Effects of the introduction of single heifers or pairs of heifers into dairy-cow herds on the temporal and spatial associations of heifers and cows

Investigations into the process of introducing heifers into dairy-cow herds may alleviate animal-welfare problems arising during social integration. The present study monitored the introduction of a single heifer and a pair of heifers known to each other on each of six dairy farms, and investigated the effects on the temporal and spatial associations of the heifers and cows. We automatically collected data on animal positions in three different functional areas (activity, lying and feeding areas) of cubicle housing systems. The positions of each animal were recorded once/min over six continuous days both before and after introduction of the heifers. A measure of synchronicity and distance within all possible dyads was averaged per cow or heifer and functional area to reflect the animals' temporal and spatial associations to other herd members. In addition, agonistic behaviour towards the heifers was recorded over the first 3 days after introduction by means of direct observations. For statistical evaluation, linear mixed-effects models were used. Heifers introduced on their own faced almost double the rate of agonistic interactions experienced by heifers introduced in pairs (7.19h⁻¹ vs. 3.79h⁻¹). They were more synchronous and at smaller distances with the cows in the activity and feeding areas than the heifers in the pairs. This pattern was inversed in the lying area, however. The same patterns also became visible from the perspective of the cows. Heifers introduced in pairs were more synchronous and at shorter distances with one another than with the cows. Cows were slightly but not significantly more affected in their dyadic synchronicity and distances after the introduction of pairs of heifers than after the introduction of single heifers. High synchronicity and small distances of the single heifers with cows coincided with more frequent agonistic interactions, whereas heifers in the pairs were more synchronous and at shorter distances with each other and may have provided some mutual social support. We would thus recommend that heifers be introduced in pairs rather than singly into dairy herds composed of up to 50 dehorned cows kept in cubicle housing systems.