Debbie Odell MSc Agric Pr. Sci. Nat
Consultant Nutritionist Bovasol cc
For many horse owners and stud farms, kikuyu grass forms an integral part if the fodder planning program. Indeed kikuyu is the mainstay of some equine operations. And why not? Its yields are good, inputs remain relatively cheap and simple and it is a hardy perennial species that can stand up to heavy grazing. Yet some folks decry the use of kikuyu for horses, claiming it to be the cause of bone development disorders and other maladies. 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. It is hardy and survives heavy grazing. All these factors make it an attractive species for cultivation. There are however some factors which may affect the value of kikuyu grass 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. The first problem therefore is that it has an inverse Ca:P ratio i.e. the phosphorus level is higher than the Ca level. Most species, horses being no exception, are sensitive to this ratio.
Kikuyu grass (as well as some other common species including Panicum and Setaria spp) is well known as an oxalic acid accumulator plant. Oxalic acid may form complexes with many elements to form oxalates. Soluble oxalate salts are formed with monovalent elements e.g. potassium and sodium, but divalent cations like calcium form highly insoluble salts. 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 in horses. Although oxalates can cause acute toxicity in horses (a single dose of 450g is considered lethal to horses) this is not usually seen under practical conditions. However nutritional secondary hyperparathyroidism (NSH) has often been reported in horses grazing certain tropical pastures including kikuyu.
Horses appear to be completely unable to utilize calcium bound to oxalic acid in pastures. Calcium in this form is passed through the digestive tract and excreted in the faeces. The availability of calcium from calcium oxalate crystals is further complicated by the specific association of these crystals with poorly digestible structural plant tissues. The total oxalate content of kikuyu appears to vary quite considerable between regions, farms and even paddocks within farms. Increasing the calcium content of the soil increases the insoluble oxalate: soluble oxalate ratio, while total oxalate remains relatively constant, and minerals such as Na, K and N do not appear to affect oxalate concentration. The greatest danger of calcium related abnormalities seems to occur on pastures containing more that 0.5% total oxalate on a dry matter basis or where the oxalate level is more than twice the calcium level.
Laboratory analyses 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.
It is well known that there is a close relationship between dietary requirements of calcium and phosphorus, and imbalances may manifest in a host of symptoms. About 98% of the calcium and 85% of the phosphorus in the horses body is present in bones and teeth. The other 2% of calcium is used for various functions including control of acid-base balance, maintaining normal muscle and nerve function, blood clotting, activation of enzymes and hormone formation. 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 or from the diet relatively quickly. It is for this reason that blood calcium levels are not a very reliable indicator of calcium status of the horse. Obviously if the diet is chronically short of calcium over an extended period, the mobilization of calcium to maintain blood homeostasis will be primarily from bone reserves, leading to a progressive weakening of the bone structure.
The remaining phosphorus in the body is involved in almost every aspect of feed metabolism and utilization. Phosphorus plays a critical role in the supply of ATP, the basic unit of energy at the cellular level. Blood levels of phosphorus may be more indicative of status than blood calcium levels, but they will not correlate well with dietary intake. Serum inorganic phosphorus levels will drop only after prolonged dietary inadequacy, and serum levels are affected by calcium and vitamin D status as well as exercise.
Calcium and phosphorus, as we well know, are intimately involved in bone formation and imbalances may cause abnormal bone formation in young horses as well as bone related symptoms and weaknesses in older horses. The ratio of calcium to phosphorus appears to be important in horse nutrition in that even if calcium requirements are ostensibly met, excessive phosphorus intake will cause skeletal abnormalities. High calcium levels do not seem to cause a problem provided that phosphorus levels are adequate, but high phosphorus levels reduce the rate and efficiency of calcium absorption, which may lead to chronic calcium deficiency.
The effects of the deficiency may go unnoticed until a period of high calcium demand is encountered, for example growth, late pregnancy 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. Even so, horses with chronic sub clinical calcium deficiency may take more time to show the clinical signs of deficiency including:
- bone formation abnormalities in young stock
- failure to reach expected weanling & yearling heights
- and ill thrift, loose teeth, shifting lameness
- increased risk of ruptured tendons and spontaneous fractures
- distinctive enlargement of the jaw bones
Occasionally these jaw bone abnormalities may cause obstruction of nasal passages and difficulty in chewing. In older animals that have had full access to kikuyu for a number of years without supplementation, it is common to attribute the poor body condition merely to old age. Sometimes, suitable supplementation can change the appearance of such horses almost miraculously.
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 has a profound effect on the growth rate of the foals. It is clear that this condition may have a profound effect on viability of a stud operation.
The off season for polo ponies coincides with peak growth of kikuyu grass. Polo ponies are usually put to pasture for the summer break and do not take long to regain condition lost in the season as their workload is reduced, and the grass provides adequate energy and protein for weight gain. During these few months on high risk pastures, unsupplemented horses continually lose bone integrity due to the mineral imbalance inherent in the pasture. Each year these horses are brought back into working condition with less bone calcium reserve. Given these facts, those unfortunate tendon injuries or spontaneous fractures in season could be more about off season management than just bad luck on the day!
Does this mean that kikuyu is unsuitable as a pasture for horses? Happily, balance studies indicate that preventative supplementation of horses with calcium and phosphorus can readily overcome the effects of high oxalate levels and is relatively inexpensive. 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.
We can conclude then that kikuyu is still a very useful pasture for horses providing that, knowing the inherent dangers of using kikuyu pastures, a prevention strategy should be implemented. 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 Calcium Lick.