|By Debbie Odell
For many horse owners, kikuyu grass forms an integral part of the annual fodder-planning program. Indeed kikuyu forms the mainstay of some equine operations. Its yields are good and inputs remain relatively cheap and simple. But is kikuyu really suitable as a pasture for horses?
Kikuyu grass (Pennisetum clandestinum) is a robust, perennial, creeping species of grass, introduced originally form East Africa, and now widely distributed throughout the country. It has become one of the most important grazing pasture grasses, being both nutritious and palatable when well fertilized and managed. It is capable of yielding in excess of 20 tons of dry matter per hectare, with a crude protein content of 12 – 18% and a TDN of 60 – 70%, depending on pasture maturity. All these factors make it an attractive species for cultivation. There is however some risk associated with the use of kikuyu for horses.
According to laboratory analysis, kikuyu contains about 2.2 g of calcium and 3.5 g of phosphorus per kg of dry matter. And therein lies the first problem – it has an inverse Ca:P relationship i.e. the phosphorus level is higher than the Ca level. The ratio of calcium to phosphorus appears to be important in horse nutrition, where even if calcium requirements are ostensibly met, excessive phosphorus intake will cause skeletal abnormalities.
A further complication is that Kikuyu grass is known to be an oxalic acid accumulator plant. Oxalic acid forms highly insoluble salts with divalent cations like calcium. Horses appear to be completely unable to utilize calcium bound to oxalic acid in pastures. The inverted Ca:P ratio combined with the presence of oxalic acid makes the grass a high risk one in terms of initiation of hypocalcaemia (calcium deficiency) in horses.
It is well known that calcium and phosphorus are intimately involved in bone formation and imbalances may cause abnormal bone formation in young horses as well as bone related symptoms in older horses. Nutritional secondary hyperparathyroidism (NSH) (also called “big head”, “bran disease” or “miller’s disease”) has often been reported in horses grazing certain tropical pastures including kikuyu. The condition has also been reported in horses fed high phosphorus diets (including those high in grain or bran). High dietary phosphorus levels reduce the rate of calcium absorption, which may lead to chronic calcium deficiency. The level of calcium in the blood is regulated by a complex set of metabolic mechanisms, and if the blood level drops, the body mobilizes calcium from the bone relatively quickly. The net effect of this isbone demineralization, the symptoms of which include bone formation abnormalities in youngstock and shifting lameness, loose teeth, loss of condition and distinctive enlargement of the jaw bones in adult animals. The risk of spontaneous bone fractures and ruptured tendons may also increase.
Lactating animals respond to calcium deficiency by decreasing their level of milk production in line with the level of the deficiency, although not by decreasing the amount of calcium and phosphorus in the milk. Reduced milk production of the dam, of course, compromises the growth rate of the foals.
It is clear that this condition may have a profound effect on viability of a stud operation. Unfortunately, the insidious effects of calcium deficiency may go unnoticed until a period of high calcium demand is encountered, for example rapid growth rate, pregnancy (especially the last trimester) or the onset of lactation. The condition may develop over a short period of time although it usually takes 6 – 8 months for the symptoms to develop.
Laboratory analysis may be undertaken to test whether pastures are high risk or not. It should be borne in mind though that oxalate levels may vary from season to season and even quite considerably between paddocks. Pastures with an oxalate level of more than twice the Calcium level are considered to be high risk
The good news is that balance studies indicate that suitable supplementation over a prolonged period may reverse some of the symptoms e.g. the lameness and ill-thrift, but the facial deformation may be permanent, depending upon the length of time spent in, and the severity of the deficiency state. Knowing the inherent dangers of using kikuyu pastures, a prevention strategy is obviously preferred. High calcium stud feeds may partly address the problem, but ideally some form of daily, balanced calcium supplementation should be made available to horses grazing kikuyu pastures.
Probably the easiest way to do this is to provide ad lib access to a suitably formulated mineral block.
© Pet’s Health