Regrouping induces anhedonia-like responses in dairy heifers

Abstract

Intensively housed dairy cattle are commonly regrouped (mixed into a new social group) as part of routine farm procedures. This stressful procedure triggers heightened levels of agonistic behaviors and disrupts animals' time budgets. However, little is known regarding the effects of regrouping on cattle's affective states. The aim of this study was to explore whether regrouping (involving a change in both the social and physical environment) triggers anhedonia (i.e., the reduced ability to experience pleasure) in 6-mo-old dairy heifers, a phenomenon associated with negative mood. In this study, we assessed anhedonia using changes in the use of a mechanical brush. Holstein heifers (n = 16) were trained to use a mechanical brush and then given the opportunity to individually brush for 10 min every 2 d. Time spent brushing (during a 10-min brush test) was collected before, during, and after regrouping (2-d interval) with the assumption that heifers would reduce their use of the brush during regrouping. Each heifer was individually regrouped into a new social group composed of 12 older and unfamiliar heifers and allowed access to the brush at 8 and 56 h after the onset of regrouping. Immediately after the last test, each heifer was brought back to her original pen and allowed to mingle with familiar pen-mates before being tested again 2 and 4 d later. When tested 8 h after regrouping, heifers reduced time spent brushing by 44 ± 27% (95% confidence interval: -96.18 to -41.8) compared with before regrouping; however, no differences were detected 56 h after regrouping. There was no relationship between the intensity of the decrease in brush use and any behaviors (number of agonistic interactions received, time heifers spent resting, or synchronization at the feed bunk) recorded for the 8 h immediately before testing (i.e., between 0 and 8 h and between 48 and 56 h after regrouping). These results indicate that regrouping induces anhedonia-like responses in dairy heifers on the day of regrouping. This routine procedure may thus induce negative mood in dairy heifers. This response was not related to behaviors typically collected to assess the negative effects of regrouping. Maintaining dairy cattle in stable social groups should be favored.

Summary: Regrouping (i.e., mixing individuals to form a new social group) is common on dairy farms. The practice is known to be stressful, but how cattle react emotionally to this stressor is still poorly understood. We studied whether heifers experience anhedonia (i.e., the reduced ability to experience pleasure) following regrouping by comparing their use of a mechanical brush before and after regrouping. Heifers reduced their use of the brush 8 h after regrouping, suggesting that they were experiencing anhedonia. This method shows promise for assessing affective states when animals are subjected to routine stressors.

Figure 1

Timeline of the experimental procedure. Heifers (n = 16) were individually tested on the brush test before (3 times), during (2 times), and after regrouping (2 times). Tests were always separated by 48 h and took place at 1600 h. Regrouping took place at 0800 h with the first and second tests taking place 8 and 56 h later, respectively. Baseline was calculated using the average brush use of the 3 pre-regrouping tests. Brush tests always occurred in the same arena and away from the other pens.

Figure 2

Change in brush use after regrouping and return to the home pen. Heifers (n = 15) were individually tested on the brush test before (3 times), during (2 times), and after regrouping (2 times). Baseline was calculated using the 3 pre-regrouping tests when animals were housed in their home pen. Percentage change was calculated for each of the post-regrouping time points. The test on d 0 was performed 8 h after regrouping and the test on d 2 was performed 56 h after regrouping. Tests on d 4 and 6 were performed 48 and 96 h after the return to the home pen, respectively. Tests were always performed 2 d apart at 1600 h. Brush tests always occurred in the same dedicated arena, away from the other pens. Boxes represent the interquartile ranges with median change; each dot is an individual point.