Standard practice in the dairy industry is to separate the calf and dam immediately after birth and raise calves in individual pens during the milk-feeding period. In nature and in extensive beef systems, the young calf lives in a complex social environment. Social isolation during infancy has been associated with negative effects, including abnormal behavior and developmental problems, in a range of species. Here, we review empirical work on the social development of calves and the effects of social isolation in calves and other species; this evidence indicates that calves reared in isolation have deficient social skills, difficulties in coping with novel situations, as well as specific cognitive deficits. We also review the practices associated with group housing of dairy calves, and discuss problems and suggested solutions, especially related to cross-sucking, competition, aggression, and disease. The studies reviewed indicate that social housing improves solid feed intakes and calf weight gains before and after calves are weaned from milk to solid feed. Evidence regarding the effects of social housing on calf health is mixed, with some studies showing increased risk of disease and other studies showing no difference or even improved health outcomes for grouped calves. We conclude that there is strong and consistent evidence of behavioral and developmental harm associated with individual housing in dairy calves, that social housing improves intakes and weight gains, and that health risks associated with grouping can be mitigated with appropriate management.

The objective of this study was to investigate how milk allowance affects body weight gains and behavior before, during, and after weaning in dairy calves. Fifty-six Holstein calves were randomly assigned at 1wk of age to 1 of 4 feeding treatments of whole pasteurized milk: 6, 8, 10, or 12 L/d. Milk allowance was reduced by 50% at d 42, and then reduced by 20% per day starting at d 50, so that calves were completely weaned at d 55. Calves were provided ad libitum access to calf starter and hay. Body weight was measured weekly from 2 to 10wk of age. Milk intake increased with milk allowance as intended, although actual intakes were lower than the treatment allowance (e.g., averaging 5.66±0.19 L on the 6-L treatment and 9.41±0.21 L on the 12-L treatment before weaning). Intake of calf starter was higher for calves that were fed less milk before weaning (averaging 0.25±0.03kg/d and 0.05±0.03kg/d on the 6-L and 12-L treatments, respectively) and also during the weaning period (averaging 1.15±0.13kg/d and 0.54±0.14kg/d on these same treatments), but we observed no effect after weaning. Time spent at the hay feeder did not differ with treatment. Average daily gain was higher for calves that were fed more milk, increasing from 0.77±0.04kg/d with the 6-L treatment to 0.90±0.03kg/d with the 12-L treatment. Before weaning, calves fed less milk engaged in more unrewarded visits to the milk feeder, averaging 11.1±0.73 visits/d with the 6-L treatment and 0.4±0.78 visits/d with the 12-L treatment. During weaning, calves fed less milk still had more unrewarded visits to the milk feeder than calves fed more milk (19.7±1.64 visits/d with the 6-L treatment versus 10.1±1.74 visits/d with the 12-L treatment), but treatments did not differ after weaning. We conclude that higher milk allowances result in weight gain advantages before weaning that can persist beyond weaning, and that the high number of unrewarded visits to the milk feeder by calves that were fed less milk are indicative of persistent hunger.

Negative emotional states in humans are associated with a negative (pessimistic) response bias towards ambiguous cues in judgement tasks. Every mammalian young is eventually weaned; this period of increasing nutritional and social independence from the dam is associated with a pronounced behavioural response, especially when weaning is abrupt as commonly occurs in farm animals. The aim of the current study was to test the effect of separation from the cow on the responses of dairy calves in a judgement task. Thirteen Holstein calves were reared with their dams and trained to discriminate between red and white colours displayed on a computer monitor. These colours predicted reward or punishment outcomes using a go/no-go task. A reward was provided when calves approached the white screen and calves were punished with a timeout when they approached the red screen. Calves were then tested with non-reinforced ambiguous probes (screen colours intermediate to the two training colours). “GO” responses to these probes averaged (± SE) 72±3.6 % before separation but declined to 62±3.6 % after separation from the dam. This bias was similar to that shown by calves experiencing pain in the hours after hot-iron dehorning. These results provide the first evidence of a pessimistic judgement bias in animals following maternal separation and are indicative of low mood.

Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, but emotional states are difficult to directly assess in animals. Researchers have assessed pain using behavioural and physiological measures, but these approaches are limited to understanding the arousal rather than valence of the emotional experience. Cognitive bias tasks show that depressed humans judge ambiguous events negatively and this technique has been applied to assess emotional states in animals. However, limited research has examined how pain states affect cognitive processes in animals. Here we present the first evidence of cognitive bias in response to pain in any non-human species. In two experiments, dairy calves (n = 17) were trained to respond differentially to red and white video screens and then tested with unreinforced ambiguous colours in two or three test sessions before and two sessions after the routine practice of hot-iron disbudding. After disbudding calves were more likely to judge ambiguous colours as negative. This ‘pessimistic’ bias indicates that post-operative pain following hot-iron disbudding results in a negative change in emotional state.

Dairy calves have traditionally been kept in individual pens throughout the milk-feeding period. Social rearing is associated with increased solid feed intake and, hence, higher weight gains before and after weaning. Little is known about the effect of the age at which social housing begins. The aim of this study was to assess the effects of early versus late pairing on feeding behavior and weight gain before and after weaning. Holstein bull calves were reared individually (n=8 calves) or paired with another calf at 6±3 d (n=8 pairs) or 43±3 d of age (n=8 pairs). All calves were fed 8 L of milk/d for 4 wk, 6 L/d from 4 to 7 wk, and then milk was reduced by 20%/d until calves were completely weaned at 8 wk of age. Calves were provided ad libitumaccess to calf starter and a total mixed ration (TMR). Body weight and feed intake were measured weekly from 3 to 10 wk of age.Intake of calf starter was significantly higher for the early-paired calves than for individually reared and late-paired calves throughout the experimental period. At 10 wk of age, starter dry matter intake averaged 2.20±0.22, 1.09±0.25, and 1.26±0.33kg/d for early-paired, late-paired, and individually housed calves, respectively. Intake of TMR did not differ among treatments, TMR dry matter intake averaged 3.27±0.72, 3.08±0.46, and 2.89±0.54kg/d for the same 3 treatments. Calves in the early paired treatment also showed significantly higher average daily gain over the experimental period (0.89±0.04 vs. 0.76±0.04 and 0.73±0.04kg/d for the early-paired, individual, and late-paired calves, respectively). These results indicate that social housing soon after birth can increase weight gains and intake of solid feed.

Rodents and primates deprived of early social contact exhibit deficits in learning and behavioural flexibility. They often also exhibit apparent signs of elevated anxiety, although the relationship between these effects has not been studied. To investigate whether dairy calves are similarly affected, we first compared calves housed in standard individual pens (n = 7) to those housed in a dynamic group with access to their mothers (n = 8). All calves learned to approach the correct stimulus in a visual discrimination task. Only one individually housed calf was able to re-learn the task when the stimuli were reversed, compared to all but one calf from the group. A second experiment investigated whether this effect might be explained by anxiety in individually housed animals interfering with their learning, and tested varying degrees of social contact in addition to the complex group: pair housing beginning early (approximately 6 days old) and late (6 weeks old). Again, fewer individually reared calves learned the reversal task (2 of 10 or 20%) compared to early paired and grouped calves (16 of 21 or 76% of calves). Late paired calves had intermediate success. Individually housed calves were slower to touch novel objects, but the magnitude of the fear response did not correlate with reversal performance. We conclude that individually housed calves have learning deficits, but these deficits were not likely associated with increased anxiety.