Automated feeding systems offer numerous advantages. Nevertheless, practical experience shows that individual distances at time-controlled hay rack for horses are frequently undercut and aggression can therefore increase. In the present study, the minimum animal-to-feeding-place ratio (AFR) was investigated at time-controlled hay racks, which would ensure sufficient individual distance between the horses. Using ethological and physiological parameters of 28 horses in a group housing system, four treatments were assessed on two observation days each: an AFR of 1:1.2 (20% more feed-throughs than horses), 1:2 (twice as many feed-throughs as horses), 1:3 (three times as many feed throughs as horses) and C (individual feeding in the grooming area with familiar conspecifics). The order of treatments was randomized, with one treatment per day and at least one day of wash-out phase between treatments, thus extended over a period of 3 weeks (C, 1:3, 1:1.2, C, 1:1.2, 1:3, 1:2, 1:2). Horses were used to daily fluctuation of the group composition, as horses were taken out of the group for riding by their owners every day and at different times, even during feeding. A preliminary test was conducted to habituate the horses. The horses showed less aggressive behavior when there were three times as many openings as horses at the time-controlled hay racks compared to 1:1.2 and 1:2 (lineal mixed model: F(3,4) = 7.411; adjusted R2 = 0.733; p(AFR_1 : 2) = 0.06, p(AFR_1 : 3) = 0.02, p(AFR_C) = 0.01). Salivary cortisol levels during feeding decreased less with low AFR (p(AFR_metric) = 0.02). The results indicate that at least three times as many feeding places as horses must be available for the investigated time-controlled hay racks so that the horses can maintain their individual distance. Further studies are required to validate the results.
