5 Publikation 2
5.3 Materials and methods
5.4.6 Correlations
The basal serum cortisol concentrations did not reveal any correlation with other pa-rameters (Table 8). However, the cortisol concentration after fixation was correlated with the heart rate during “fixation 1) (P=0.05). Also the relative increase of serum cortisol was positively correlated with the heart rate at "fixation 1" and its AUC (P = 0.001 and P = 0.04). Likewise, the behaviour under "B total" was positively
noCS tCS nCS nnCS a
ab ab b
(B2) was positively correlated with the percentage increase of the cortisol concentra-tion (P = 0.04).
Table 8: Correlations (Spearman-Rank) between serum cortisol concentration, heart rate and behaviour of calves during a standardized stress test (correlation co-efficient in upper line, P-value in the second line)
Basal
cortisol Cortisol after
Fix 1: Section “fixation 1” in the stress response test (min 37 – 41): period when calf was haltered and led to the tethering area
B2: Behaviour of the calf during “fixation 1” (min 37 – 41) evaluated with Score points
B3: Behaviour of the calf during tethering period (min 42 – 46) evaluated with Score points
B total: Sum of all Behaviour Scores (B1 (behaviour of the calf without any stressors
5.5 Discussion
The aim of the present study was to investigate the background of nnCS which oc-curs in a considerable extent in group-housed calves. Although up to 70% of all CS activities occur as nnCS (Roth et al., 2009), this abnormal behaviour has not been studied extensively.
In general, stressors can be divided into three categories. Firstly, psychological stressors are caused by procedures such as transport, handling, re-grouping and separation. Secondly, physical stressors are extreme temperatures or lack of food (Kim et al., 2011). Thirdly, a combination of both types of stressors has been sug-gested including noise, pain, tethering and weaning (Kelley, 1980). Responses to such stressors manifest themselves, by activation of the sympathetic nervous sys-tem, in heart rate increases and changes in hormone levels (increased release of catecholamines and glucocorticoids) as well as behavioural adaptations up to the eventual occurrence of stereotypies. The capacity to cope with stress varies greatly between individuals, and is influenced by factors such as genotype, rearing, devel-opment and early experiences of each animal (Dantzer and Mormède, 1983; Kool-haas et al., 1999; Van Reenen et al., 2005).
The measurement of the heart rate is a suitable parameter to represent the reaction potential to short-term stress exposure and to determine the stress level in animals (Pollard and Littlejohn, 1995; Boissy and Le Neindre, 1997; Visser et al., 2002). In this context, the use of a POLAR® heart rate monitor has already been described as a useful tool for measuring heart rate in cattle (Hopster and Blokhuis, 1994). During the experiment, the calves were confronted with different stressors. Besides fixation for blood sampling and the blood sampling itself, the halter, the leading and the sub-sequent isolation from the familiar environment provoked stress. Handling and fixa-tion, occasionally accompanied with pain (e.g. zootechnical interventions such as dehorning) and restriction of movement and isolation have already been used as stressors in previous experiments with calves for stress-, behaviour- and tempera-ment studies (Dellmeier et al., 1985; Friend et al., 1985; De Passillé et al., 1995;
Gauly et al., 2001; Sisto and Friend, 2001; Steinhardt and Thielscher, 2001; Gauly et al., 2002; Hickey et al., 2003). Within the scope of this experiment, increases of the heart rate during the blood sampling and the leading to the tethering area were de-termined in all calves. The extent of the increase in heart rate was found to be highly variable between individuals.
Beside the heart rate, also the cortisol concentration in serum is a suitable parameter to evaluate the stress level and is used as an indicator for stress and for evaluation of temperament (Cooper et al., 1995; Grandin, 1997; Gauly et al., 2002; Van Reenen et al., 2005). In order to measure variations in the cortisol concentration, one blood sample was obtained before and one after exposure to the stressors mentioned above. As expected, in 97% of the calves serum cortisol concentration rose after tethering compared to the basal concentration. This result is consistent with findings from other studies in which increases in blood cortisol concentration, due to stress exposure, were observed (Wohlt et al., 1994; Petrie et al., 1995; Hickey et al., 2003;
Kim et al., 2011). The increase in cortisol concentration was positively correlated with the heart rate during the leading phase (“fixation 1”). Such correlations between heart rate and cortisol concentrations have already been described (Van Reenen et al.
2005). Acute stress leads to the release of catecholamines which causes an immedi-ate increase in heart rimmedi-ate, while cortisol in turn increases and extends the metabolic effects of the catecholamines (Dantzer and Morméde, 1983). Thus, the assessment of cortisol concentration before and after stress exposure is useful to verify the meth-od of the heart rate measurement for quantification of the stress response.
Furthermore, correlations between the calves’ behaviour during stress-inducing situa-tions and their blood cortisol concentrasitua-tions have been reported (Hickey et al., 2003;
Van Reenen et al., 2005). Thus, the behaviour in response to a stressful situation with high cortisol concentrations, can either be active (flight, increased physical ac-tivity) or passive (immobility), but is always fear-induced. In calves, it has been shown, that there are positive correlations between the cortisol concentration and the duration up to approach to an unfamiliar object (Van Reenen et al., 2005). In this
and the calves’ behaviour, but the relative increase between both blood samples and the behavioural score of B2 were positively correlated, suggesting that calves which achieved higher behavioural scores also showed greater increases in cortisol con-centration after exposure to stress.
Correlations were also found between the behaviour and heart rate. Accordingly, there were positive correlations between the behaviour (B3) and the heart rate (heart rate fix 1) during the tethering period. This result may be the effect of animal’s indi-vidual physical fitness or physical activity during sympathetic stimulation (Le Neindre, 1989; De Passillé et al., 1995; Steinhardt and Thielscher, 2001). Since under B3 pre-dominately the physical activity and willfulness of the calf were assessed, it is likely that this correlation is due to the physiologically-based heart rate increases due to increased muscle activity.
For studies focusing on animal behaviour, reliable observations are essential. In or-der to get the observations as accurate and reliable as possible, the group with the tested calves had to be observed at least twice daily at least 30 min.
Validation of farmers’ observation was carried out on four of the 16 farms. The extent of agreement (21 of 25 observed calves) of self-conducted observations with those of the farmers was found to be sufficient for further evaluation.
Interestingly, Simmental calves, compared to pure dairy breeds and their cross-breeds, tended to perform nnCS to a greater extent which has already been sug-gested by Keil and Audigé (1999). Results support a genetic background. On the other hand, nCS occurred independently of the breed but has been observed signifi-cantly more frequently in groups of calves with more than ten, than in groups with fewer than ten members. One possible explanation for this might be the reduced competition for milk in smaller groups (Veissier et al., 1994; De Paula Vieira, 2007;
Eriksson, 2009).
The absolute results for the heart rate of calves either performing CS or not in later life did not differ significantly, but a tendency was obvious comparing nCS and nnCS calves. Relative increases, however, revealed evidence that nnCS calves exhibited stronger and longer-lasting increases of the heart rate during stressful situations, than calves which did not show CS in later life. Although the behaviour of calves dur-ing fixation was not found to correlate to CS, the measurement of cortisol concentra-tion turned out as useful parameter, i. e., a more pronounced cortisol release was found after stress exposure.
Despite the relatively small number of experimental animals, the results support the hypothesis that calves with a more pronounced stress response than others, tend to perform nnCS more frequently than stress-resistant calves. Since also in this study purebred Simmental calves were frequently noticed performing non-nutritive cross-sucking, a breed disposition with regard to increased stress sensitivity seems possi-ble. Although there were no significant differences in measured parameters between calves which performed nCS and nnCS, it is obvious, due to the statistical tenden-cies, that the latter showed the highest response to the stressors during the test. Un-fortunately, so far there are no comparable studies, so further research will be need-ed. If nnCS is confirmed as a behaviour that serves stress compensation, an proach for breeding and selection of stress-resistant animals may be a promising ap-proach to reduce this abnormal behaviour in calves.
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