Seasonal changes in body composition of growing Merino sheep

Circannual patterns in feed intake, metabolism and growth for sheep appear to be endogenous rhythms that are entrained to a seasonal photoperiodic pattern. It was considered that body composition, a resulting outcome of the input and utilisation of feed (metabolism), would also demonstrate a seasonal pattern. This study was aimed at establishing the presence of seasonal oscillations in body components of growing sheep, using total body fat, muscle and empty body weight data from a long term serial slaughter experiment in growing Merino sheep from weaning through to maturity. Changes in total fat and muscle weight relative to empty body weight were analysed using the additive allometric model in the non-linear form, to which a sine function was added to account for any seasonal oscillations in the residuals. For total body fat weight, the addition of a sine oscillation to the allometric model was significant for both rams (P < 0.05) and ewes (P < 0.10) with mean amplitudes of 1.01 and 0.58 kg, respectively. For muscle weight, the addition of the sine function was significant only for the rams (P < 0.01), with a mean amplitude of 0.664 kg. In rams, the peak in the oscillation for total fat occurred in mid summer, whilst in ewes the peak occurred 111 days later in autumn. In rams, the oscillation for total muscle was approximately six months out of phase with that for total body fat. Therefore in rams, after adjustment for empty body weight, fat weight increased during summer, whilst muscle weight increased during winter. These oscillations indicate that there is a seasonal effect on the priorities for muscle and fat accretion in growing sheep. Oscillations in body components will have implications for assessing the energy requirements of growth and effect the ability to finish animals to fatness/weight specifications. Oscillations in body components will effect the maintenance energy costs of the mature breeding unit.