Pasture Turnout during Periods of Exercise Layoff Helps to Maintain Equine Fitness
Weather conditions or injury often puts an end to riding and training for many horses. No matter the reason for exercise layoff, what does the end of scheduled exercise mean to the fitness level of horses? Low exercise intensity often leads to loss of muscle mass and bone density, as well as decreases in measures of aerobic capacity. Furthermore, horses on complete stall rest may experience even greater losses of fitness. Certainly these losses would delay the return to training and affect progress. However, the voluntary exercise associated with complete (24-hour) pasture turnout may offset fitness losses during periods of layoff.
Dr. Patricia Graham-Thiers from the Equine Studies Department at Virginia Intermont College recently examined fitness maintenance of horses that were laid off their exercise regimens for 14 weeks from April to August. All horses used in the study were exercised 1 – 2 hours per day in the 12 weeks prior to layoff. After this period, six horses were turned out on a 100-acre moderately hilly pasture for 14 weeks (P group). Five horses were stalled and received 1 – 2 hours of light-to- moderate exercise five days per week (SE+ group), and five additional horses were stalled without exercise (SE- group). Both of the stalled groups were fed grass hay plus concentrate and were kept in their stalls during the day and turned out in 1-acre paddocks at night.
Horse body weights and body condition scores were measured and evaluated every two weeks. Every four weeks, horses wore halters fitted with GPS units to quantify distances travelled for 24 hours on pasture or during the night turnouts. Measurements taken at the beginning and end of the study included: 1) rump fat, which was used to estimate body fat, 2) radiographs of the left cannon bone to estimate bone mineral content, and 3) a standardized exercise test (SET) to evaluate fitness. The SET was conducted in an indoor arena and consisted of 5 minutes of walking, trotting for 5 minutes, 3 minutes of cantering, 5 minutes of trotting, hand-galloping for 2 minutes, and 10 minutes of walking for recovery. Heart rate was monitored during the SET. In addition, blood samples and rectal temperatures were collected prior to exercise, at the peak of exercise (within one minute of the hand-gallop), and after the 10-minute recovery period.
Horses did not lose weight or body condition during the 14-week layoff period. Not surprisingly, horses in the P group traveled greater voluntary distances (6.7 miles/day) than horses in the SE+ group (3.2 miles/day) or SE- group (2.8 miles/day). Since exercise was conducted indoors, GPS units could not be used to determine distance travelled. Based on arena size and training duration, horses in the SE+ group covered roughly 5 miles during forced exercise.
Horses in the SE- group lost fitness during the 14-week layoff period. This is supported by the fact that it took longer for their heart rates to return to normal after the SET than horses in the P or SE+ groups. Heart rate in conditioned horses is lower during exercise because training improves the stroke volume of the heart, which means that more blood is pumped to tissues with each heart beat and also allows quicker recovery time after workout ends. In comparison, heart rates of horses in the P group were similar to heart rates of horses in the SE+ group at comparable exercise intensities, indicating that horses maintained fitness during pasture turnout. Furthermore, plasma lactate concentrations and rectal temperatures at the peak of exercise and the 10-minute recovery period were lower in horses in the P and SE+ groups. Lower lactate levels indicate improved aerobic capacity and metabolism. Since aerobic enzyme concentrations quickly return to pretraining levels when conditioning stops, these results signify that horses in the SE- group lost fitness while horses in the P and SE+ groups maintained fitness. Body temperature is another indicator of fitness; therefore, horses with lower rectal temperatures working at similar exercise intensities have improved fitness as a result of improved heat removal. Radiographs showed that bone mineral content was not different in horses at the beginning of the study. However, bone mineral content of the lateral cannon bone increased in horses in the P group after 14 weeks of pasture turnout but did not change in horses in the SE+ or SE- groups.
In summary, voluntary exercise of horses with access to pasture helps to maintain exercise fitness as well as improves bone strength during a training layoff.
Graham-Theirs, P.M. and L. K. Bowen. 2012. Improved ability to maintain fitness in horses during large pasture turnout. J. Equine Vet. Sci. Article in Press.