Energy balance and its relationship to body weight and body condition in grazing horses

Equine obesity currently represents a significant welfare concern, with its development influenced by seasonal changes in the nutritional value of pasture, which substantially affect the animals' energy balance. The aim of this study was to evaluate the seasonal dynamics of pasture nutritional composition, assess the balance between energy requirements and actual intake of horses during the grazing season, and determine the impact of these factors on body weight and body condition. Pasture was analysed across three seasonal periods: T1 (May), T2 (July), and T3 (September). The highest concentration of digestible energy in pasture was recorded in T1 (10.81 MJ/kg DM), while the lowest was observed in T2 (9.67 MJ/kg DM). In T1, digestible energy intake (DEi; 98.02 MJ/day) was markedly higher than the energy requirement (61.94 MJ/day), resulting in a positive energy balance (36.08 MJ/day). This surplus was accompanied by a significant increase in body weight from 444.6 kg to 534.7 kg (P = 0.006 1) and an increase in body condition score (BCS) from 5.1 to 6.7 (P = 0.000 1). In the subsequent periods (T2 and T3), when the energy balance was close to equilibrium (-0.02 and 0.14 MJ/day), no further significant changes were observed in body weight (534.7 vs 535.0 kg; P > 0.05) or BCS (6.7 vs 6.6; P > 0.05). Changes in BCS showed a strong positive relationship with the difference between DE intake and requirement (r = 0.696 1; P = 0.000 7). The results confirm that seasonal energy surplus, particularly at the beginning of the grazing season, represents a significant risk factor for body fat accumulation in adult horses maintained on ad libitum pasture.