Effects on exercise response, fluid and acid‐base balance of protein intake from forage‐only diets in Standardbred horses

Abstract

High-energy forage might be an alternative to concentrates for performance horses and such forage can be produced by an early cut. However, early cut forage is high in crude protein (CP), which may result in an excessive CP intake. To investigate how CP intake affects nitrogen (N), fluid and acid-base balance, and exercise response in horses fed high-energy forage diets. The hypothesis was that high CP intake causes acidosis, and alters fluid balance and response to intensive exercise. Two forage-only diets based on high-energy grass forage were fed for 23 days in a crossover design to 6 Standardbred horses in racing condition. One forage diet provided a high (HP) CP (16.6%) intake and the other diet provided recommended intake (RP) of CP (12.5%) for racehorses. The horses had intensive exercise twice and slow exercise 1-3 times every week. At the beginning and end of each period, faeces and urine were collected for 48 and 72 h, respectively and analysed for dry matter, pH and N content. At Days 19 and 23 in each period 2 race-like exercise tests were performed, a standardised treadmill test and a field test on a race track. Blood samples were taken before, during and after the tests and analysed for sodium (Na), potassium (K), chloride (CI), total plasma proteins (TPP), TCO2, urea, pH and lactate. The strong ion difference (SID) was calculated and heart rate and respiratory rate was also recorded. There was a decrease in urinary pH and an increase in N excretion, blood urea, water intake, urine volume and faecal water content on the HP diet. Total water intake was higher than the increase in urinary and faecal water loss indicating increased evaporative losses on the HP diet. During the exercise tests there were no significant differences between diets in TPP, plasma lactate, blood Na, K, Cl, TCO2, pH, SID and respiratory or heart rates. Feeding a forage-only diet with a CP intake corresponding to 160% of the requirement caused an increase in N excretion within 36-48 h after the diet was introduced and alterations in fluid and acid base balance at rest. The increased urine and probably also evaporative fluid losses suggest that feeding a HP diet will cause an unnecessary challenge for horses during prolonged exercise.