Investigation of Time of Hay Feeding on Plasma Volume and Exercise Response in Thoroughbred Horses
A common question asked regarding feeding the performance horse is when should the horse be fed hay relative to grain and exercise. Should the horse receive hay before, with, or after a grain meal?
Four mature Thoroughbred horses (2 mares and 2 geldings) were used in a 4×4 Latin square design experiment to determine whether method of feeding prior to a standardized exercise test (SET) will affect plasma volume and metabolic response to exercise. The four dietary treatments included FAST (12-hour fast prior to SET), AD LIB HAY + GRAIN (ad lib hay from 6 pm the night before and up to SET and 5 lb of a sweet feed mix 2 h prior to exercise), HAY + GRAIN (5 lb of hay fed 3 h prior to SET and 5 lb grain fed 2 h prior to exercise), and GRAIN (5 lb of hay fed at 10 pm the night prior to SET and 5 lb of grain fed 2 h prior to exercise on the morning of the SET).
The SET was performed on an inclined treadmill (3°) and consisted of a 10 min walk, 10 min trot, 2 min gallop, 10 min walk, 10 min trot, 10 min walk, and 8 min canter. Three hours before the SET, the horses were fitted with an indwelling catheter in the jugular vein. Blood samples were taken hourly before exercise and plasma volume was determined immediately before thee SET using an indocyanine green clearance method. During the exercise test, blood samples were taken during the last 30 seconds of each step of the test and 15, 30, 60,120, and 180 min after exercise. The blood samples were analyzed for glucose, lactate, total plasma protein, PCV, Na+, Cl-, and K+.
The horses were significantly heavier at the beginning of the SET after having ad libitum access to hay. Their plasma volumes were also about 9% smaller compared to the other three treatments. Feeding hay and/or grain 3 hours before the SET did not affect plasma volume. Fasted horses had lower blood lactate after the 8 min canter compared to the other three treatments. Heart rate was significantly different between the treatments. During the 2 min gallop, FAST heart rate averaged 191 while AD LIB HAY + GRAIN equaled 206. During the 8 min canter, FAST heart rate averaged 176 while the heart rate for the AD LIB HAY + GRAIN equaled 191. Heart rates for GRAIN and HAY + GRAIN were intermediate, averaging 183 and 189, respectively. The results of this experiment suggest that an overnight fast before an extended bout of exercise may be beneficial.
Pagan, J.D., and P. Harris. 1997. Investigation of time of hay feeding on plasma volume and exercise response in Thoroughbred horses. In: Proc. 15th Equine Nutr. and Physiol. Soc. Symp. pp. 244-247.