Reduced stocking density mitigates the negative effects of regrouping in dairy cattle.

In freestall systems, cows are frequently moved among pens and regrouped. This practice involves mixing unfamiliar cows, and can result in changes in stocking density after regrouping. Both regrouping and changes in stocking density can affect cow welfare, but no study to date has assessed the combined effects. The aim of this study was to test if reductions in stocking density can mitigate the responses to regrouping. By manipulating group size (6 vs. 12 cows) and pen size (12 vs. 24 stalls), 3 different stocking densities were created (25, 50, and 100%). Four groups of Holstein cows were regrouped weekly for 4wk and the stocking density changed at regrouping. The change in density varied as follows: a decrease by a factor of 4 (100 to 25%), a decrease by a factor of 2 (100 to 50% or 50 to 25%), no change (50 to 50%), an increase by a factor of 2 (25 to 50% or 50 to 100%), and an increase by a factor of 4 (25 to 100%). Displacements at the feeding area, feeding time, and lying time were scored. The daily means for each group were used to calculate the differences in responses from 1d before to 1d after each regrouping. The number of displacements at the feed bunk decreased and lying time increased when stocking density decreased at regrouping. In conclusion, increases in competitive behavior and the associated decrease in lying times can be mitigated by reducing stocking density when regrouping dairy cows.