By Sara Muhonen
PhD-student, Dept. of Animal Nutrition and Management, Swedish University of Agricultural Sciences (SLU)


A project financed by the Swedish Farmers' Foundation for Agricultural Research

Diets for racing horses often contain large amounts of concentrates to provide enough energy. Feeding concentrates increases the risk for metabolic disorders1 and stereotypic behaviour2 and high-energy forage might be a better alternative. Early cut forage is high in energy as well as in crude protein (CP), which will result in an excessive CP intake. In the body, the degradation and metabolism of protein generates heat and hydrogen ions, which might negatively affect exercise performance3. In a recently published study4 the effects of high crude protein intake on exercise response in trotters eating only high-energy roughage (100-120 MJ ME/day) was investigated.

The study: background and methodology
Six trotters in racing condition were fed two roughage diets, mostly grass silage with high energy content (11.1 MJ/kg DM). One of the roughages had a high CP content (16-17.5 %) and the other provided a recommended5 intake of CP (10.5-11.5 %). Three horses started on the high protein diet and the remaining three on the recommended protein diet; the diets were then switched over so that all the horses were fed both diets for 23 days. In the end of each feeding period two exercise tests similar to trotting races were performed. In one of the tests the horses ran on a treadmill and the other was performed as a simulated race on a racetrack. After the simulated race the drivers ranked the horses as: very sluggish, sluggish, alert, very alert or pulling. The same driver tested both horses but was not aware of what diet the horses were fed.

The results: horses can handle excess forage protein
During and after the exercise tests there were no differences between diets in breathing frequency, heart rate, plasma lactate and pH in blood (Table 1). There were no differences in the drivers ranking of the horses after the simulated race.

Table 1 - Heart rate, plasma lactate and pH in blood after exercise tests on treadmill and simulated race on racetrack (recommended and high protein intake)

   Immediately after exercise   After 15 minutes recovery 
  recommended high recommended high
  exercise test on treadmill   
Heart rate1
(beats/minute)
213 ± 4 216 ± 6 70 ± 2 72 ± 4
Plasma lactate
Millimole/L
(mmol/L)
17.6 ± 2.8 18.3 ± 2.2 11.7 ± 3.3 11.1 ± 2.1
Blood pH 7.32 ± 0.03 7.29 ± 0.03 7.38 ± 0.04 7.38 ± 0.01
  simulated race on racetrack   
Heart rate1
(beats/minute)
222 ± 5 215 ± 4 80 ± 2 78 ± 7
Plasma lactate
(mmol/L)
20.2 ± 1.5 22.9 ± 2.8 18.7 ± 2.7 20.5 ± 2.9
Blood pH 7.28 ± 0.03 7.26 ± 0.03 7.32 ± 0.04 7.30 ± 0.03

 1 Highest heart rate noted

The results suggest that racing horses are able to handle excess forage protein (>60% of requirements) during race-like exercise and recovery. The horses had no problem maintaining body weight, which shows that roughage with high energy content can sufficiently fill the energy needs for racing horses. However, an effect on the fluid balance during rest was measured. On the high protein diet the horses lost more fluid via the skin (evaporation) than on the recommended protein diet. This indicates an increased heat production and might be an unnecessary challenge during more prolonged exercise, for example endurance riding, when fluid losses might be a limiting factor.

References:

  1. MacLeay, J., Valberg, S., Pagan, J. D., De La Corte, F., Roberts, J., Billstrom, J., McGinnity, J. and Kaese, H. (1999) Equine vet. J., Suppl. 30, 458-462.
  2. Redbo, I., Redbo-Torstensson, P., Ödberg, F. O., Hedendahl, A. & Holm, J. 1998. Anim. Sci. 66, 475-481.
  3. Glade, M. J. (1983) Equine vet. J. 15, 31-36.
  4. Connysson, M., Muhonen, S., Lindberg, J. E., Essén-Gustavsson, B., Nyman, G., Nostell, K. & Jansson, A. (2006) Equine vet. J., Suppl. 37, 648-653. 
  5. NRC. 1989. Nutrient requirements of horses. 5th ed National academy press, Washington D. C. USA.

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