grazed on high oxalate pasture 24/7 then the mineral supplement will need to be increased by 50% e.g. a 60gm recommended dose will be a 90gm dose.

The Importance of Keeping a Balance of Minerals

Increasing calcium alone should never be done without understanding the need to keep the phosphorus level up as well and even then, the maximum tolerable amount is 2%. The relationship to Phosphorus is crucial to the physiological functioning of the horse. High calcium can create ulcers. Also osteochondritis is a possibility if the calcium is high and the phosphorus not increased, imbalances in that ratio have been linked to OCD.

Calcium uptake will be reduced too if there is inadequate Vitamin D – that’s the vitamin that horses naturally obtain by being in sunlight and also from sun dried hay. So if he is outside, not rugged from head to foot, ie leave the legs & head  free, given some time in the winter without a rug if he isn’t being  ridden and feed good hay – then those levels will be fine. Don’t rush off to overdose that vitamin either as high levels are also detrimental.

If the horse is in an unusual (and not recommended) environment where he grazes oxalate grasses, has high grain feeds and little hay and no lucerne – then that is when he will need a lot more calcium. This should be done with the input of a veterinarian experienced in nutrition as other adjustments may need to be made too and the horse monitored to keep the calcium intake to safe levels. High calcium is better tolerated when the phosphorus is correctly increased so the overall volume of both is higher.   


So the real answer is not to listen to offhand advice and start tweaking with extra bits and pieces, but to be careful with minerals and vitamins. More is not better, the key is balance; for example add too much zinc and you interfere with copper, too much copper can interfere with selenium, sulphur can prevent copper absorption and so on. Full detail on the interrelationships of minerals can be found in the article on Supplements.

Know what your horse needs, look at labels, provide balanced supplements – especially in minerals. If you think you need to give more, then increase the amount of a balanced mineral supplement that has the correct ratios – don’t just pick out one mineral and increase that. A mineral supplement that shows exactly what it contains and the amounts and has been formulated for horses in New Zealand conditions by a reputable manufacturer is what you need. Vetpro Everyday Minerals provides the correct formulation for resting and light working sport horses and Vetpro Performance Minerals provide for the needs of competition horses and pregnant and lactating broodmares.

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