Locomotor activity measured by actigraphy as a means of estimating social mixing and animal density-induced stress in lambs

This study aimed to determine whether social mixing and density-induced stress affect locomotor activity in lambs. Eighteen 50-70 d lambs on a commercial farm were fitted with accelerometers for nine days. Lambs were housed with another 25 lambs in a 64- m2 pen (density: 0.40 lambs/m2; farm phase, FP). After nine days, lambs were transported to a feedlot and had the sen sors affixed for another nine days. The 43 lambs were pooled with another 25 lambs that came from other farms, and were housed in a 10 x 10-m pen (density: 0.67; low-density phase, L-DP). After 5 d, another 49 lambs were added to the pen (density: 1.21; high-density phase, H-DP). The mean 24-h cosinor curve was calculated, and the cosinor values of each phase were compared statistically by Paired-sample t-tests. The effects of phase, lamb sex, time of day (day or night) on activity-related variables were evaluated statistically based on a multifactorial model; within fixed effects, significant differences were identified by an analysis of variance. Activity rates did not differ between FP and L-DP; however, lambs were less active in the H-DP than in the L-DP. Activity did not differ between males and females. The proportion of the lambs that exhibited a 24-hour circadian rhythm was higher in the FP than in the L-DP and the H-DP, which differed from each other. In conclusion, social stress, caused by social mixing and high densities can affect the locomotor activity and disrupt the circadian rhythms of lambs. This study demonstrated the usefulness and feasibility of using sentinel animals (fitted with sensors) in commercial feedlots to monitor changes in ac tivity so that measures can be taken to improve animal welfare if necessary.