Discussion

The present study aimed to identify the most important risk factors triggering CS in Simmentaler calves raised on dairy farms. The combination of a questionnaire and a visit to the farm to assess the specific housing and feeding conditions enabled a standardized and comparable evaluation of the risk factors present on each farm.

Moreover, including farms from different regions gave an impression about the typical and relevant raising conditions on dairy farms in all parts of Germany. There were, however, some methodological constraints that affect the reliability of the results.

First of all, remarkably, the number of farms struggling with CS (n=41) was more than threefold higher than those that did not experience this behavioural disorder between their calves (n=12). This indicates the severity of the problem in the dairy industry, in agreement with reports from experienced producers and cattle practitioners. Due to the limited number of farms included in this study, however, some risk factors may be overlooked due to the concomitant low power of the statistical analysis. Secondly,

some management tools were implemented on almost all farms (e.g., > 90% of the farms used straw for bedding) which made it impossible to evaluate their impact on the incidence of CS. Conversely, because only a very limited number of farmers used other management tools (e.g., environmental stimuli in the pens), conclusions about their value in coping with CS are not possible. Thirdly, all farms raised exclusively calves that were born at that dairy; thus, conclusions from this study cannot be une-quivocally transferred to farms fattening calves bought from the market.

The nutrient intake during the entire milk-feeding period proved to be paramount to affecting the incidence of CS, while the source of nutrients (milk or milk replacer) and the feeding technique (bucket or automatic feeding system) seem to be less im-portant. It has to be emphasized that beef calves consume an average of 10 L milk per day in their first two weeks of life (Odde et al., 1985). The much lower amounts usually fed under the established rearing conditions (Maccari et al., 2012) are drunk within a few minutes. Thus, the sucking motivation is not satisfied. Consistent with previous studies (Jung and Lidfors, 2001; De Paula Vieira et al., 2008; Roth et al., 2009), the results of our study indicate that restrictive feeding of milk increases the probability of CS.

Moreover, the restrictive feeding of milk is accompanied by longer periods where the calves experience hunger. Hunger is known to enhance the sucking motivation (De Passillé and Rushen, 1997) and acts as a stressor. Sucking stimulates the se-cretion of CCK, gastrin, insulin and oxytocin; thus, it leads to saturation but has also a stress-lowering effect (Kessen et al., 1967; Field and Goldson, 1984; De Passillé, 2001). The plasma oxytocin concentration rises markedly in sucking calves during milk intake (Lupoli et al., 2001), inducing a reduction in blood pressure and lower cor-tisol levels (Uvnäs-Moberg et al., 2001). This may explain why calves drinking milk from teat buckets appear to be more relaxed and lay down faster after milk ingestion than calves that received the milk from open buckets or troughs (Veissier et al., 2002). Consequently, it seems reasonable to assume that nnCS is also triggered by lack of nutrition.

Another distinctive factor was the duration of the milk feeding period. Previous stud-ies demonstrated that short milk feeding periods are associated with an increased occurrence of nCS and nnCS (Roth et al., 2009; De Passillé et al., 2010). Early and, in particular, abrupt weaning is accompanied by increased plasma concentrations of cortisol and norepinephrine (Hickey et al., 2003) and at least a short period is charac-terized by a negative energy balance, frustration, hunger, loss of body weight and stress-induced nnCS (Jung and Lidfors, 2001; De Paula Vieira et al., 2008; Roth et al., 2009; De Passillé et al., 2010; Khan et al., 2010). Thus, a gradual reduction of the milk offered daily (“step down”) is recommended to avoid these problems (Roth et al., 2008; Sweeney et al., 2009; Khan et al., 2010).

A striking result of our study was that the stocking density was a decisive factor af-fecting the incidence of CS and was found to have a greater effect than group size, bedding and further environmental factors. These results are in keeping with other findings (Keil et al., 2000). In fact, calves raised in pens with a high stocking density have significantly higher plasma cortisol levels than calves in groups with a low stocking density (Schlichting et al., 1990). Accordingly, a high stocking density is considered a strong stressor (Friend et al., 1977; Le Neindre, 1993; Nielsen et al., 1996). Furthermore, increased competition for milk can occur, which can cause addi-tional stress, especially in subdominant calves on farms with restrictive milk feeding (Jensen and Budde, 2006).

The following factors also affected the probability of CS, which should be the focus of farms struggling with this behavioural disorder:

 It is well known that an adequate colostrum supply has positive effects on the calf’s health and development (Robison et al., 1988; Wittum and Perino, 1995).

The results of this study indicate that a failure of passive transfer may also repre-sent a risk factor for CS.

 nnCS was more frequently found on farms where calves were raised in pens with dirty or insufficient straw bedding. Thus, uncomfortable housing conditions may play a pivotal role as stressors (Maccari et al., 2012).

 The lighting conditions should be considered. High brightness enhanced the oc-currence of nnCS in group-raised calves. Obviously, brightness leads to more so-cial and exploratory behaviour, including the licking of objects occurs more fre-quently (Dannenmann et al., 1985). A low light intensity, on the other hand, in-creases the duration and frequency of resting and reduces stress.

 The availability of tap water in the first weeks of life reduces the incidence of nnCS due to an increased intake of solid feedstuff (Hepola et al., 2007) and re-duced behavioural disorders (De Passillé et al., 2011b). Calves with permanent access to water show lower plasma cortisol levels than calves without a continu-ous water supply. In addition, water enriches the environment and improves the welfare of calves (Gottardo et al., 2002).

 Particularly in view of the stronger temperament of Simmentals compared to other breeds, such as German Angus or Hereford (Gauly et al., 2002; Hoppe et al., 2010), frequent and intensive contact with humans was found to be advisable.

Calves that have frequent contact with humans, such as stroking or finger suck-ling, are more sociable and show less stress-related behaviour (increased defeca-tion, escape) whether in contact with strangers or familiar persons. This makes handling easier, increases the calves’ welfare and reduces the probability for CS (Waterhouse, 1978; Grandin, 1997; Lensink et al., 2000).

 The separation of the newborns from their dams after calving tended to be earlier in farms without CS than in farms with this behavioural disorder. Later separation from the dam (> 24 h until one week vs. immediately after birth) increases loco-motor activity, the frequency of vocalization and exploration and the occurrence of oral behaviours, such as sniffing and licking walls and objects (Lidfors, 1996;

Stĕhulováa et al., 2008). This increased restlessness, combined with an en-hanced duration of suckling at the dam’s udder, impedes habituation to the

artifi-number of leukocytes in the blood of calves performing nnCS may indicate previ-ously experienced stress (Hickey et al., 2003; Kim et al., 2011).

 A marginal supply of fat-soluble vitamins seems to represent another predispos-ing factor for CS (Table 5).

The results indicate that nCS and nnCS should be considered as two forms of CS with a common etiology. While it may be correct to assume that, in respect to nCS, an unsatisfied sucking motivation is the most important factor, the results of this study and other cited experiments demonstrate that hunger as well stress seem to be a decisive factor for nnCS. Thus, nCS and nnCS seem to represent two sides of the same coin. Furthermore, according to corresponding reports of the farmers, almost all calves exhibiting nnCS show nCS as well. This indicates that nnCS may represent a behavioural disorder caused by a more pronounced stress than that causing nCS.

It is concluded that, on farms struggling with CS, changes in the management are a precondition to solving the problem. In particular, the quantity of milk fed should con-sider the natural demand of the calf (i. e., 10 L milk/day), a stocking density ensuring 3 m² in the group pen per calf and a step-down protocol for weaning should be im-plemented. In addition, good colostral supply, plenty of straw bedding, access to wa-ter in the first weeks of life and an appropriate supply of fat-soluble vitamins should be ensured.

Acknowledgements

The authors wish to thank all the farmers who participated in the interviews and all the people who assisted in the organization and implementation of the study. Fur-thermore, the financial support by the Karl Eibl Foundation, Neustadt a. d. Aisch is gratefully acknowledged.