by Dr Jennifer Stewart CEO BVSc BSc PhD Dip BEP
... and your horse’s health Good pasture represents one of the best and least expensive sources of food for horses - and it provides the most natural and healthy environment for exercise and rest. Productive, well-managed pastures can provide most of the feed requirements of horses at the lowest cost. Kikuyu pastures provide good grazing for many animals and are widespread throughout the world - including high-rainfall and coastal areas of Australia (see map) and New Zealand. Imported, exotic grasses have become the basis for intensive cattle grazing in tropical and sub-tropical regions of Australia. G r a s s e s of African origin include kikuyu, paspalum, Rhodes, green panic and setaria. Kikuyu (Cenchrus clandistinus) is so-named because it is native to the highland regions of East Africa that are home to the Kikuyu people. An important range grass, especially for areas not suited for other grasses or where weed control is difficult, it is valued for growing and fattening beef cattle and for dairy production – yielding enough energy for one cow to produce 20 litres of milk a day.
For horses however, there are three main concerns with kikuyu grazing that need to be taken into consideration. Energy – the high level of sugars and starches, which can increase the risk of laminitis in horses and ponies.
Oxalate – makes the plant calcium unavailable to the horse, which then draws calcium from bones and can result in lameness, facial swelling (Big Head) and possible increased bone fractures. Potassium - young kikuyu leaves are high in potassium, which can create nervous/excitable behaviour and affect the uptake of magnesium. It is important to give a high-quality, magnesium and specific calcium and bone mineral supplement to horses grazing on kikuyu. August/September 2021 - Page 42
ENERGY LEVELS
With all grasses, the levels of energy vary between seasons and kikuyu is no different (Table 2).The energy levels can be higher in autumn/winter than in spring or summer.
The trouble for horse owners and vets is to work out when grass starch/sugar levels, and hence grazing, will be dangerous – especially for horses and ponies with insulin-resistance, equine metabolic syndrome or PPID (Cushings). Plants are basically sugar factories. When the sun’s shining, they photosynthesise madly to make lots of sugar. At night they use that sugar to survive until they can begin making more sugar when the sun is out again. Many environmental factors change plant sugar levels – even if we haven’t noticed or we think ‘nothing’s changed’. It may be just that it was a bit cooler overnight and the plants didn’t use as much sugar…or there may have been a little more or stronger sunlight during the day and the grass made more sugar. The amount of energy in kikuyu is determined primarily by sunlight, which allows photosynthesis (which the plant uses to make sugar), and temperature (which the plant needs to grow). The balance between photosynthesis and growth determines how much sugar is stored. On sunny days, the grasses can make lots of sugar and when it is warm, they can use up that sugar by growing.
Kikuyu
SUGAR LEVELS
Sugar levels rise through the sunny days, reaching a maximum at sundown. They then fall through the night as the plant draws on stored sugars to continue to grow even though there is no sunlight.
Sugar levels are lowest before sunrise. However, if the day is cloudy, sugar production is reduced. If the weather is cool, growth is reduced, less sugar is used by the plant – and sugar levels in the grass remain high.
Environmental factors can change the amount of starch/sugar through the seasons and even daily and hourly! These variables have a profound effect on grass starch/sugar levels and hence how safe it is for horses and ponies to graze. The list below is applicable to many grasses – not just kikuyu, and it highlights how sugar levels in different grasses can fluctuate widely. Looking through the list gives us some basis upon which to decide if, when and for how long horses can have access to grass. It also becomes clear how changeable it all is. • gradual onset of droughts can double the sugar and starch in setaria • 45 days of drought can double the sugars in cocksfoot and ryegrass • ryegrass has particularly high sugar levels in July regardless of the stage of growth • even stubble can be really high in starch because the grass stores it in the base of the plant • in kikuyu (and legumes like clover and lucerne) the leaves are the primary sites of starch accumulation • cold temperatures and frost can increase plant carbs especially in cool, sunny weather • sunlight also affects plant sugar levels – which can double in perennial rye grass after just 3 hours of sunlight • in warm temperatures, sugar levels can range from 95 to 560g/kg • when sugar levels are low, horses consume around 750g of sugar per day, compared to 5—10kg when levels are high! • dry, brown, stressed grass can have a dangerously high starch/sugar level
18kg BLOCKS
Contact us for advice on equine nutrition and equine clinical nutrition Products available around Australia in produce stores through AIRR and at Veterinarians and Vet Clinics through Cenvet & Provet
• colour has nothing to do with starch/sugar content. Continued
Table 1. ADVANTAGES AND LIMITATIONS OF KIKUYU GRASS ADVANTAGES
LIMITATIONS
High-quality feed
Rank and unpalatable if not grazed
Long growing season
Tops killed by frost
Survives drought
Poor winter growth
Frost-tolerant
Crowds out other grasses
Erosion control
Can become a weed
Suppresses weeds
Needs highly fertile soil
Rapid summer growth
Overgrows legumes
Table 2. LEVELS OF NUTRIENTS IN KIKUYU IN DIFFERENT SEASONS* Summer
Autumn/winter
Spring
SUGAR (g/kg)
42
37
57
STARCH (g/kg)
60
40
low
ENERGY (MJ/kg)
8
9.3
9
3cm
6cm
12cm
84
95
106
HEIGHT OF GRASS SUGAR & STARCH (g/kg)
*Varies with weather, rainfall, temperature and fertiliser
August/September 2021 - Page 43
Kikuyu and your horse’s health continued...
Figure 2
Susceptible horses should not graze when light intensity is high and temperatures low - conditions often found in spring and autumn.
Over-grazed kikuyu – down to stem bases, where starch is stored.
What can we do to reduce grass sugar intake in pastured horses? Restrict intake – this can be challenging! And assumes that they eat at the same rate every hour. But studies have found they accelerate eating during the first four hours at pasture and take in 67% more than in the next four hours. When given the opportunity horses, and especially ponies, can eat grass to between 1.5 and 5% of their bodyweight each day more grass than they need. Some gobble down a massive 3.3% of their body weight (15-20kg for a 400kg equid) while grazing. Others can eat over 50% of their daily energy requirements during three hours of pasture access. This finding suggests that restricting time at pasture may not always be an effective means of reducing energy intake.
Grazing muzzles - these
reduce bite mass and restrict intakes to the tops of leaves where the concentrations of NSC tend to be lowest.
Time of day sugar concentrations can easily double in four hours of sunlight. Levels are highest in the late afternoon and early evening and lowest at sunrise. Restricting access to between 3am and 11am is safest. Daily weather - grass growing in shade has been found to have lower sugar levels (reduced photosynthesis due to lack of sunlight). Cold, sunny days are nearly always dangerous because sugar production exceeds plant growth and unused sugar is stored in the leaves. Season - pastures shouldn’t be over-grazed (Figure 2) because
sugar is stored as starch low in the stem. This reservoir of stored sugar can be especially high in autumn or winter when grasses do not go completely dormant.
Rotational grazing – section off the paddock if possible and graze to a height of 10-15cm, then move on to another section. Pastures should be about 18 – 25cm tall before horses are turned out to graze a new paddock. Horses should be removed from paddocks when the pasture stand is grazed to about 7 – 10cm. Mowing - constant mowing may actually increase starch/sugar concentration. Short grass is more exposed to sunlight, which produces sugar. Also, plants whose leaves lay flat on the ground out of reach of the mower blades like clover, dandelion and plantain can be very high in sugar. August/September 2021 - Page 44
Horses on restricted grazing need an alternative source of forage – usually hay/straw – and a balanced mineral/vitamin supplement. Hay can be high in sugar/fructan/starch and needs to be analysed and/or soaked (which reduces minerals that need to be replaced).
Oxalates and calcium deficiency osteoporosis
Like most other introduced tropical/subtropical grasses, kikuyu contains oxalate which makes the plant calcium unavailable to the horse. This can lead to osteoporosis and your equine veterinarian can provide advice on managing calcium intake. A calcium deficiency disease from a persistent lack of calcium, excess of phosphorus and/or imbalanced calcium to phosphorus ratio is Big Head (nutritional secondary hyperparathyroidism NSH). This disease is caused by generalised osteoporosis and has severe effects on horses including difficulty breathing, swelling of the facial bones and painful movement and lameness. Early studies used mixtures of rock phosphate (only low in fluorine), dicalcium phosphate, lime and 1.5kg of molasses to treat Big Head. However, cases still occurred, and the amount fed had to be doubled. The high molasses may be contraindicated in horses with metabolic or endocrine conditions. More recent studies in Brazil have demonstrated the benefits of organic forms of calcium when oxalates are part of the diet. Laminitis has plagued domesticated horses since the beginning of recorded history and osteoporosis secondary to dietary calcium deficiency has been recognised for centuries. Understanding the risks, working closely with your veterinarian to keep up-to-date, and your district agronomist re local conditions, can reduce the risk and incidence of both and allow horses to grow and thrive on kikuyu pasture. All content provided in this editorial is for general use and information only and does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your equine veterinarian. Dr Jennifer Stewart CEO BVSc BSc PhD Dip BEP Equine Veterinarian and Consultant Nutritionist. Dr Jen Stewart is currently the only practicing equine veterinarian and clinical nutritionist in Australia with more than 40 years experience. Jen has been developing premium formulas for studs, trainers and feed companies - such as Mitavite - in Australia and around the world. Consulting to leading international studs and trainers in various countries while working on research projects and being involved in nutritional management of a variety of equine clinical conditions, including colic, tying-up, laminitis, performance problems, developmental orthopaedic diseases and postsurgery. Jen’s vision is to provide a world best-practice in equine veterinary nutrition and to BRING SCIENCE TO YOUR FEED BIN.
Dr Jennifer Stewart www.jenquine.com
Equine Veterinarians Australia www.ava.com.au