Effects of stocking density on growth, accelerometer-derived locomotor activity and circadian rhythms in lambs during suckling and fattening

This study assessed the impacts of stocking density on growth and locomotor activity (LA) of lambs in the suckling and fattening periods, integrating production variables with behavioural and circadian rhythm analyses. Lambs were reared under either high or low stocking densities (suckling period: high, 2.1; low, 7.3 m2/ewe; fattening period: high 1.4; low 2.6 m2/lamb), and growth performance was assessed separately for suckling and fattening periods. LA was recorded by accelerometers, and circadian rhythms were characterised by a 24-h cosinor analysis. In the suckling period, stocking density had a significant effect on amplitude and acrophase of LA. In the fattening period, lambs exhibited a robust circadian rhythm in LA at both densities, similar mean MESOR and acrophases; however, lambs housed at high density at both studied periods had a significantly higher amplitude than did lambs at low density. Differences between groups in growth performance observed in the suckling period tended to diminish in the fattening period, suggesting that compensatory growth occurred once nutritional constraints were removed. The results have demonstrated that stocking density influences LA in a phase-dependent manner and that circadian indicators provide complementary insights into the biological responses of lambs to housing conditions, beyond those obtained from growth performance alone.