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Keeping an Eye on Calcium Balance in Australian Horse Nutrition

It is pretty much common knowledge that calcium is an essential mineral for strong and healthy bones and teeth in horses. It’s importance in the nutrition of the late pregnant mare, weanling and growing horse, especially young racehorses is well known, and most horse people are aware of the increased requirements for calcium in these types of horses. Most people also know that you need the correct ratio ie 2:1 of calcium to phosphorus for good health and balanced nutrition but generally, if questioned, the average horse person couldn’t explain why this ratio is important. Furthermore, it is not enough to make sure that calcium and phosphorus levels are correct in the diet if availability of these minerals is compromised. There are naturally occurring chemicals which may be available to the horse in everyday feeding regimes which can bind calcium and phosphorus and prevent it from being absorbed by the horse.

It is not possible to discuss calcium balance in the body without mentioning the relationship between calcium and phosphorus. In the horses’ bones, the ratio of calcium to phosphorus is 2:1. Bone acts as a reservoir for calcium and phosphorus that can be tapped when dietary intake falls short of requirements. Calcium is also used in the body in a soluble form (ie as Ca+ ions) for nerve and muscle function, therefore the body maintains rigid and controlled levels of calcium in the blood (2.9-3.9mmol/litre) for these processes by a mechanism known as homeostasis. On the other hand, blood levels of phosphorus can fluctuate throughout the day and in response to exercise with no adverse effects.

This article will explain some of the danger spots to look for when you are considering the calcium availability of your horses’ diet. It will outline some of the dangers of calcium deficiency and how you can manipulate your management program to deal with problems associated with calcium being tied up in the diet by chemicals which make it less available to the horse.

Calcium Deficiency

In days of old, when man still relied heavily on the horse for transport, for working the fields and as a beast of burden it was not uncommon to reward ones trusty steed with a nice big warm bran mash at the end of the day. The practice is still upheld by many experienced horsemen today. Although wheat-milling processes were less efficient in those days, meaning that bran had slightly more nutritive value than the bran of today, as a foodstuff it still posed a problem with regard to calcium and phosphorus ratios. In those days the disease colloquially known as Big Head was associated with workhorses and was something of a mystery. Big Head was eventually linked to nutrition and in particular to dietary calcium balance, and hence became known as Bran Disease. These names were given to the syndrome associated with calcium deficiency (nutritional secondary hyperparathyroidism) because of the fibrous growths responsible for swelling the facial bones of affected horses to unnatural proportions. These days it is generally only seen occasionally and in cases of extreme calcium deficiency caused by dietary deficiencies or in the presence of calcium binding agents in the diet.

Wheat bran is detrimental to calcium balance in two ways: Firstly, 90% of the phosphorus in wheat-bran exists as calcium phytate, binding both calcium and phosphorus and preventing their absorption into the horses’ blood stream. The phytic acid involved in forming this compound can also bind to other dietary nutrients, which also reduces their ability to be absorbed into the horses’ bloodstream. These nutrients include copper, zinc and manganese. Secondly, wheat bran has ten times as much phosphorus as calcium. The combination of excessive dietary phosphorus and little available dietary calcium causes calcium to be leached from the bones to maintain and balance blood levels of calcium ions.

Wheat-bran however, is not the only culprit and there are other foodstuffs available to horses that limit the amount of calcium that can be absorbed from the diet. Many grains are also high in phosphorus and low in calcium, and the majority contain some level of phytic acid. In addition many introduced species of tropical and sub-tropical grasses exist in pastures used for horse grazing or hay making in Australia. Some of these grasses contain high levels of oxalic acid. Oxalic acid binds up calcium by forming crystals of calcium oxalate in the grass stem and leaf in much the same way as phosphorus (phytic acid) forms phytates and prevents calcium absorption. Oxalic acid forms compounds with many elements to produce oxalates, some soluble, some insoluble. Levels of oxalates vary between plants but in plants where calcium is present oxalic acid forms an insoluble compound of calcium oxalate which reduces the amount of available calcium in the plant. Horses are completely unable to digest any of the calcium associated with calcium oxalate crystals. Some oxalate containing plants also contain plenty of calcium, meaning that they are safe to feed despite their oxalate content. For a grass of this nature to be safe for feeding to horses without the risk of causing calcium deficiency it must have a calcium to oxalate ratio of at least 0.5:1. Calcium deficiency caused by consumption of tropical grasses high in oxalates is also known as chronic oxalate poisoning.

Oxalate poisoning can be acute where grasses or weeds contain high concentrations of soluble oxalates which are absorbed rapidly into the bloodstream binding up calcium in the blood and rapidly reducing blood calcium levels. In this case, calcium oxalate crystals are formed in the kidney tubules and interfere with kidney function. Affected horses may have muscle tremors, staggering gait, may appear lethargic, and stop eating. Twitching of the muscles of the face may be seen, and death may occur if signs go unnoticed. Plants containing more than 2% soluble oxalate have the potential to cause acute oxalate poisoning, but horses usually have to be very hungry before considering these essentially unpalatable grasses and weeds as a food source.

Interestingly, many horsemen report increased incidence of calcium deficiency and cases of big head within a few weeks of rainfall. It may be that rainfall triggers new growth that the horses find more palatable, or that the increased moisture increases the oxalate content of tropical grasses. Whatever the reason, reports suggest that increased cases of big head after rainfall are seen with in a matter of two to three weeks. Further research in this area could help horse owners to develop new pasture and grazing management practices to prevent horses from grazing dangerous pastures at times of high risk.

Clinical Signs of Calcium Deficiency

Although the clinical signs of chronic and extreme calcium deficiency are less frequently seen today than in the past, it can still be a significant problem for horse owners, and it is important to be aware of the signs and be able to recognise them if you see them. The disease can develop within two months of putting horses out on high oxalate pastures but more often takes six to eight months before symptoms are noticed.

Low blood calcium levels (known clinically as hypocalcaemia) stimulates the release of parathyroid hormone from the parathyroid gland. This hormone is responsible for triggering the release of calcium from bones, particularly from the large bones of the head and limbs. This calcium is released into the blood to bring blood calcium levels back to within normal ranges for optimal nerve, heart and muscle function. If the horse has a high requirement for calcium, as in pregnant and lactating mares, growing horses and heavily sweating performance horses, these effects will occur more readily and to a greater extent that in horses at maintenance levels of requirement.

As the bones become demineralised, they become weak and fragile, the horse may become lame and may start to drop weight even though his feeding regime has not altered. He will probably have a ‘dull’ depressed countenance and may appear to have swollen lower jawbones and/or loose teeth. The horse will appear stiff and have a shortened gait when trotted up, the stiffness often only increasing as the horse is exercised.

As the condition progresses, both upper and lower jaws and sometimes other facial bones become swollen (hence the name Big Head), and badly affected horses may suffer fractures and break down.

On examination post-mortem, the swellings comprise mostly of fibrous tissue with small sparse fragments of bone. The surfaces of the joints appear pitted and rough, and the parathyroid glands of the throat and lower neck are visibly enlarged and distended. If the condition goes unnoticed and untreated, it is likely that the horse will suffer fatal or irreparable fractures and will need to be destroyed.

Treatments For Horses With Big Head

Examination by a veterinarian may be needed to determine the severity of the disease, perhaps involving X-Ray examination, blood and urine tests. These may need to be repeated throughout treatment to examine the efficacy of the treatment program. Although the swellings of the facial bones may never completely disappear in severely affected horses, it is possible to get a horse back to full health after an episode of big head with time and correct attention to calcium: phosphorus ratios. It can take up to 12 months for re-mineralisation of bone to occur, and horses must be maintained at rest for this period of recovery. Suggested treatments include 2kg of rock phosphate mixed with 3kg of molasses or 2kg of a combination of 1/3 ground limestone and 2/3 dicalcium phosphate (DCP) mixed with 3kg molasses. These supplements need feeding weekly (either over a couple of days or split into daily feeds) for a period of at least 6 months. Commercially available supplements such as EQUIVIT Bone Food can also be used and fed daily in smaller quantities than the above mixtures (around 120g per day) to provide the same benefit.

The Prime Suspects!

There are a few prime suspects to be investigated if a case of calcium deficiency has been diagnosed. The risk is greatest when these grass types make up all or almost all of the pasture available to the horse. All grasses are introduced sub-tropical species; no native grasses have been responsible for any documented cases of the disease. These grasses should be identified and curbed from causing any further damage as soon as possible.

Kikuyu Grass has a calcium to oxalate ratio of 0.23:1. Grows very rapidly in summer and becomes rank and unpalatable relatively quickly. Can enhance the development of horse-sick pastures.
Buffel Grass has a calcium to oxalate ration of 0.22:1. Has a tufted appearance, often forming dense tussocks. A number of varieties vary from 0.2-1.5m tall. Leaves vary in colour from yellowish to bluish green, are thin and narrow and taper to a long point. Occurs in all mainland Australian states.
Pangola Grass has a calcium to oxalate ratio of 0.37:1.
Green Panic Grass has a calcium to oxalate ratio of 0.32:1. Very green, lush growth, highly palatable used in pastures and in hay.
Para Grass has a calcium to oxalate ratio of 0.29:1. A course vigorous trailing grass grows well in wet and flooded soils. Grows up to 1m tall. Occurs in the Northern Territory. More often used in hay as opposed to pasture.
Setaria Grass has a calcium to oxalate ratio of 0.15:1. Grows up to 3m tall with erect stems and leaves 15-30cm long and 0.3-1.7cm wide. Seedheads are spike-shaped and cylindrical. Occurs in QLD, NSW and WA.
Pigweed Fleshy low to the ground weed with yellow flowers in the summer. Rarely consumed by horses, but can cause acute oxalate poisoning if horses are hungry enough to eat large quantities. Oxalate concentrations range between 4.5-9.4% of the dried plant. Occurs both coastally and inland.

 

Prevention

To prevent calcium deficiency caused by grazing of sub-tropical grasses, look for and avoid these grasses in potential horse pastures. Use native pastures where possible, and if pastures containing the sub-tropical grasses mentioned above must be used, avoid grazing them for longer than one month.

If established pastures contain a large proportion of sub-tropical grass, calcium to phosphorus ratios of dietary intake should exceed 2:1 and may need to be as high as 3:1 to counterbalance the oxalate effect. It is also useful to encourage the growth of leguminous plans such as lucerne, which is high in calcium, and gives the horses an oxalate free alternative forage. If your pasture contains grasses such as kikuyu, it is wise to avoid using fertilisers with high phosphorus levels such a poultry manure and superphosphate.

It may be necessary to also make available a calcium and phosphorus supplement, and if so, using half the amount of the supplements mentioned in the treatment section weekly should prevent any problems from occurring. Alternatively, feeding 20kg of good quality lucerne hay per horse per week ensures adequate calcium intake to guard against oxalate poisoning.

Conclusion

Although extreme calcium deficiencies are seen less today than in the past, the introduction of foreign sub-tropical grasses to Australian pastures has meant that horse owners have another danger spot to look out for when investigating potential horse pastures. It is important for horse owners to know that wheat bran is not the only culprit when it comes to Big head or Bran disease. It is a good idea to thoroughly check the species of grass in pastures to be used for horses, and take measures to prevent extended periods of grazing on unsafe pastures. If hazardous pastures make up the only grazing available, then supplements should be used to ensure adequate calcium intake to balance the effects of oxalic acid.

By Sonja Gardner
Kentucky Equine Research
Brighton, Victoria, Australia