SEASONAL-CHANGES IN BODY-COMPOSITION OF GROWING MERINO SHEEP

Circannual patterns in feed intake, metabolism and growth for sheep ap pear to be endogenous rhythms that are entrained to a seasonal photope riodic pattern. It was considered that body composition, a resulting o utcome of the input and utilisation of feed (metabolism), would also d emonstrate a seasonal pattern. This study was aimed at establishing th e presence of seasonal oscillations in body components of growing shee p, 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 we ight, the addition of a sine oscillation to the allometric model was s ignificant for both rams (P < 0.05) and ewes (P < 0.10) with mean ampl itudes of 1.01 and 0.58 kg, respectively. For muscle weight, the addit ion 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 oscillati on for total fat occurred in mid summer, whilst in ewes the peak occur red 111 days later in autumn. In rams, the oscillation for total muscl e was approximately six months out of phase with that for total body f at. Therefore in rams, after adjustment for empty body weight, fat wei ght increased during summer, whilst muscle weight increased during win ter. These oscillations indicate that there is a seasonal effect on th e priorities for muscle and fat accretion in growing sheep. Oscillatio ns in body components will have implications for assessing the energy requirements of growth and effect the ability to finish animals to fat ness/weight specifications. Oscillations in body components will effec t the maintenance energy costs of the mature breeding unit.