Ensuring our horses are obtaining the correct nutrients to prevent disease can be both worrying and daunting. In this article we cover the role of calcium, the importance of a correct calcium to phosphorus ratio and a correct calcium to oxalate ratio and how to achieve them. We hope you find it helpful for choosing what to feed and managing horses on high oxalate, subtropical pastures.
What is Nutritional Secondary Hyperparathyroidism and how does it occur?
Nutritional Secondary Hyperparathyroidism (NSH) occurs as a result of calcium deficiencies within the diet. Up to 99% of calcium within the body is found within the bones and teeth and comprises about 35% of the equine bone. Calcium also has a significant role in muscle contraction, neuromuscular activity, cell membrane permeability, blood coagulation and enzyme regulation. Blood calcium levels are tightly regulated by the body through the hormones parathyroid hormone (PTH) and Calcitonin (CT) in a process known as homeostasis. When horses are maintained on calcium deficient diets, bones become the main source of calcium and the body secretes PTH from the parathyroid gland which helps to mobilize calcium out of the bones so that it can maintain normal blood ionized calcium concentrations.
Signs of NSH and diagnosis
In early stages of disease horses may show signs of shifting or difficult to diagnose lameness in one or more legs, joint tenderness, changes in behaviour, general body soreness, strange breathing noises, reluctance to move and a stiff, stilted gait. These can be associated with bone demineralization, the development of microfractures, loss of bone integrity and the disruption of articular cartilage. In advanced cases the horse may suffer from spontaneous fractures within the limbs and/or pelvis. The bones of the head may also be visibly affected, as mineral content of the facial bones and mandible are replaced with increased amounts of osteoid and fibrous material resulting in the physical distortion of the head – hence the characteristic name ‘big head.’ Affected horses may have upper airway breathing difficulties or noise, watery nasal discharge or swelling of the distal pasterns. Horses whose mandibles (jaws) have been affected may have difficulty chewing which can result in subsequent weight loss and a poor body condition. NSH without obvious clinical signs may be difficult to diagnose, as blood tests will often show normal blood calcium levels, due to calcium being drawn out of the bones to maintain blood calcium levels. However, increased PTH concentrations with normal to slightly low blood calcium with normal to slightly high blood phosphate concentrations may be indicative of NSH. If a horse is showing clinical signs, evaluating their diet is going to be the best method to determine a calcium deficiency and make appropriate changes. Unfortunately, horses may not show subtle signs at all, and the first sign may be a broken leg or pelvis, so prevention through nutritional management is key in the prevention of disease.
Is your horse at risk of a calcium deficiency?
There are two main causes of calcium deficiencies 1) inadequate dietary calcium or an incorrect calcium to phosphorus ratio (Ca:P) in the diet and 2) grazing pasture containing oxalate
1) Inadequate dietary calcium or an incorrect Ca:P
Horses require an adequate amount of both calcium and phosphorus within the diet, but the intake of these minerals also needs to be within the correct ratio, with the ideal ratio being between 1 to 2.5:1. The absorption efficiency of calcium decreases as the amount of phosphorus within the diet increases, due to the competitive nature of absorption of these minerals in the small intestine. A ratio less than 1:1 (Ca:P) may impair calcium absorption and if fed for an extended period can lead to chronic calcium deficiency and NSH. Horses particularly at risk are those fed diets comprised of mature grass hay, which can vary in calcium content, with large amounts of cereal grains or grain by products such as wheat and rice bran, which contain high amounts of phosphorus and low amounts of calcium. Without the addition of an adequate vitamin and mineral supplement, balancer pellet or fortified feed, these horses are at risk of calcium deficiencies.
Horses maintained on pasture may also be at risk of calcium deficiencies, particularly pregnant and lactating mares and growing horses that have higher mineral requirements. Lush, rapidly growing pastures tend to be low in calcium, as calcium moves quite slowly through the plants root system into the leaves and stem, while phosphorus moves quite quickly. This may result in either an incorrect Ca:P ratio or simply not enough calcium or phosphorus in the diet.
Both scenarios can be corrected by adding in a sufficient source of calcium in the diet, such as the inclusion of lucerne hay, a correct vitamin and mineral or balancer pellet or the use of a fortified feed.
2) Grazing high oxalate pasture
In subtropical high oxalate grass, the major portion of calcium is present as calcium-oxalate, meaning the dietary calcium is bound to the oxalate and it is unable to be absorbed across the wall of the small intestine. This dramatically decreases calcium absorption, with grasses containing more than 0.5% oxalate capable of causing a chronic calcium deficiency and NSH. NSH has been reported in horses consuming oxalate containing plants as their primary forage within 2-7 months with horses at risk when the Calcium:Oxalate ratio is 0.5:1 or less. While oxalate induced calcium deficiency can occur in any horse, lactating mares and growing horses are at particularly high risk, with horses from weaning to 7 years old most affected.
The first step to preventing oxalate induced calcium deficiencies is to identify if your horse is grazing oxalate containing pasture. High oxalate species include: Setaria, Kikuyu, Buffel Grass, Green Panic, Pangola Grass, Guinea Grass, Purple Pigeon Grass, Para Grass and Signal Grass. Any horse grazing these pastures is at risk and requires specific supplementation to prevent disease. If you are unsure of your pasture type, speak with your local rural store, DPI office or agronomist who should be able to assist you in identifying your pasture.
If your horse is grazing one of these pasture types, the aim is to increase their calcium intake, decrease their oxalate intake and maintain a correct Ca:P and Ca:Oxalate ratio, below are some tips to achieve this:
- Feed lucerne hay – the addition of 2-4kg of lucerne hay (for a 500kg horse) will increase your horses calcium intake and decrease their pasture and hence oxalate intake. This helps to balance the Calcium to oxalate ratio.
- Avoid feeds high in phosphorus – as discussed above high amounts of phosphorus and an incorrect Ca:P ratio will decrease calcium absorption. Hence, it is helpful to avoid unfortified high phosphorus products such as wheat and rice bran. Rather it is best to choose a fortified, complete feed with a balanced Ca:P ratio.
- Feed a balanced ration – ensure that you are feeding a diet with sufficient calcium to maintain the correct calcium to oxalate and calcium to phosphorus ratio. Supplementation with limestone alone is not enough to correct calcium intake, nor will this meet phosphorus or other lacking nutrient requirements. While correctly formulated mineral blocks may be beneficial, studies have shown that mineral intake can be erratic and it is likely they do not meet horses’ requirements, compared with daily supplementation that results in better mineral intake and retention. Hence, the recommendation to provide horses on these pastures with a correctly formulated balancer pellet or complete fortified feed to ensure these requirements are met and prevent the occurrence of disease. Below are some example diets that contain the correct ratios for a 500kg horse:
A note on Setaria: Setaria contains very high amounts of oxalate, horses grazing this pasture need to be managed differently and carefully. Ideally their pasture intake is limited as much as possible with the provision of high quantities or ad lib hay, with the addition of a balanced ration. If your horse is grazing Setaria, please get in contact for a specific strategy for your horse.
Written by Bethanie Clark BAnVetBioSc (Hons I)
National Research Council. (2001). Nutrient Requirements of Horses. Washington, D.C., The National Academies Press.