Growth, training response and health in Standardbred yearlings fed a forage-only diet.

The aim of this study was to, from a holistic perspective, describe the effects of a forage-only feeding system and a conventional training program on young Standardbred horses and compare data with similar observations from the literature. Sixteen Standardbred colts fed a forage-only diet for 4 months from breaking (August to December) and with the goal to vigorously trot 5 to 7 km at a speed of 5.6 m/s (3 min/km) were studied. The horses were fed grass haylage (56 to 61% dry matter (DM), 2.80 to 3.02 Mcal DE/kg DM and 130 to 152 g CP/kg DM) ad libitum, 1 kg of a lucerne product and minerals. The amount of training and number of training sessions were documented daily, and feed intake and body development were measured once every month. Heart rate (HR) was measured during and after a standardized exercise test in October and December. In December, a postexercise venous blood sample was collected and analyzed for plasma lactate concentration. Muscle biopsies (m. gluteus medius) were taken and analyzed for glycogen and fiber composition. Health was assessed in October and November by an independent veterinarian using a standardized health scoring protocol. BW and height at withers increased from 402 to 453 kg (root mean square error (RMSE) 6) and from 148.7 to 154.1 cm (RMSE 0.7), respectively, and the body condition score was 4.9 (RMSE 0.2) at the end of the study. Muscle glycogen content was 532 mmol/kg dry weight (s.d. 56). There was a significant decrease in postexercise HR (81 v. 73 bpm, RMSE 8), and the individual amount of training was negatively correlated with HR during and after exercise. Health scores were high and similar at both assessments (8.4 and 8.4 (RMSE 1.0) out of 10; P > 0.05), and the number of lost training days per month due to health problems was <0.9, with the exception of November (5.3 days). It is concluded that yearlings in training fed high-energy forage ad libitum can reach a conventional training goal and grow at least as well as earlier observations on yearlings of other light breeds.