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Age at introduction to the group affects dairy calves’ use of a computer-controlled milk feeder
Applied Animal Behaviour Science 107 (2007) 22–31 www.elsevier.com/locate/applanim
Age at introduction to the group affects dairy calves’ use of a computer-controlled milk feeder Margit Bak Jensen * Department of Animal Health, Welfare and Nutrition, Danish Institute of Agricultural Sciences, Research Centre Foulum, 8830 Tjele, Denmark Accepted 13 September 2006 Available online 13 October 2006
Abstract Use of computer-controlled milk feeders in dairy calves often involves large group sizes and continuous introduction of new calves, which may cause problems due to competition especially for young calves. Ninetyeight dairy calves were paired (one 6-day-old and one 14-day-old) and introduced into an existing group (group size median 18 (interquartile range 16–24)). During the first week calves were guided to the feeder by the stockperson if they had consumed less than half of their daily milk allowance. Number of days guided to the feeder and milk feeder visits were recorded. Due to the dynamic group structure, blocks of calves were created and the dataset was divided into two halves. Each half dataset contained four blocks, and calves in separate blocks were independent within each half dataset. Calves introduced at day 6 had a higher probability of requiring guidance to the feeder than calves introduced at day 14 (P < 0.001). Calves introduced at day 6 spent less time in the milk feeder during days 1–12 after introduction (P < 0.01). This was mainly due to these calves spending less time during unrewarded visits to the milk feeder (P < 0.01). However, calves introduced at day 6 also spent less time in the milk feeder after ingesting milk (P < 0.01), and ingested less milk (P < 0.01). The results show that the younger calves had more difficulties in getting access to the milk feeder during the first period after introduction and required more guidance by the stockperson. It is suggested that 6-day-old calves are less able to compete for milk feeder access in large groups. # 2006 Elsevier B.V. All rights reserved. Keywords: Calves; Computer-controlled milk feeder; Groups housing
1. Introduction Feeding dairy calves by computer-controlled milk feeders often involves a large group size and a dynamic group structure. In large groups there is more competition for access to the milk * Tel.: +45 89 99 13 25. E-mail address: [email protected]. 0168-1591/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2006.09.017
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feeder (Jensen, 2004), and in groups with continuous introduction of new calves, the youngest calves also visit the milk feeder during the night, where other calves are resting (Zerbe and Schlichting, 1993), which could suggest that young calves are less able to compete for milk feeder access. O’Driscoll et al. (2006) reported that newly introduced calves were rarely displaced from the milk feeder and that reduction in milk intake was only seen just after introduction, but the calves in their study were 18 days old when introduced into a group of only four calves. Under commercial conditions the age at introduction to a group with a computercontrolled milk feeder has been found to vary from days 2 to 21 (Beckman, 1993). Computercontrolled milk feeders are intended to need relatively little labour (Kung et al., 1997), but if newly introduced calves do not ingest enough milk, the stockperson needs to spend a considerable amount of time guiding these calves to the milk feeder, ensuring that they ingest milk. The aim of this study was to investigate the effect of dairy calves’ age at introduction to a group on their milk feeder use, milk intake, live weight gain, and required guidance by the stockperson. It is predicted that young calves are less able to get access to the milk feeder for a period of time after introduction than calves introduced at an older age.
2. Material and methods Ninety-eight male and female calves born on the Cattle Research Centre at Foulum were used. The calves were Jersey, Danish Red or Holstein–Friesian calves and the birth weight of the calves was 30 (S.D. 4.3), 44 (S.D. 7.7) and 45 (S.D. 4.6) kg for Jersey, Danish Reds and Holstein–Friesians, respectively. Details on the number of calves of each sex and breed are given in Table 1. The calves were moved to one of two straw-bedded pens (9.30 m 4.35 m), each of which had one milk feeder equipped with one teat and connected to a computer-controlled unit. The calves were introduced into one of two existing dynamic groups in pairs. In each pair one calf was 6 days old and the other calf was 14 (14.8 (1.7)) days old. In both groups the group size was 6 when the first pair of experimental calves was introduced. The group size thus gradually increased over time and the group size median was 18 (interquartile range 16–24). The calves of the heavy breeds (Holstein–Friesian and Danish Red) were offered a milk allowance of 6.4 L/day, while the calves of the Jersey breed were offered 5.2 L/day. During the first 12 days in the group pen, 50 calves received their daily milk allowance in 8 daily milk portions, while 48 calves received the daily milk allowance in 4 daily portions (Table 1). For calves of the heavy breeds the allowance of 6.4 L was
Table 1 The number of calves of each sex and breed, and the number of calves offered four and eight daily milk portions Pen
1 1 1 1 1 2 2 2
Blocka
1 2 3 4 5 6 7 8
Total a
Number of calves
Sex Male
Breed Female
Jersey
Milk portions Danish Red
Holstein–Friesian
4
8
10 8 10 18 10 14 12 16
6 4 3 8 5 9 7 9
4 4 7 10 5 5 5 7
3 1 0 2 8 4 2 2
4 5 5 7 0 7 3 9
3 2 5 9 2 3 7 5
4 4 4 10 4 8 6 8
6 4 6 8 6 6 6 8
98
51
47
22
40
36
48
50
The definition of even and odd numbered blocks is explained in the text.
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M.B. Jensen / Applied Animal Behaviour Science 107 (2007) 22–31
divided into either 4 portions of 1.6 L or 8 portions of 0.8 L. Similarly, for the Jersey calves the 5.2 L was divided into either 4 large portions (1.2 and 1.4 L) or 8 small portions (0.6 and 0.8 L). From day 13 in the group, all the calves were offered the daily milk allowance in four milk portions. The number of milk portions was the minimum number of portions, and if the calves consumed less than their allowed milk per portion, they could get the remaining milk in an additional portion. Each 24-h period was subdivided into two 12-h feeding periods, which started between 0500 and 0900 h, and between 1700 and 2100 h, respectively, and the calves were allocated at random to start their feeding period within this range. Within each 12-h period the calves could consume half their daily milk allowance in either two or four milk portions according to treatment, but at least 30 min had to pass between successive milk portions. All calves were weighed at day 40 of age. Irrespective of the age at introduction all calves spent 6 weeks in the group after which they were moved to a non-experimental pen and weaned. Prior to introduction all calves had been housed with their dam in a large straw-bedded pen for 24-h after birth and then moved to individual straw-bedded calf hutches (1.75 m 1.10 m) with an outdoor yard (1.60 m 1.00 m), where they were fed colostrum on days 2–4 and on day 5 a mixture of whole milk and milk replacer via teat buckets. From day 6 and onwards all milk was milk replacer (Friska Sød, DLG, Denmark, based on defatted milk powder (60%), whey powder (18%) and vegetable fat (18%), and containing 24% CP and 19% fat (145 g replacer per litre water)). All calves could ingest milk from a teat bucket and all calves ingested their full allowance before the time of introduction to the group. Concentrates (Grøn Kalv Valset, DLG, Denmark (18% CP, 4% fat and 6% fibre)) and hay were available to all calves ad libitum from day 2 and throughout the experimental period. 2.1. Data collection and statistical analyses To reduce the dependency between calves in the same dynamic group in the statistical analyses, the dataset was split into two. Blocks of calves introduced in succession were defined within each of the two pens. Even and odd numbered blocks were defined in such a way that the calves in one odd numbered block had all left the pen before the first calves of the next odd numbered block entered the pen. Similarly, the calves in an even block had all left the pen before calves of the next even block entered the pen. Thus, calves in one odd numbered block could be regarded as independent of calves in any other odd numbered block, because calves in different odd numbered blocks were never in the same pen at the same time. Similarly, calves in one even block could be regarded as independent of calves in any other even block. According to this criterion five blocks were defined in pen 1 and three blocks in pen 2 (Table 1). The statistical analyses were performed firstly by analysing the complete dataset and secondly by analysing data for odd and even numbered blocks, respectively. For all variables the effects were considered significant if a P-value less than 0.05 was found based on the analyses of the complete data and if a P-value of 0.10 or less was found for both the analysis of even and the analysis of odd numbered blocks. 2.2. Guidance to the milk feeder During the first week after introduction to the group pen, it was monitored daily if a calf had consumed less than half of its allowance for two successive 12-h feeding periods. The stockperson guided those calves, for which this was the case, to the milk feeder in the morning. The stockperson ensured that the calves found the teat and ensured that no other calves disturbed them for 5 min. The calves were encouraged to ingest milk, but not forced to ingest milk. For each calf the number of days with guidance divided by all 7 days of the first week was calculated. These data were analysed by logistic regression, fitting a binominal distribution and a logit link function (Dobson, 2002). The model included age at introduction (6 or 14), and number of milk portions (4 or 8) as fixed effects, while number of calves in the group was included as a covariate. An initial analysis included breed and sex, but as these effects were found to be non-significant (P > 0.25), they were excluded from the final model. In the model including the complete dataset, pen was included as an additional fixed effect,
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while in the analyses performed for even and for odd numbered blocks separately, block was included as a fixed effect in addition to age at introduction and number of milk portions. 2.3. Milk feeder data During the 6 weeks the calves were in the pen, the computer-controlled milk feeder unit recorded the start and end time of three different types of visits to the feeder: rewarded visits, unrewarded visits and visits where the calf had access to milk, but did not consume any. Rewarded visits, where the calf had milk in the feeder, included time spent ingesting milk (defined by the presence of milk in the mixer bowl) and time spent in the feeder just after ingesting milk (after the mixer bowl had been emptied). Milk intake was recorded for each rewarded visit. For each calf and each day the number and duration of the three types of visits were calculated. Furthermore, the duration of all visits to the feeder, the duration of ingesting milk during rewarded visits, the milk intake and the duration of time spent in the feeder after ingesting milk were calculated for each calf and each day. For each of the variables, an average across the first 12 days, as well as an average across days within week for the last 4 weeks, was calculated for each calf and this was used as input data in the statistical analyses. Some of the variables were transformed by the square root, or by natural logarithm, to meet the requirement of normal distribution of the data, but for those variables that were transformed before analysis, the mean and approximate standard errors were back-transformed and presented on the measured scale. Data were analysed by variance component analysis (Littell et al., 1996). Initially, two analyses were performed; one including the first 12 days (the period where the calves had their milk in either four or eight portions) and one including the last 4 weeks in the pen. However, in the analysis of the last 4 weeks in the pen no interaction between age at introduction and week, or number of milk portions and week was found for any of the variables. Therefore, 1 week was chosen for analysis. The fourth week (days 22–28 after introduction) was chosen to represent a period in the middle of the milkfeeding period. Regarding the first 12 days after introduction, the statistical model included age at introduction (6 or 14), number of daily milk portions (4 or 8), age at introduction number of milk portions, breed, sex, age at introduction breed, age at introduction sex, number of milk portions breed and number of milk portions sex as fixed effects. For the fourth week after introduction the statistical model included as fixed effects age at introduction (6 or 14), breed, sex, age at introduction breed, and age at introduction sex. The final analyses were performed first for the complete dataset, and pair, pen and group size were included as random effects. Random effects, which were estimated to zero, were removed from the model. The analyses of ‘all visits’, ‘with access no milk ingested’ and ‘milk intake’ included all random effects. In the analysis of ‘unrewarded visits’ and frequency of ‘rewarded visits’ the random effect of pen was estimated to zero and excluded, while in the analysis of ‘after ingesting milk’ the random effect of pair, pen and group size were estimated to zero and excluded. For those variables where significant effects were found in the analysis of the complete dataset, analyses were subsequently repeated for odd numbered and for even blocks, respectively, and pair, block and group size were included as random effects. Also here random effects, which were estimated to zero, were excluded. Because Satterthwaite’s approximation was used to calculate the denominator degrees of freedom (Littell et al., 1996), and because random effects estimated to zero were removed from the analyses, the degrees of freedom may vary between the variables (see Tables 2 and 3). 2.4. Live weight Data for live weight on day 40 of age were analysed by variance component analysis including effects of age at introduction (6 or 14), number of daily milk portions (4 or 8), age at introduction number of milk portions, breed, sex, age at introduction breed, age at introduction sex, number of milk portions breed, and number of milk portions sex as fixed effects, and pair and pen were included as random effects. Satterthwaite’s approximation was used to calculate the denominator degrees of freedom (Littell et al., 1996).
26
Age at introduction 6
Complete dataset 14
Odd numbered blocks
Even blocks
F-value
P
F-value
P
F-value
P
Mean
S.E.
Mean
S.E.
Duration (min/24 h) All visits
57.8
3.00
66.8
3.23
F1,44 = 8.40
<0.01
F1,17 = 3.10
<0.10
F1,42 = 8.84
<0.01
Rewarded visits Ingesting milk After ingesting milk
19.9 12.8 6.0
0.92 0.56 0.57
21.4 13.1 7.5
0.95 0.57 0.64
– – F1,84 = 6.69
NS NS <0.01
F1,29 = 5.61
<0.05
F1,43 = 9.73
<0.01
Unrewarded visits With access, no milk ingested
33.4 2.5
2.79 0.25
43.2 2.00
3.17 0.20
F1,45 = 10.7 F1,41 = 5.93
<0.01 <0.05
F1,17 = 4.00 F1,17 = 4.93
<0.10 <0.05
F1,21 = 7.29 –
<0.01 NS
F1,17 = 4.66
<0.05
F1,20 = 10.56
<0.01
Frequency (number/24 h) Rewarded visits
5.4
0.23
5.6
0.24
–
NS
Milk ingested (L/24 h)
5.0
0.13
5.4
0.13
F1,44 = 10.43
<0.01
Estimates for analysis including the complete dataset are given.
M.B. Jensen / Applied Animal Behaviour Science 107 (2007) 22–31
Table 2 The effect of age at introduction on use of milk feeder during the first 12 days after introduction
Number of milk portions
Complete dataset
4
F-value
Mean
8 S.E.
Mean
P
Odd numbered blocks
Even blocks
F-value
P
F-value
P
S.E.
Duration (min/24 h) All visits
60.6
3.21
63.9
3.30
–
NS
Rewarded visits Ingesting milk After ingesting milk
19.9 13.1 5.7
1.15 0.91 0.57
21.4 12.8 7.9
1.19 0.90 0.67
– – F1,84 = 13.05
NS NS <0.001
F1,29 = 6.10
<0.05
F1,43 = 8.89
<0.01
Unrewarded visits With access, no milk ingested
36.5 2.5
3.04 0.25
39.8 2.0
3.18 0.20
– F1,52 = 4.46
NS <0.05
F1,16 = 5.86
<0.05
F1,22 = 3.55
<0.10
Frequency (number/24 h) Rewarded visits
4.6
0.26
6.4
0.30
F1,50 = 41.65
<0.001
F1,18 = 25.98
<0.001
F1,24 = 10.57
<0.05
Milk ingested (L/24 h)
5.3
0.19
5.1
0.19
–
NS
Estimates for analysis including the complete dataset are given.
M.B. Jensen / Applied Animal Behaviour Science 107 (2007) 22–31
Table 3 The effect of number of milk portions on use of milk feeder during the first 12 days after introduction
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3. Results 3.1. Guidance to the milk feeder The calves introduced at day 6 were guided to the feeder on 2 (1–4) days (median and interquartile range), whereas the calves introduced at day 14 were guided to the feeder on 1 (0–2) days (median and interquartile range) during the first week in the group pen. The number of calves in the two age groups guided to the feeder for 0–7 days during the first week after introduction is shown in Fig. 1. The analysis of the complete data set showed that the calves introduced on day 6 had a greater probability of requiring guidance than the calves introduced on day 14 (Odds ratio 2.34 (95% confidence limits: 1.58–3.42); x21 ¼ 13:04; P < 0.001), also for odd numbered blocks (Odds ratio 3.67 (95% confidence limits: 1.97–6.75); x21 ¼ 8:52; P < 0.01), and for even blocks (Odds ratio 2.08 (95% confidence limits: 1.20–3.60); x21 ¼ 5:63; P < 0.05). There was no effect of the number of milk portions on the number of calves requiring guidance to the milk feeder. The analysis of the complete dataset showed that the larger the group size, the greater the probability of the calves requiring guidance to the milk feeder (Odds ratio 1.09 (95% confidence limits: 1.03–1.16); x21 ¼ 6:40; P < 0.01). However, the analyses of odd and even blocks separately showed no effect of group size in odd numbered blocks (Odds ratio 1.24 (95% confidence limits: 0.99–1.56); x21 ¼ 1:26; NS), and only a tendency in even blocks (Odds ratio 1.20 (95% confidence limits: 1.09–1.33); x21 ¼ 3:21; P < 0.10), and therefore this effect is not considered further. 3.2. Milk feeder data during days 1–12 after introduction to the group The effects of age at introduction on the use of the milk feeder during the first 12 days are shown in Table 2. The calves introduced at day 6 spent less time in the milk feeder. This was mainly due to these calves spending less time during unrewarded visits, but these calves also spent less time during rewarded visits after ingesting milk. There was no effect of age at introduction on the time spent ingesting milk, but the calves introduced at day 6 ingested less milk.
Fig. 1. Number of calves guided to the milk feeder for 0–7 days during the first week after introduction to the pen in calves introduced at day 6 (&) and calves introduced at day 14 (&).
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The effects of the number of milk portions on the use of the milk feeder during the first 12 days are shown in Table 3. As imposed by treatment, the number of rewarded visits was lower for the calves offered the milk allowance in four daily portions. The calves offered their milk allowance in four daily portions spent less time in the milk feeder after ingesting milk than the calves offered their milk allowance in 8 daily portions. They also spent more time in the feeder with access, but without ingesting any milk (Table 3). 3.3. Milk feeder data for the fourth week after introduction No effects of age at introduction were found for any of the variables. The calves of the Jersey breed spent more time in the milk feeder after ingesting milk (8.3 (0.90), 5.8 (0.56) and 5.8 (0.61) min for Jersey, Danish Reds, and Holstein–Friesians, respectively; F 2,81 = 4.74; P < 0.01); odd numbered (P < 0.10) and even blocks (P < 0.10). 3.4. Live weight There was no effect of age at introduction, or number of milk portions during days 1–12, on the live weight on day 40 of age. However, the male calves were heavier than the female calves (63 (S.E. 1.0) kg versus 56 (S.E. 1.2) kg; F 1,84 = 19.14; P < 0.001) and the calves of the heavy breeds were heavier than the Jersey calves (46 (S.E. 1.6), 66 (S.E. 1.2) and 67 (S.E. 1.2) kg for Jersey, Danish Reds, and Holstein–Friesians, respectively; F 2,73 = 58.05; P < 0.001). 4. Discussion During the first week in the group more calves introduced at day 6 failed to ingest half their milk allowance and required guidance to the feeder by the stockperson than those introduced at day 14. When guided by the stockperson, the calves were ensured 5 min of undisturbed feeder access. Although they were helped in this way, the calves introduced at day 6 ingested less milk during days 1–12 after introduction than the calves introduced at day 14. All calves ingested their allowance before introduction to the group pen, and it is unlikely that the difference is due to the young calves not being able to ingest their allowance. The calves introduced at day 6 also spent less time in the computer-controlled milk feeder during the first 12 days. The main reason for the lower time spent in the feeder by the calves introduced at day 6 was a lesser duration of unrewarded visits. Unrewarded visits have been related to low milk allowances and are suggested to be partly motivated by hunger (Jensen and Holm, 2003; Jensen, 2006). However, unrewarded visits may also be due to inexperience in predicting when milk is available, as unrewarded visits were seen more often during the first few weeks after introduction to the feeder (Jensen and Holm, 2003). Both age groups were equally inexperienced and the difference in the present experiment may be due to the younger calves having more difficulties in competing for access to the feeder. Calves introduced at day 6 also spent less time during rewarded visits after they had finished ingesting milk. Non-nutritive sucking typically occurs after the ingestion of milk (Rushen and De Passille´, 1995) and time spent in the milk feeder after the available milk has been ingested is likely to be spent on non-nutritive sucking. Rasmussen et al. (2006) recorded the behaviour of a subset of the calves included in this study and showed that sucking the empty teat in the milk feeder after ingesting a milk portion was less in the calves introduced at day 6. During the first 12 days in the group, the calves introduced on day 6 may have been displaced more often before they
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had finished their milk portion, or before they had had the opportunity to perform non-nutritive sucking after finishing the milk meal. However, Rasmussen et al. (2006) did not find calves introduced at day 6 to be displaced more. An alternative explanation may be that the stockperson ensured that the calves could ingest their milk, but the stockperson did not ensure that they performed non-nutritive sucking after ingesting the milk for as long as they liked to. Displacement from the milk feeder can be reduced by fitting a closing gate on the feeder in groups of 15 calves (Weber and Wechsler, 2001), but it is important to note that the gate increases the occupancy of the feeder (Wendl et al., 1998) and that using a gate with more calves per feeder may mean that it becomes even more difficult for weak calves to get access. Used in small groups the gate has the advantage that the calves are not disturbed during milk intake, but gates may require extra guidance of calves to learn to use the feeder. Introducing the calves at the age of 2 weeks appeared to be a better strategy than introducing them before 1 week old. Increasing the introduction age means that calves have to be fed manually for longer before introduction. It could be argued that the more manual milk feeding requires as much labour as guiding the calves to the computer-controlled milk feeder. However, before introduction all the calves drank from teat buckets without any aid and all calves ingested their allowance of 6.4 or 5.2 L/day according to breed. Introducing calves at a later age has the advantage that it becomes possible under production conditions to house calves in pairs, or small subgroups, before introduction and thus to introduce them with familiar calves of their own age. Calves can distinguish familiar from unfamiliar calves after only 3 weeks of grouping (Færevik et al., 2006), and possibly social bonds form even sooner. How introduction in subgroups affects milk feeder use has not been investigated, but the calves of the same age that were introduced together rested more together (Søfteland, 1999) and introduction of subgroups rather than individuals may improve the social environment. Number of milk portions affected the time spent in the feeder after ingesting milk. The calves offered four milk portions during days 1–12 after introduction spent less time in the milk feeder after ingesting milk. Rasmussen et al. (2006) found that less time was spent sucking on the empty teat in the calves offered four daily milk portions compared to eight, and again, the correspondence between the behaviour observed and the variable recorded by the milk feeder supports that the time in the feeder after ingesting milk was spent on non-nutritive sucking. It was expected that a higher number of milk portions during the first period after introduction would facilitate learning to use the feeder, however, this was not the case. During the fourth week in the group no effects of age at introduction were found on the use of the computer-controlled milk feeder, which illustrates that the problems of the young calves do not persist throughout the milk-feeding period. 5. Conclusion When calves were introduced to a large group with computer-controlled milk feeders on day 6, they required more guidance to the feeder by the stockperson, they spent less time in the feeder and they ingested less milk compared to calves introduced on day 14. This suggests that 6-dayold calves are less able to compete for milk feeder access in large groups. Acknowledgements This study was recommended by the Nordic Joint Committee for Agricultural Research and supported by The Danish Ministry of Food, Agriculture and Fisheries and The Danish Livestock
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Federation. Thanks to Jane Eriksen, Helle Møller Nielsen, Erik L. Decker and John Misa Obidah for assistance. References Beckman, S., 1993. Computer-controlled milk feeders—a questionaire on expirences with calf rearing (Transponderstyrda kalvammor-en intervjuunderso¨kning om erfarenheter vid uppfo¨dning av kalvar). Examwork no. 54. Swedish University of Agricultural Sciences, Department of Animal Nutrition and Management, 40 pp. Dobson, A., 2002. An introduction to generalised linear models. In: Texts in Statistical Science, Chapman and Hall/CRC, Boca Raton, USA, 225 pp. Færevik, G., Jensen, M.B., Bøe, K.E., 2006. Dairy calves social preferences and the significance of a companion during separation from the group. Appl. Anim. Behav. Sci. 99, 205–221. Jensen, M.B., 2006. Computer-controlled milk feeding of group-housed calves, the effect of milk allowance and weaning type. J. Dairy Sci. 89, 201–206. Jensen, M.B., 2004. Computer controlled milk feeding of dairy calves: the effects of number of calves per feeder and number of milk portions on use of feeder and social behavior. J. Dairy Sci. 87, 3428–3438. Jensen, M.B., Holm, L., 2003. The effect of milk flow rate and milk allowance on feeding related behaviour in dairy calves fed by computer controlled milk feeders. Appl. Anim. Behav. Sci. 82, 87–100. Kung Jr., L., Demarco, S., Siebenson, L.N., Joyner, E., Haenlein, G.F.W., Morris, R.M., 1997. Evaluation of two management systems for rearing calves fed milk replacer. J. Dairy Sci. 80, 2529–2533. Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., 1996. SAS1 system for mixed models. SAS Inst., Cary, NC, USA, pp. 1–134. O’Driscoll, K., Keyserlingk, M.A.G., Weary, D.M., 2006. Effects of mixing on drinking and competitive behavior of dairy calves. J. Dairy Sci. 89, 229–233. Rasmussen, L., Jensen, M.B., Jeppesen, L.L., 2006. The effect of age at introduction and number of milk-portions on calves responses to integration into a dynamic group of dairy calves fed by computer controlled milk feeder. Appl. Anim. Behav. Sci. 100, 153–163. Rushen, J., De Passille´, A.M., 1995. The motivation of non-nutritive sucking in calves, Bos taurus. Anim. Behav. 49, 1503–1510. Søfteland, T., 1999. How do young calves react when they are introduced into a dynamic group? (Hvordan pa˚virkes unge ˚ s, 67 kalve na˚r de bliver introdusert in en dynamisk gruppe?) M.Sc. Thesis at the Norwegian Agricultural University, A pp. Weber, R., Wechsler, B., 2001. Reduction in cross-sucking in calves by the use of a modified automatic teat feeder. Appl. Anim. Behav. Sci. 72, 215–223. Wendl, G., Schuch, S., Callian, B., Wendling, F., 1998. Besaugen Verhu¨ten. Ein verschliessbarer Tra¨nkestand fu¨r Ka¨lbertra¨nkeautomaten zur Verringerung des gegenseitigen Besaugens. Jahrgang Landtechnik 4, 264–265. Zerbe, F., Schlichting, M.C., 1993. Drinking behaviour and activities of rearing calves kept in groups and fed with an automatic-milk-feeder. In: Nichelmann, M., Wierenga, H.K., Braun, S. (Eds.), Proceedings of the International Congress on Applied Ethology Berlin 3rd Joint Meeting, Humboldt Universitat, Berlin, Germany, pp. 483–486.
Age at introduction to the group affects dairy calves’ use of a computer-controlled milk feeder Margit Bak Jensen * Department of Animal Health, Welfare and Nutrition, Danish Institute of Agricultural Sciences, Research Centre Foulum, 8830 Tjele, Denmark Accepted 13 September 2006 Available online 13 October 2006
Abstract Use of computer-controlled milk feeders in dairy calves often involves large group sizes and continuous introduction of new calves, which may cause problems due to competition especially for young calves. Ninetyeight dairy calves were paired (one 6-day-old and one 14-day-old) and introduced into an existing group (group size median 18 (interquartile range 16–24)). During the first week calves were guided to the feeder by the stockperson if they had consumed less than half of their daily milk allowance. Number of days guided to the feeder and milk feeder visits were recorded. Due to the dynamic group structure, blocks of calves were created and the dataset was divided into two halves. Each half dataset contained four blocks, and calves in separate blocks were independent within each half dataset. Calves introduced at day 6 had a higher probability of requiring guidance to the feeder than calves introduced at day 14 (P < 0.001). Calves introduced at day 6 spent less time in the milk feeder during days 1–12 after introduction (P < 0.01). This was mainly due to these calves spending less time during unrewarded visits to the milk feeder (P < 0.01). However, calves introduced at day 6 also spent less time in the milk feeder after ingesting milk (P < 0.01), and ingested less milk (P < 0.01). The results show that the younger calves had more difficulties in getting access to the milk feeder during the first period after introduction and required more guidance by the stockperson. It is suggested that 6-day-old calves are less able to compete for milk feeder access in large groups. # 2006 Elsevier B.V. All rights reserved. Keywords: Calves; Computer-controlled milk feeder; Groups housing
1. Introduction Feeding dairy calves by computer-controlled milk feeders often involves a large group size and a dynamic group structure. In large groups there is more competition for access to the milk * Tel.: +45 89 99 13 25. E-mail address: [email protected]. 0168-1591/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2006.09.017
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feeder (Jensen, 2004), and in groups with continuous introduction of new calves, the youngest calves also visit the milk feeder during the night, where other calves are resting (Zerbe and Schlichting, 1993), which could suggest that young calves are less able to compete for milk feeder access. O’Driscoll et al. (2006) reported that newly introduced calves were rarely displaced from the milk feeder and that reduction in milk intake was only seen just after introduction, but the calves in their study were 18 days old when introduced into a group of only four calves. Under commercial conditions the age at introduction to a group with a computercontrolled milk feeder has been found to vary from days 2 to 21 (Beckman, 1993). Computercontrolled milk feeders are intended to need relatively little labour (Kung et al., 1997), but if newly introduced calves do not ingest enough milk, the stockperson needs to spend a considerable amount of time guiding these calves to the milk feeder, ensuring that they ingest milk. The aim of this study was to investigate the effect of dairy calves’ age at introduction to a group on their milk feeder use, milk intake, live weight gain, and required guidance by the stockperson. It is predicted that young calves are less able to get access to the milk feeder for a period of time after introduction than calves introduced at an older age.
2. Material and methods Ninety-eight male and female calves born on the Cattle Research Centre at Foulum were used. The calves were Jersey, Danish Red or Holstein–Friesian calves and the birth weight of the calves was 30 (S.D. 4.3), 44 (S.D. 7.7) and 45 (S.D. 4.6) kg for Jersey, Danish Reds and Holstein–Friesians, respectively. Details on the number of calves of each sex and breed are given in Table 1. The calves were moved to one of two straw-bedded pens (9.30 m 4.35 m), each of which had one milk feeder equipped with one teat and connected to a computer-controlled unit. The calves were introduced into one of two existing dynamic groups in pairs. In each pair one calf was 6 days old and the other calf was 14 (14.8 (1.7)) days old. In both groups the group size was 6 when the first pair of experimental calves was introduced. The group size thus gradually increased over time and the group size median was 18 (interquartile range 16–24). The calves of the heavy breeds (Holstein–Friesian and Danish Red) were offered a milk allowance of 6.4 L/day, while the calves of the Jersey breed were offered 5.2 L/day. During the first 12 days in the group pen, 50 calves received their daily milk allowance in 8 daily milk portions, while 48 calves received the daily milk allowance in 4 daily portions (Table 1). For calves of the heavy breeds the allowance of 6.4 L was
Table 1 The number of calves of each sex and breed, and the number of calves offered four and eight daily milk portions Pen
1 1 1 1 1 2 2 2
Blocka
1 2 3 4 5 6 7 8
Total a
Number of calves
Sex Male
Breed Female
Jersey
Milk portions Danish Red
Holstein–Friesian
4
8
10 8 10 18 10 14 12 16
6 4 3 8 5 9 7 9
4 4 7 10 5 5 5 7
3 1 0 2 8 4 2 2
4 5 5 7 0 7 3 9
3 2 5 9 2 3 7 5
4 4 4 10 4 8 6 8
6 4 6 8 6 6 6 8
98
51
47
22
40
36
48
50
The definition of even and odd numbered blocks is explained in the text.
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M.B. Jensen / Applied Animal Behaviour Science 107 (2007) 22–31
divided into either 4 portions of 1.6 L or 8 portions of 0.8 L. Similarly, for the Jersey calves the 5.2 L was divided into either 4 large portions (1.2 and 1.4 L) or 8 small portions (0.6 and 0.8 L). From day 13 in the group, all the calves were offered the daily milk allowance in four milk portions. The number of milk portions was the minimum number of portions, and if the calves consumed less than their allowed milk per portion, they could get the remaining milk in an additional portion. Each 24-h period was subdivided into two 12-h feeding periods, which started between 0500 and 0900 h, and between 1700 and 2100 h, respectively, and the calves were allocated at random to start their feeding period within this range. Within each 12-h period the calves could consume half their daily milk allowance in either two or four milk portions according to treatment, but at least 30 min had to pass between successive milk portions. All calves were weighed at day 40 of age. Irrespective of the age at introduction all calves spent 6 weeks in the group after which they were moved to a non-experimental pen and weaned. Prior to introduction all calves had been housed with their dam in a large straw-bedded pen for 24-h after birth and then moved to individual straw-bedded calf hutches (1.75 m 1.10 m) with an outdoor yard (1.60 m 1.00 m), where they were fed colostrum on days 2–4 and on day 5 a mixture of whole milk and milk replacer via teat buckets. From day 6 and onwards all milk was milk replacer (Friska Sød, DLG, Denmark, based on defatted milk powder (60%), whey powder (18%) and vegetable fat (18%), and containing 24% CP and 19% fat (145 g replacer per litre water)). All calves could ingest milk from a teat bucket and all calves ingested their full allowance before the time of introduction to the group. Concentrates (Grøn Kalv Valset, DLG, Denmark (18% CP, 4% fat and 6% fibre)) and hay were available to all calves ad libitum from day 2 and throughout the experimental period. 2.1. Data collection and statistical analyses To reduce the dependency between calves in the same dynamic group in the statistical analyses, the dataset was split into two. Blocks of calves introduced in succession were defined within each of the two pens. Even and odd numbered blocks were defined in such a way that the calves in one odd numbered block had all left the pen before the first calves of the next odd numbered block entered the pen. Similarly, the calves in an even block had all left the pen before calves of the next even block entered the pen. Thus, calves in one odd numbered block could be regarded as independent of calves in any other odd numbered block, because calves in different odd numbered blocks were never in the same pen at the same time. Similarly, calves in one even block could be regarded as independent of calves in any other even block. According to this criterion five blocks were defined in pen 1 and three blocks in pen 2 (Table 1). The statistical analyses were performed firstly by analysing the complete dataset and secondly by analysing data for odd and even numbered blocks, respectively. For all variables the effects were considered significant if a P-value less than 0.05 was found based on the analyses of the complete data and if a P-value of 0.10 or less was found for both the analysis of even and the analysis of odd numbered blocks. 2.2. Guidance to the milk feeder During the first week after introduction to the group pen, it was monitored daily if a calf had consumed less than half of its allowance for two successive 12-h feeding periods. The stockperson guided those calves, for which this was the case, to the milk feeder in the morning. The stockperson ensured that the calves found the teat and ensured that no other calves disturbed them for 5 min. The calves were encouraged to ingest milk, but not forced to ingest milk. For each calf the number of days with guidance divided by all 7 days of the first week was calculated. These data were analysed by logistic regression, fitting a binominal distribution and a logit link function (Dobson, 2002). The model included age at introduction (6 or 14), and number of milk portions (4 or 8) as fixed effects, while number of calves in the group was included as a covariate. An initial analysis included breed and sex, but as these effects were found to be non-significant (P > 0.25), they were excluded from the final model. In the model including the complete dataset, pen was included as an additional fixed effect,
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while in the analyses performed for even and for odd numbered blocks separately, block was included as a fixed effect in addition to age at introduction and number of milk portions. 2.3. Milk feeder data During the 6 weeks the calves were in the pen, the computer-controlled milk feeder unit recorded the start and end time of three different types of visits to the feeder: rewarded visits, unrewarded visits and visits where the calf had access to milk, but did not consume any. Rewarded visits, where the calf had milk in the feeder, included time spent ingesting milk (defined by the presence of milk in the mixer bowl) and time spent in the feeder just after ingesting milk (after the mixer bowl had been emptied). Milk intake was recorded for each rewarded visit. For each calf and each day the number and duration of the three types of visits were calculated. Furthermore, the duration of all visits to the feeder, the duration of ingesting milk during rewarded visits, the milk intake and the duration of time spent in the feeder after ingesting milk were calculated for each calf and each day. For each of the variables, an average across the first 12 days, as well as an average across days within week for the last 4 weeks, was calculated for each calf and this was used as input data in the statistical analyses. Some of the variables were transformed by the square root, or by natural logarithm, to meet the requirement of normal distribution of the data, but for those variables that were transformed before analysis, the mean and approximate standard errors were back-transformed and presented on the measured scale. Data were analysed by variance component analysis (Littell et al., 1996). Initially, two analyses were performed; one including the first 12 days (the period where the calves had their milk in either four or eight portions) and one including the last 4 weeks in the pen. However, in the analysis of the last 4 weeks in the pen no interaction between age at introduction and week, or number of milk portions and week was found for any of the variables. Therefore, 1 week was chosen for analysis. The fourth week (days 22–28 after introduction) was chosen to represent a period in the middle of the milkfeeding period. Regarding the first 12 days after introduction, the statistical model included age at introduction (6 or 14), number of daily milk portions (4 or 8), age at introduction number of milk portions, breed, sex, age at introduction breed, age at introduction sex, number of milk portions breed and number of milk portions sex as fixed effects. For the fourth week after introduction the statistical model included as fixed effects age at introduction (6 or 14), breed, sex, age at introduction breed, and age at introduction sex. The final analyses were performed first for the complete dataset, and pair, pen and group size were included as random effects. Random effects, which were estimated to zero, were removed from the model. The analyses of ‘all visits’, ‘with access no milk ingested’ and ‘milk intake’ included all random effects. In the analysis of ‘unrewarded visits’ and frequency of ‘rewarded visits’ the random effect of pen was estimated to zero and excluded, while in the analysis of ‘after ingesting milk’ the random effect of pair, pen and group size were estimated to zero and excluded. For those variables where significant effects were found in the analysis of the complete dataset, analyses were subsequently repeated for odd numbered and for even blocks, respectively, and pair, block and group size were included as random effects. Also here random effects, which were estimated to zero, were excluded. Because Satterthwaite’s approximation was used to calculate the denominator degrees of freedom (Littell et al., 1996), and because random effects estimated to zero were removed from the analyses, the degrees of freedom may vary between the variables (see Tables 2 and 3). 2.4. Live weight Data for live weight on day 40 of age were analysed by variance component analysis including effects of age at introduction (6 or 14), number of daily milk portions (4 or 8), age at introduction number of milk portions, breed, sex, age at introduction breed, age at introduction sex, number of milk portions breed, and number of milk portions sex as fixed effects, and pair and pen were included as random effects. Satterthwaite’s approximation was used to calculate the denominator degrees of freedom (Littell et al., 1996).
26
Age at introduction 6
Complete dataset 14
Odd numbered blocks
Even blocks
F-value
P
F-value
P
F-value
P
Mean
S.E.
Mean
S.E.
Duration (min/24 h) All visits
57.8
3.00
66.8
3.23
F1,44 = 8.40
<0.01
F1,17 = 3.10
<0.10
F1,42 = 8.84
<0.01
Rewarded visits Ingesting milk After ingesting milk
19.9 12.8 6.0
0.92 0.56 0.57
21.4 13.1 7.5
0.95 0.57 0.64
– – F1,84 = 6.69
NS NS <0.01
F1,29 = 5.61
<0.05
F1,43 = 9.73
<0.01
Unrewarded visits With access, no milk ingested
33.4 2.5
2.79 0.25
43.2 2.00
3.17 0.20
F1,45 = 10.7 F1,41 = 5.93
<0.01 <0.05
F1,17 = 4.00 F1,17 = 4.93
<0.10 <0.05
F1,21 = 7.29 –
<0.01 NS
F1,17 = 4.66
<0.05
F1,20 = 10.56
<0.01
Frequency (number/24 h) Rewarded visits
5.4
0.23
5.6
0.24
–
NS
Milk ingested (L/24 h)
5.0
0.13
5.4
0.13
F1,44 = 10.43
<0.01
Estimates for analysis including the complete dataset are given.
M.B. Jensen / Applied Animal Behaviour Science 107 (2007) 22–31
Table 2 The effect of age at introduction on use of milk feeder during the first 12 days after introduction
Number of milk portions
Complete dataset
4
F-value
Mean
8 S.E.
Mean
P
Odd numbered blocks
Even blocks
F-value
P
F-value
P
S.E.
Duration (min/24 h) All visits
60.6
3.21
63.9
3.30
–
NS
Rewarded visits Ingesting milk After ingesting milk
19.9 13.1 5.7
1.15 0.91 0.57
21.4 12.8 7.9
1.19 0.90 0.67
– – F1,84 = 13.05
NS NS <0.001
F1,29 = 6.10
<0.05
F1,43 = 8.89
<0.01
Unrewarded visits With access, no milk ingested
36.5 2.5
3.04 0.25
39.8 2.0
3.18 0.20
– F1,52 = 4.46
NS <0.05
F1,16 = 5.86
<0.05
F1,22 = 3.55
<0.10
Frequency (number/24 h) Rewarded visits
4.6
0.26
6.4
0.30
F1,50 = 41.65
<0.001
F1,18 = 25.98
<0.001
F1,24 = 10.57
<0.05
Milk ingested (L/24 h)
5.3
0.19
5.1
0.19
–
NS
Estimates for analysis including the complete dataset are given.
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Table 3 The effect of number of milk portions on use of milk feeder during the first 12 days after introduction
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3. Results 3.1. Guidance to the milk feeder The calves introduced at day 6 were guided to the feeder on 2 (1–4) days (median and interquartile range), whereas the calves introduced at day 14 were guided to the feeder on 1 (0–2) days (median and interquartile range) during the first week in the group pen. The number of calves in the two age groups guided to the feeder for 0–7 days during the first week after introduction is shown in Fig. 1. The analysis of the complete data set showed that the calves introduced on day 6 had a greater probability of requiring guidance than the calves introduced on day 14 (Odds ratio 2.34 (95% confidence limits: 1.58–3.42); x21 ¼ 13:04; P < 0.001), also for odd numbered blocks (Odds ratio 3.67 (95% confidence limits: 1.97–6.75); x21 ¼ 8:52; P < 0.01), and for even blocks (Odds ratio 2.08 (95% confidence limits: 1.20–3.60); x21 ¼ 5:63; P < 0.05). There was no effect of the number of milk portions on the number of calves requiring guidance to the milk feeder. The analysis of the complete dataset showed that the larger the group size, the greater the probability of the calves requiring guidance to the milk feeder (Odds ratio 1.09 (95% confidence limits: 1.03–1.16); x21 ¼ 6:40; P < 0.01). However, the analyses of odd and even blocks separately showed no effect of group size in odd numbered blocks (Odds ratio 1.24 (95% confidence limits: 0.99–1.56); x21 ¼ 1:26; NS), and only a tendency in even blocks (Odds ratio 1.20 (95% confidence limits: 1.09–1.33); x21 ¼ 3:21; P < 0.10), and therefore this effect is not considered further. 3.2. Milk feeder data during days 1–12 after introduction to the group The effects of age at introduction on the use of the milk feeder during the first 12 days are shown in Table 2. The calves introduced at day 6 spent less time in the milk feeder. This was mainly due to these calves spending less time during unrewarded visits, but these calves also spent less time during rewarded visits after ingesting milk. There was no effect of age at introduction on the time spent ingesting milk, but the calves introduced at day 6 ingested less milk.
Fig. 1. Number of calves guided to the milk feeder for 0–7 days during the first week after introduction to the pen in calves introduced at day 6 (&) and calves introduced at day 14 (&).
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The effects of the number of milk portions on the use of the milk feeder during the first 12 days are shown in Table 3. As imposed by treatment, the number of rewarded visits was lower for the calves offered the milk allowance in four daily portions. The calves offered their milk allowance in four daily portions spent less time in the milk feeder after ingesting milk than the calves offered their milk allowance in 8 daily portions. They also spent more time in the feeder with access, but without ingesting any milk (Table 3). 3.3. Milk feeder data for the fourth week after introduction No effects of age at introduction were found for any of the variables. The calves of the Jersey breed spent more time in the milk feeder after ingesting milk (8.3 (0.90), 5.8 (0.56) and 5.8 (0.61) min for Jersey, Danish Reds, and Holstein–Friesians, respectively; F 2,81 = 4.74; P < 0.01); odd numbered (P < 0.10) and even blocks (P < 0.10). 3.4. Live weight There was no effect of age at introduction, or number of milk portions during days 1–12, on the live weight on day 40 of age. However, the male calves were heavier than the female calves (63 (S.E. 1.0) kg versus 56 (S.E. 1.2) kg; F 1,84 = 19.14; P < 0.001) and the calves of the heavy breeds were heavier than the Jersey calves (46 (S.E. 1.6), 66 (S.E. 1.2) and 67 (S.E. 1.2) kg for Jersey, Danish Reds, and Holstein–Friesians, respectively; F 2,73 = 58.05; P < 0.001). 4. Discussion During the first week in the group more calves introduced at day 6 failed to ingest half their milk allowance and required guidance to the feeder by the stockperson than those introduced at day 14. When guided by the stockperson, the calves were ensured 5 min of undisturbed feeder access. Although they were helped in this way, the calves introduced at day 6 ingested less milk during days 1–12 after introduction than the calves introduced at day 14. All calves ingested their allowance before introduction to the group pen, and it is unlikely that the difference is due to the young calves not being able to ingest their allowance. The calves introduced at day 6 also spent less time in the computer-controlled milk feeder during the first 12 days. The main reason for the lower time spent in the feeder by the calves introduced at day 6 was a lesser duration of unrewarded visits. Unrewarded visits have been related to low milk allowances and are suggested to be partly motivated by hunger (Jensen and Holm, 2003; Jensen, 2006). However, unrewarded visits may also be due to inexperience in predicting when milk is available, as unrewarded visits were seen more often during the first few weeks after introduction to the feeder (Jensen and Holm, 2003). Both age groups were equally inexperienced and the difference in the present experiment may be due to the younger calves having more difficulties in competing for access to the feeder. Calves introduced at day 6 also spent less time during rewarded visits after they had finished ingesting milk. Non-nutritive sucking typically occurs after the ingestion of milk (Rushen and De Passille´, 1995) and time spent in the milk feeder after the available milk has been ingested is likely to be spent on non-nutritive sucking. Rasmussen et al. (2006) recorded the behaviour of a subset of the calves included in this study and showed that sucking the empty teat in the milk feeder after ingesting a milk portion was less in the calves introduced at day 6. During the first 12 days in the group, the calves introduced on day 6 may have been displaced more often before they
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had finished their milk portion, or before they had had the opportunity to perform non-nutritive sucking after finishing the milk meal. However, Rasmussen et al. (2006) did not find calves introduced at day 6 to be displaced more. An alternative explanation may be that the stockperson ensured that the calves could ingest their milk, but the stockperson did not ensure that they performed non-nutritive sucking after ingesting the milk for as long as they liked to. Displacement from the milk feeder can be reduced by fitting a closing gate on the feeder in groups of 15 calves (Weber and Wechsler, 2001), but it is important to note that the gate increases the occupancy of the feeder (Wendl et al., 1998) and that using a gate with more calves per feeder may mean that it becomes even more difficult for weak calves to get access. Used in small groups the gate has the advantage that the calves are not disturbed during milk intake, but gates may require extra guidance of calves to learn to use the feeder. Introducing the calves at the age of 2 weeks appeared to be a better strategy than introducing them before 1 week old. Increasing the introduction age means that calves have to be fed manually for longer before introduction. It could be argued that the more manual milk feeding requires as much labour as guiding the calves to the computer-controlled milk feeder. However, before introduction all the calves drank from teat buckets without any aid and all calves ingested their allowance of 6.4 or 5.2 L/day according to breed. Introducing calves at a later age has the advantage that it becomes possible under production conditions to house calves in pairs, or small subgroups, before introduction and thus to introduce them with familiar calves of their own age. Calves can distinguish familiar from unfamiliar calves after only 3 weeks of grouping (Færevik et al., 2006), and possibly social bonds form even sooner. How introduction in subgroups affects milk feeder use has not been investigated, but the calves of the same age that were introduced together rested more together (Søfteland, 1999) and introduction of subgroups rather than individuals may improve the social environment. Number of milk portions affected the time spent in the feeder after ingesting milk. The calves offered four milk portions during days 1–12 after introduction spent less time in the milk feeder after ingesting milk. Rasmussen et al. (2006) found that less time was spent sucking on the empty teat in the calves offered four daily milk portions compared to eight, and again, the correspondence between the behaviour observed and the variable recorded by the milk feeder supports that the time in the feeder after ingesting milk was spent on non-nutritive sucking. It was expected that a higher number of milk portions during the first period after introduction would facilitate learning to use the feeder, however, this was not the case. During the fourth week in the group no effects of age at introduction were found on the use of the computer-controlled milk feeder, which illustrates that the problems of the young calves do not persist throughout the milk-feeding period. 5. Conclusion When calves were introduced to a large group with computer-controlled milk feeders on day 6, they required more guidance to the feeder by the stockperson, they spent less time in the feeder and they ingested less milk compared to calves introduced on day 14. This suggests that 6-dayold calves are less able to compete for milk feeder access in large groups. Acknowledgements This study was recommended by the Nordic Joint Committee for Agricultural Research and supported by The Danish Ministry of Food, Agriculture and Fisheries and The Danish Livestock
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Federation. Thanks to Jane Eriksen, Helle Møller Nielsen, Erik L. Decker and John Misa Obidah for assistance. References Beckman, S., 1993. Computer-controlled milk feeders—a questionaire on expirences with calf rearing (Transponderstyrda kalvammor-en intervjuunderso¨kning om erfarenheter vid uppfo¨dning av kalvar). Examwork no. 54. Swedish University of Agricultural Sciences, Department of Animal Nutrition and Management, 40 pp. Dobson, A., 2002. An introduction to generalised linear models. In: Texts in Statistical Science, Chapman and Hall/CRC, Boca Raton, USA, 225 pp. Færevik, G., Jensen, M.B., Bøe, K.E., 2006. Dairy calves social preferences and the significance of a companion during separation from the group. Appl. Anim. Behav. Sci. 99, 205–221. Jensen, M.B., 2006. Computer-controlled milk feeding of group-housed calves, the effect of milk allowance and weaning type. J. Dairy Sci. 89, 201–206. Jensen, M.B., 2004. Computer controlled milk feeding of dairy calves: the effects of number of calves per feeder and number of milk portions on use of feeder and social behavior. J. Dairy Sci. 87, 3428–3438. Jensen, M.B., Holm, L., 2003. The effect of milk flow rate and milk allowance on feeding related behaviour in dairy calves fed by computer controlled milk feeders. Appl. Anim. Behav. Sci. 82, 87–100. Kung Jr., L., Demarco, S., Siebenson, L.N., Joyner, E., Haenlein, G.F.W., Morris, R.M., 1997. Evaluation of two management systems for rearing calves fed milk replacer. J. Dairy Sci. 80, 2529–2533. Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., 1996. SAS1 system for mixed models. SAS Inst., Cary, NC, USA, pp. 1–134. O’Driscoll, K., Keyserlingk, M.A.G., Weary, D.M., 2006. Effects of mixing on drinking and competitive behavior of dairy calves. J. Dairy Sci. 89, 229–233. Rasmussen, L., Jensen, M.B., Jeppesen, L.L., 2006. The effect of age at introduction and number of milk-portions on calves responses to integration into a dynamic group of dairy calves fed by computer controlled milk feeder. Appl. Anim. Behav. Sci. 100, 153–163. Rushen, J., De Passille´, A.M., 1995. The motivation of non-nutritive sucking in calves, Bos taurus. Anim. Behav. 49, 1503–1510. Søfteland, T., 1999. How do young calves react when they are introduced into a dynamic group? (Hvordan pa˚virkes unge ˚ s, 67 kalve na˚r de bliver introdusert in en dynamisk gruppe?) M.Sc. Thesis at the Norwegian Agricultural University, A pp. Weber, R., Wechsler, B., 2001. Reduction in cross-sucking in calves by the use of a modified automatic teat feeder. Appl. Anim. Behav. Sci. 72, 215–223. Wendl, G., Schuch, S., Callian, B., Wendling, F., 1998. Besaugen Verhu¨ten. Ein verschliessbarer Tra¨nkestand fu¨r Ka¨lbertra¨nkeautomaten zur Verringerung des gegenseitigen Besaugens. Jahrgang Landtechnik 4, 264–265. Zerbe, F., Schlichting, M.C., 1993. Drinking behaviour and activities of rearing calves kept in groups and fed with an automatic-milk-feeder. In: Nichelmann, M., Wierenga, H.K., Braun, S. (Eds.), Proceedings of the International Congress on Applied Ethology Berlin 3rd Joint Meeting, Humboldt Universitat, Berlin, Germany, pp. 483–486.