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Forage-only diets have positive effects on performance

Forage-only diets have positive effects on performance

A Swedish study has examined the impacts on body weight and exercise response in trotters in training when fed a forage-only diet compared to a 50:50 forage:concentrate diet. The forage-only diet consisted of early harvested energy rich haylage and the forage:concentrate diet consisted of late harvested haylage from the same ley. The concentrate consisted of mainly oats and soybean meal and both diets were supplemented with minerals and salt. The two diets, forage-only and forage:concentrate, provided the same daily energy and protein intake.

Three horses started on the forage-only diet and three on the forage:concentrate diet and then they switched so all horses were tested on both diets. The horses were fed the diets for 29 days before exercise tests on a treadmill were performed. The standardised exercise tests on the treadmill started with a warm up phase and then a second phase with stepwise increased speed on an incline until the horses reached or exceeded a heart rate of 200 beats/minute.

The horses’ body weights did not differ between diets before or after the exercise test (Table 1). In this study the horses did not get heavier by consuming a lot of forage. This can be due to that the forage was harvested early which implies a high energy content, more soluble fibre and less insoluble fibre e.g. less bulk. On the forage-only diet the horses had lower concentrations of total plasma proteins (TPP) (Table 1) which indicates a larger plasma volume; that means that the higher water-holding capacity of the forage has worked as a fluid reservoir in the hindgut so the horses had better fluid status on the forage-only diet.

Text 33.1 tabell 1 eng

Rectal temperature, heart rate and breathing frequency did not differ between diets during the exercise test (Table 1). However, there was a tendency to a higher VLa4 when the horses consumed the forage-only diet compared to the forage:concentrate diet (Table 1). This means that on the forage-only diet the horses reached a higher speed before they reached the lactate threshold (4 mmol/l) than on the forage:concentrate diet. This is probably due to a change in energy utilization with a higher utilization of short-chain fatty acids on the forage-only diet and thereby an improved aerobic energy metabolism during exercise. The horses could increase their speed a little bit more before they had to switch from aerobic work to anaerobic work. The concentration of plasma lactic acid was also significantly lower after exercise and blood pH higher during exercise when the horses consumed the forage-only diet compared to the forage:concentrate diet (Table 1).

Another effect of the forage-only diet was a higher concentration of plasma bicarbonate (Table 1). This can be explained by grass containing anions like malic, aconitic and citric acids in the form of potassium, calcium and magnesium salts. When the anions are oxidised, hydrogen ions are removed and bicarbonate ions accumulate. Bicarbonate has an alkalizing effect and increases pH in body fluids, e.g. makes the horse more basic.

The concentration of muscle glycogen (gluteus medius) was lower before and after exercise when the horses consumed the forage-only diet compared to the forage:concentrate diet (Table 1). This could be due to that the sugar intake was lower on the forage-only diet (water soluble carbohydrates ~ 860 g/day) compared with the forage:concentrate diet (starch 3100 g/day). This might have affected the production of muscle glycogen.

The staff that worked with the horses on a daily basis had the impression that the horses were more aggressive when fed the forage:concentrate diet. Especially in the afternoon when horses were fetched from the paddock to be put in their boxes where fed was available.

To summarize, the results from this study shows that a diet consisting of only early harvested high energy forage compared to a more traditional forage:concentrate diet does not affect body weight but gives a different metabolic exercise response which implies an improved aerobic energy metabolism. A lower TPP and higher plasma bicarbonate on the forage-only diet showed positive effects also on fluid and acid-base balance. Lower concentration of muscle glycogen on the forage-only diet could be a limitation but another study shows that high crude protein intake from forage gives just as high muscle glycogen levels. In addition, a long-term study with young racing trotters show high muscle glycogen levels on forage-only diets.

Sara Muhonen, AgrD

Jansson A & Lindberg JE. 2012. A forage-only diet alters the metabolic response of horses in training. Animal 6, 1939-1946.

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