SLOWING THE HANDS OF TIME

Originally, Dr Stewart wrote this article for the popular Australian equine magazine, Hoofbeats. We were thrilled with an overwhelming response and it was evident the ageing horse makes up a significant percentage of the equine population. This lead us to share it again for those who may have missed out the first time, we hope you enjoy and take away some information to help your loved ones live a long and happy life. 

Many horses continue to be ridden, compete and reproduce well into their twenties. In horses, as with humans, there is no set age at which horses become ‘old’. For disciplines requiring years of training and physical development, peak performance often occurs in the mid to late teens. Many showjumpers and dressage horses don’t reach their prime until they are 12-14 years old. However, from around 15 years of age, the athletic ability and needs of horses begin to change. The reserve capacity of many organ systems narrows and the safety margin of many common practices is reduced – however, certain dietary inclusions can smooth the passage and slow the hands of time.

Age (Table 1.) – ‘..an irreversible, progressive and time-dependent decline of overall body functions, resulting from the interaction of genetic and random factors’. Seen (and experienced!) as changes in body composition, a decrease in muscle mass and a reduction in aerobic and anaerobic exercise capacity. Less visible are the age-related decline in the immune system (immunosenescence), inflamm-aging (systemic low-grade chronic inflammation), reduced ability to respond to heat and cold (thermoregulation) and insulin-resistance (reduced ability to load glucose into muscles). Key aspects of ‘healthy’ aging for which nutrition can have a protective effect include: muscle mass, immunity, insulin-resistance and body weight.

Speed, strength and endurance can decline with advancing years. This is partly because of a reduction in muscle mass, which tends to be replaced with fat, resulting in a loss of muscle strength. The age-related decrease in muscle mass (sarcopaenia) seen in all species is thought to be due to reduced ability to synthesize protein. The good news is that this defect can been overcome by the essential amino acid leucine - which is high in lucerne. The number of new cells the body can build – whether blood, muscle, bone cells – depends on the amount and the amino acid profile of the protein in the diet. Most prepared feeds for adult horses contain 10—14% protein. But, horses don’t need a % protein, they need a number of grams of protein each day. For example, a horse fed 3kg of a 10% protein feed, or 1½ kg of a 20% protein feed gets 300g of protein in both cases.

Whether this protein is ‘useable’, depends on its amino acid profile. Proteins are made of chains of amino acids – good quality protein is high in essential amino acids. Of the 22 amino acids in nature, 10 are essential, cannot be synthesized in the body and must be provided by the diet. All the different feeds (Table 2) and all the different tissues in the body have their own formula or recipe amino acids. And a bit like a wooden water barrel, or making chocolate cupcakes — it doesn’t matter how much flour, sugar and eggs you have, once you run out of chocolate (or wood for the barrel), cake production stops. Similarly, a deficiency of any single amino acid will limit new cell production. Unusable amino acids convert to fat. So % protein is not useful in terms of knowing how much usable protein

Lysine and threonine are considered the first and second limiting amino acids in horses. Leucine stimulates protein synthesis, and lysine and threonine make up a big part of muscle. Feeding lucerne or a protein supplement in the 2 hours before work ensures the amino acids are high in the blood stream when the muscle demands them during exercise. Feeding a small meal of lucerne and protein before work is also protective against stomach ulcers – which are increased in horses worked on an empty stomach. Feeding a small grain/protein meal within one hour of finishing exercise and hourly for the next six hours, provides the raw materials and the fuel for muscles to best respond, repair, rebuild, recover and refuel after work.

The immune system becomes less competent with increasing age (immuno-senescence), increasing susceptibility to infections, cancer and autoimmune conditions. The numbers of white blood cells and their response (killing capacity) to infectious agents reduces and the amount of inflammatory compounds increases with advancing years. Vitamin C (25grams of ascorbyl palmitate/day) and vitamin E (160iu/kg of diet) have been shown to increase killing capacity of white blood cells and the immune response to vaccination in aged horses. This daily dose of vitamin C has also reduced the incidence and severity of “shipping fever” (respiratory conditions associated with travel).

Aging is also associated with increased markers of chronic inflammation. Inflamm-aging is thought to be a result of the cumulative effects of oxidants – those radical, altered molecules of oxygen produced during the metabolic processes of life and increased after exercise – when oxygen consumption and therefore free-radical waste products are increased. The body has an anti-oxidant defence system – involving vitamins and minerals, including vitamins E and C, selenium and zinc. For older horses in work, correct nutritional support can make a significant difference and certain specific compounds have demonstrated benefits. The ability to synthesize anti-oxidants declines with age and suggestions for older exercising horses include: alpha-lipoic acid (ALA) 2–4g/day; biotin 10 to 20 mg/day; acetyl-L-carnitine 10–20g/day; coenzyme Q10 400 to 600 mg daily and twice the recommended daily intakes for selenium, vitamin A and vitamin E. Recent studies found supplementing aged horses for 4 weeks with resveratrol (2g/day) reduced inflammatory markers and increased white blood cell numbers and killing capacity.

Advancing age also increases insulin resistance (IR). Feeds with low starch/sugar/NSC are essential. Extrusion improves starch digestibility in the small intestine and, although this helps avoid starch overload of the large intestine for horses needing high grain intakes, it exacerbates IR. Most commercial feeds contain grains or grain by-products (bran, pollard, hominy meal, mill-run) high in NSC (Table 3). Heat processing increases small intestinal digestion by 446% and speeds caecal fermentation to yield over 200% more lactic acid than unprocessed grains – and many “senior” feeds contain 3–5% molasses, pushing up the NSC content. Prepared feeds should be used with caution.

The highly popular use of ‘senior’ feeds is evidence that we are willing to spend extra to maintain our old horse’s health. But a recent study found however no differences in digestibility and body condition between 5–12 yo and 19–28yo horses with good worm and dental care on the same diet. Dental abnormalities and declining pancreatic function may reduce absorptive capacity, but no age differences in digestibility of minerals, energy, neutral detergent fiber, protein, fat, calcium or phosphorus has been found between adult horses and ageing horses. Although more work is needed in this area, current evidence suggests that advanced age (>20 years) alone does not significantly affect digestive efficiency in horses. As long as they are fed to meet their requirements, old horses can easily maintain good body condition well into their 20s and even 30s.

Healthy older horses generally do well on a ‘normal’ diet ie hay/roughage (good quality and 30% to 35% acid detergent fibre). A mix of around 50% lucerne + 50% grassy/oaten hay or chaff at 1.5-2.5% of bodyweight, ad lib water and salt, plus a well-formulated balancer to provide vitamins and minerals, is usually appropriate. Feeding lucerne to older horses is controversial, but with higher leucine and crude protein than grass hay, it has been shown to prevent loss of muscle mass. Beet pulp (fed at 1­-3kg a day) is a good source of energy and readily fermentable structural carbohydrates – it is also a probiotic fermented primarily in the caecum. Oils provide concentrated energy and those rich in omega-3 fatty acids (canola, linseed, omega 3 supplements – start at 50ml per day, increase gradually to 500ml) are anti-inflammatory.

Aging can compromise the ability to handle the combined demands of exercise and thermoregulation, thought to be due to a decreased blood volume. However, 20–30 year old horses can still complete strenuous standardized treadmill tests with no special feeds or training regimens. This underscores the fact, that despite reduced exercise tolerance, many old horses can be maintained in good body condition and continue to be used for athletic endeavours well into their 20s and even 30s. Clipping in summer and clipping and rugging in winter, as well as ensuring daily intake of salt/electrolytes meets requirements, helps maintain hydration and plasma volume. Administering an electrolyte paste with free access to water four hours before travel promotes hydration and reduces the risk of impaction colic.

Body condition should be monitored. Cold conditions are harder on horses with a BCS <4/9, and loss of social position and bullying can occur in groups of horses. Managing horses individually, providing appropriate nutrition and veterinary care, with regular anthelmintic (horses 20–33yo have higher strongyle egg counts than 5–15yo) and dental treatment is a sound approach. Selecting feedstuffs carefully and feeding 3–4 meals improves digestibility, better matches normal feeding behaviour and minimises post-prandial glucose and insulin responses.

Weight loss can be a problem for some horses. Fat is the energy source of choice - high energy density and minimal risk of digestive upset. Canola and linseed provide omega-3 benefits and up to 800ml/day can be fed.  Horses may require 21 days to adjust to a fat-supplemented diet.  Omega 3 oils improve blood oxygen levels and reduce the incidence and severity of arthritis and inflammatory skin conditions in both humans and animals.

With few exceptions and as in most species, whatever gets worse with age gets better with exercise. Provided horses are free from lameness and in good health, they should be able to sustain a moderate conditioning program.  Experienced, well-schooled horses can often maintain fitness with a lower volume of work and this helps reduce stress on the musculo-skeletal system. If a horse has been spelled or retired for some time, it’s worth having a veterinary examination before beginning a training program.

Exercise helps maintain joint function and slows the progress of certain diseases, whereas inactivity can worsen the severity. Although some level of aches and pains is normal with advancing years, there are medications and specialist shoeing techniques that can minimise the impact of age-related degenerative conditions such as navicular disease and arthritis. Osteoarthritis is a common cause of lameness and poor performance in horses of all ages, and degenerative joint disease a frequent reason for euthanasia. There are demonstrated benefits in providing chondroproetective compounds such as glucosamine, chondroitin and hyaluronic acid, and recently turmeric has consistently been found to be very useful in slowing all kinds of age-related changes.

Many old horses do not need special feeds or rations as a consequence of age alone and most old horses, especially those in their early 20s, can be fed adult (not geriatric or senior) prepared feeds. With thought and attention to detail, nutrition can be used to assist in maintaining health and exercise capacity. Managing horses individually, providing appropriate nutrition and veterinary care, with regular anthelmintic (horses 20–33yo have higher strongyle egg counts than 5–15yo) and dental treatment is a sound approach. Selecting low NSC feedstuffs carefully and feeding 3–4 meals/day, better matches normal feeding behaviour and minimises glucose and insulin responses to a meal. For performance horses, maintaining body condition is the minimum requirement of feeding. But the effects of ageing can be minimised and performance can be improved through nutrition. 

EQUINE CLINICAL NUTRITION
Dr Jennifer Stewart