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Calf mortality in Norwegian dairy herds

S. M. Gulliksen^*,,1, K. I. Lie, T. Løken^* and O. Østerås^*

^* Department of Production Animal Clinical Sciences, Norwegian School of Veterinary Science, Oslo, Norway
Department of Cattle Health Services, TINE Norwegian Dairies, Ås, Norway
Department of Basic Sciences and Aquatic Medicine, Norwegian School of Veterinary Science, Oslo, Norway

¹ Corresponding author: stine.gulliksen{at}veths.no

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 
The aims of this study were to estimate mortality rates in Norwegian dairy calves and young stock up to 1 yr of age, identify risk factors for calf mortality, and evaluate the etiology of calf mortality based on postmortem analyses. The material comprised 3 data sets. The first data set included information on 289,038 offspring in 14,474 dairy herds registered in the Norwegian Dairy Herd Recording System (NDHRS) in 2005. The second included recordings on 5,382 offspring in 125 Norwegian dairy herds participating in a survey on calf health, and the third included results from postmortem analyses of 65 calves from 37 of the survey herds. The calf mortality rate during the first year of life in all herds registered in the NDHRS was 7.8%, including abortion (0.7%) and stillbirth (3.4%). The overall calf mortality rate in liveborn calves in the survey herds was 4.6%. Cows with severe calving difficulties had an odds ratio (OR) of 38.7 of stillbirth compared with cows with no calving difficulties. Twins and triplets showed an increased risk of stillbirth compared with singletons (OR = 4.2 and 46.3, respectively), as did calves born in free stalls compared with tie stalls (OR = 1.9). Respiratory disease increased the risk of death in all age groups with hazard ratios (HR) of 6.4, 6.5, 7.4, and 5.6 during the first week of life, 8 to 30 d of age, 31 to 180 d of age, and 181 to 365 d of age, respectively. Diarrhea increased the risk of death among calves younger than 180 d of age, but the influence was only significant during the first week of life and between 8 to 31 d of age (HR = 2.4 and 2.9, respectively). Calves born during the winter were more likely to die during the first week of life than calves born during the summer (OR = 1.2), and were more likely to die during the first month of life than calves born during the autumn (OR = 1.2). Calf mortality rates in all age groups increased with increasing herd size. Calves housed in a group pen from 2 wk of age were more likely to die during the first month of life than calves housed individually (HR = 1.5). Bronchopneumonia and enteritis were the most frequent postmortem diagnoses, with proportional rates of 27.7 and 15.4%, respectively.

Key Words: calf • mortality • dairy • risk factor

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 
Increased mortality rates are one of several indicators of poor welfare resulting in economic losses in cattle production (Martin and Wiggins, 1973). In the Norwegian dairy cow population, which includes approximately 280,000 calvings per year, deaths among calves and young stock cause annual losses up to 100 million Norwegian kroner (>US$15 million) for Norwegian dairy farmers (Østerås et al., 2007a).

Mortality rate is defined as the number of animals that die in a designated period divided by the number of animal-time units at risk during that period (Dohoo et al., 2003). According to Heinrichs and Radostitis (2001), calf mortality can be divided into the following 4 groups according to age at time of death: abortions or prenatal deaths (stillborn from 40 to 270 d of gestation), perinatal mortality (stillborn after 270 d of gestation or until 24 h after birth), neonatal mortality (death between 1 and 28 d of age), and older calf mortality (death between 1 and 6 mo of age). Calf mortality rates reported from European countries vary from 4% between 1 to 210 d of life (Svensson et al., 2006) to 7% during the first 180 d of life (including stillbirths) (Agerholm et al., 1993). The National Animal Health Monitoring System (NAHMS) in the United States reported an overall abortion percentage of 4.5% during 2006, and mortality rates of 7.8 and 1.8% in unweaned and weaned heifers, respectively (USDA, 2007). Simensen (1982a) reported a mortality rate of 4.0%, including stillbirth, during the first 30 d of life in Norwegian dairy herds.

Calves are at highest risk of death during the first 4 wk of life (Agerholm et al., 1993). Dystocia has been reported to be the most important cause of perinatal mortality (Chassagne et al., 1999) and pneumonia and diarrhea the most common causes of neonatal death (Agerholm et al., 1993; Virtala et al., 1996). However, in most cases, the cause of calf mortality is multifactorial, often due to a combination of dam factors, various infective agents, and nonoptimal housing and management. Among the factors reported to influence the risk of calf mortality are herd size, type of housing, personnel in charge of calf management, colostrum feeding regimen, sex, and time of separation from dam after calving (Simensen, 1982b; Lance et al., 1992).

Cattle production in Norway has changed considerably over time, and during the last 10 yr the average herd size in Norway has increased from approximately 14 cow-years to approximately 20 (cow-year = the sum of days from first calving to culling within one year, divided by 365). Changes in cattle production are continuing today; herd size is increasing, there are more cooperative farms with several owners, and starting in 2004, the authorities demanded that all new stalls built are loose-housing or free-stall systems.. These changes could influence the risk of disease and mortality in calves and young stock.

The aims of the present study were to estimate the mortality rate in calves and young stock of up to 1 yr of age in Norwegian dairy herds, to identify risk factors for calf mortality, and to evaluate the etiology of calf death by postmortem analyses.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 
Herds Registered in the Norwegian Dairy Herd Recording System
Approximately 97% of all Norwegian dairy herds are included in the Norwegian Dairy Herd Recording System (NDHRS). In the current study, information on 309,361 offspring in 15,050 Norwegian dairy herds in 2005 was extracted from this database. The following calves were excluded: born in herds for which herd size was not registered (n = 9,665); born in a herd other than the current herd (n = 10,440); unknown date of birth, but registered in further production (n = 191); registered as being both aborted and stillborn (n = 2); without registration of life status after birth (n = 20); and sold or slaughtered, but dead within 20 d of life (n = 5). Castrated calves were defined as males (n = 661). The final population in this data set included 289,038 offspring in 14,474 Norwegian dairy herds in 2005 (Table 1).

The farmers participating in the survey received a questionnaire comprising 55 closed questions on housing, management, and feeding routines. All questions referred to the situation in the herd during the study year. A total of 127 (94.1%) farmers completed and returned the questionnaire. Two of the herds were excluded from further analysis because of poor recording, and therefore poor data quality. All data were transferred to a separate database by project personnel. The final population included 5,382 offspring in 2005. Data were extracted from the NDHRS (Table 2), and information on housing, feeding, and management in the 125 Norwegian dairy herds was extracted from the questionnaire (Table 3).

Health Recordings and Calf Mortality in the NDHRS
In the NDHRS, calf mortality is classified as abortion (stillborn before 260 d of gestation), stillbirth (death after 260 d of gestation or with 24 h after birth), or death between 24 h after birth and registration in the NDHRS, mandatory within 21 d of age, all as binary traits (0/1). Abortions and stillbirths are recorded as a part of the disease history of the dam. Death later in life is recorded as an individual event of the calf.

Information on diseases and preventive treatments for each animal is regularly reported through the Norwegian Cattle Health Recording System (NCHRS). A 3-point scale is used to document and describe the occurrence of calving difficulties, as follows: 1 = calving requiring no assistance, 2 = slight calving problems requiring some assistance, and 3 = difficult calving requiring considerable assistance from the owner or a veterinarian. The occurrence of congenital defects is recorded as a binary trait (0/1). The most frequently reported diseases of the dam from 15 d precalving and 5 d postcalving were extracted from the NDHRS. This included indigestion, laminitis, metritis, prolapse of the vagina or uterus, prolonged gestation period (>296 d of gestation), retained placenta, ketosis, milk fever, and clinical mastitis. In addition, recordings in calves of respiratory disease, diarrhea, and arthritis were extracted from the NDHRS. Further details regarding the health recording system are described by Østerås et al. (2007b).

Statistical Analysis
NDHRS Herds.
Data were stored in a central database managed by TINE Norwegian Dairies and transferred to SAS version 9.1 (SAS Institute Inc., Cary, NC) for descriptive statistics and statistical analyses. Separate multivariable models were fit with abortion, stillbirth, and death from 1 to 7 d of age, 8 to 30 d of age, 31 to 180 d of age, and 181 to 365 d of age as dependent variables, all as binary traits (0/1). PROC GENMOD with binomial distribution and logit link function with Wald’s statistics for type 3 contrasts (Dohoo et al., 2003) was used with herd as random effect. By applying alternative logistic regression, the random effects were expressed as log (odds ratio) by using the LOGOR statement (Carey et al., 1993). Two different models were fit in case of death in the different age groups, except abortion. The first model included all offspring in 2005, whereas the second included offspring defined as normal; that is, a singleton calf born without congenital defects or calving difficulties. The potential risk factors included in the statistical analysis are presented in Tables 1 and 2. In addition, prolonged gestation period, occurrence of disease in the dam from 15 d before and 5 d after calving, and the occurrence of respiratory disease, diarrhea, or arthritis in the calf were included as fixed effects. A season variable was created for each calf based on the date of birth. Spring, summer, autumn, and winter were defined as March through May, June through August, September through November, and December through February, respectively. Hierarchical dummy variables were created for herd size in cow-years, divided into >5, >10, >15, >20, >30, and >50 cow-years. The significance of the different variables was initially evaluated by univariate analysis. If the P-value was <0.1 the variable was included in a final multivariable model. The overall significance of each variable was evaluated by using manual forward stepwise selection with inclusion criteria of P < 0. 05. Possible interactions between significant fixed effects were tested. The model fit was evaluated by deviance residuals (Dohoo et al., 2003).

Survey Herds.
Data were stored in MS SQL Server (Microsoft, Redmond, WA) or a central database managed by TINE Norwegian Dairies and transferred to SAS version 9.1 (SAS Institute Inc.) for descriptive statistics and statistical analyses. In the analysis of risk factors for abortion and stillbirth, PROC GENMOD with binomial distribution and logit link function with Wald’s statistics for type 3 contrasts (Dohoo et al., 2003) was used with herd as random effect. Separate multivariable models were fit, with abortion (0/1) and stillbirth (0/1) as dependent variables. By applying alternative logistic regression, the random effects were expressed as log (odds ratio) by using the LOGOR statement (Carey et al., 1993). In cases of death during the first year of life, survival analysis was performed in the following groups: death from 1 to 7 d, 8 to 30 d, 31 to 180 d, and 181 to 365 d of age. Separate Cox proportional hazard models (Cox, 1972) were fitted by using the PROC PHREG statement with death (0/1) as dependent variable, including the positive stable frailty models in the SAS macro available from Shu and Klein (1999, 2005). The significance of the frailty effect was assessed by the likelihood ratio test of independence [H₀: Theta () = 1]. The frailty effect equals the strength of association between 2 individuals within the same herd, measured by Kendall’s (1 – ) and was considered significant at P < 0.05. The end of the observation period in a calf was defined as 365 d of age, if death had not occurred. Data were censored when the animal left the herd due to sale or slaughter. To include all calves born in 2005, the observation period in the survey was defined as January 1, 2005, to December 31, 2006. As with the analyses performed on offspring in the NDHRS herds, 2 different models were fit in case of death in all age groups except abortion. The first model included all calvings in 2005, and the second only included calvings defined as normal. In addition to the fixed effects included in the analysis of calf mortality in all NDHRS herds (Table 1), variables on housing, feeding, and management were extracted from the survey questionnaire and included (Table 3). Nonsignificant variables were removed one by one by backward stepwise elimination, with an inclusion criterion of P < 0.05. Possible interactions between significant fixed effects were tested.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 
NDHRS Herds
The mortality rate during the first year of life was found to be 7.8%, including abortion (0.7%) and stillbirth (3.4%). The mortality rates in the different age groups are presented in Table 1.

The factors that had a significant effect on the survival of offspring in the NDHRS herds are presented in Tables 4, 5, 6, 7, 8, and 9.

Dystocia, congenital defects, and being born as a twin or a triplet were also found to increase the risk of death, during both the first week and the first month of life. Severe calving difficulties increased the risk of death, OR = 4.6 (95% CI: 4.1–5.1) and OR = 1.9 (95% CI: 1.6–2.2), during the first week and first month, respectively, compared with normal calvings. Twins and triplets had an increased risk of death during the first week [OR = 1.3 (95% CI: 1.2–1.5) and OR = 4.9 (95% CI: 1.8–13.6), respectively], compared with singletons, but once the calves were older than 1 wk, there was no significant effect on the risk of death for calves born as twins or triplets compared with calves born as singletons. Calves born with malformations had an increased risk (OR) of death during the first week and first month of life of 16.9 (13.8–20.9) and 5.1 (3.7–7.1), respectively, compared with normal calves. The factors influencing the risk of death in calves born from normal calvings from 1 to 365 d of age are presented in Tables 6 to 9. No associations between occurrence of disease in the dam and calf mortality were found in any age group.

Survey Herds
The overall calf mortality rate during the first year of life was found to be 9.5%, including abortion (0.9%) and stillbirth (3.9%). By organizing the herds in descending order by mortality rate, the mortality rate in each herd is presented in Figure 1. The mortality rates in the different age groups are presented in Table 2.

View larger version (23K): [in this window] [in a new window]   Figure 1. Calf mortality rates (%), including abortion and stillbirth, in 125 Norwegian dairy herds participating in a survey on calf health in 2005. The herds are organized in descending order by mortality rate.

View larger version (12K): [in this window] [in a new window]   Figure 2. Survival distribution function for calf mortality in 125 Norwegian dairy herds including calves (n = 5,104) previously registered with (...) or without (—) respiratory disease during their first year of life. The periods used in the analysis (1 to 7 d, 8 to 30 d, 31 to 180 d, and 181 to 365 d of age) are marked with arrows.

View larger version (12K): [in this window] [in a new window]   Figure 3. Survival distribution function for calf mortality in 125 Norwegian dairy herds including calves (n = 5,104) born during 2005 previously registered with (...) or without (—) diarrhea. The periods used in the analysis (1 to 7 d, 8 to 30 d, 31 to 180 d, and 181 to 365 d of age) are marked with arrows.

Significant cluster effects on herd level were found in all age groups <180 d, with frailty effects varying from 5.0 to 7.2%.

Postmortem results of 65 calves from 37 herds represented 25.9% (65/251) of all calves that died during this period. Of these, 55 (84.6%) examinations were performed by veterinarians in the field, and the other 10 (15.4%) were performed at the National Veterinary Institute or the Norwegian School of Veterinary Science. Bronchopneumonia was the main diagnosis in 18 (27.7%) of the 65 calves (Table 10). In animals older than 1 mo, bronchopneumonia was the main diagnosis in 17 (41.5%) of the 41 cases. Bronchopneumonic changes (gross or histological changes) from very mild to severe lesions were detected in 36 (55.0%) calves. Enteritis was the second most common finding and was considered the main or primary diagnosis in 10 (15.4%) of 65 calves and in 5 (20.8%) of 24 calves who died at less than 1 mo of age.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

Mortality Rate
The mortality rate estimated for the herds registered in the NDHRS was similar to mortality rates reported from other European countries (Olsson et al., 1993; Esslemont and Kossaibati, 1996), lower than that reported in the United States (USDA, 2007), but slightly higher than that previously reported from Norway (Simensen, 1982a). A lack of a standardized definition of calf mortality means that inter-study comparisons are difficult. Results from the NAHMS in 2006 indicate an abortion rate of 4.5% (USDA, 2007), which is substantially higher than the abortion rate of 0.74% reported from the NDHRS (Table 1). This might be partly due to the considerably smaller herds in Norway, and the fact that Norway is free from several important infectious diseases of cattle such as brucellosis, leptospirosis, infectious bovine rhinotracheitis, and bovine virus diarrhea. Svensson et al. (2006) reported a mortality rate of 3% in Swedish dairy calves before 90 d of age, which is similar to the 3.1% for the same age group in the herds registered in the NDHRS. It might be anticipated that calf mortality rates in these 2 countries would be comparable because of similar infectious status, although the average dairy herd size in Sweden is slightly larger (Pettersson et al., 2001).

The mortality rate in the survey herds was higher than that of the herds registered in the NDHRS. Mortality rate increased in all age groups with increasing herd size, which could be due in part to the larger size of the survey herds [average size of survey herds is 46.8 (SD 27.6) cow-years; average size of herds registered in the NDHRS is 21.6 (SD 13.4) cow-years]. The mortality rates in the 3 largest herd-size groups were similar in NDHRS herds and survey herds, demonstrating that the survey herds could be considered as representative of large Norwegian dairy herds. Abortion and stillbirth are recorded as a part of the disease history of the dam. Hence, these recordings are expected to be very accurate for all herds in the NDHRS. Also, as the reporting of dead farm animals is mandatory according to tax authorities, differences between mortality rates in the NDHRS herds and the survey herds are most likely not due to underreporting from the NDHRS herds.

Risk Factors
Disease and Postmortem Findings.
Calves previously diagnosed with arthritis had the highest risk of death during the first 6 mo of life, although diarrhea and respiratory disease were the most frequently reported diseases. Infectious arthritis could be secondary to an umbilical infection in young calves, and a concurrent septicemia might result in death (Agerholm et al., 1993; Virtala et al., 1996). Few studies have examined risk factors influencing mortality after 180 d of age. Similar to findings of Waltner-Toews et al. (1986), calves that had been treated for respiratory disease had significantly increased risk of death in the current study (Table 9). Calf diseases in the NCHRS are unfortunately underestimated by approximately 40% (Gulliksen et al., 2009). Therefore, it seems probable that the effect of disease on calf mortality rates is more pronounced than could be demonstrated in this study.

Pneumonia was the most common cause of death among all animals necropsied in the survey, whereas enteritis was the most common diagnosis in the youngest calves. Other studies investigating calf mortality have reported similar findings (Agerholm et al., 1993; Svensson et al., 2006; USDA, 2007), and these data are similar to registrations in the NDHRS.

Housing.
Calves born in free stalls had a higher risk of stillbirth than calves born in tie stalls. In 19 of 53 (35.8%) free stalls, more than 80% of all calvings occurred in the cubicle section or in the alley area. Previous studies of calving management in Norwegian dairy herds have also reported this observation (Kjæstad and Simensen, 2001). If the calf were not removed from the alley area immediately after birth, it would be exposed to high levels of infectious agents from manure and bedding. This is expected to increase the risk of disease and death in the newborn calves. All tie-stall herds included in the survey (n = 71) reported that >80% of all calvings happened while the cow was restrained. Supervision of each cow is easier in tie stalls than in loose housing systems. This could be part of the reason that calving in a tie stall was not found to be a risk factor for stillbirth. According to Norwegian regulations, one maternity pen is required per 25 cows. It is recommended that cows be transferred to the maternity unit at least 24 h before expected calving to avoid prolonged calvings due to elevated stress levels and to fulfill the natural, isolation-seeking behavior of calving cows (Bao and Giller, 1991). Larger herd sizes and new housing systems are likely to reduce the possibility of individual supervision of cows in late gestation, and thereby the risk of calvings happening outside maternity pens will increase. Insufficient monitoring around parturition and a high number of unassisted calvings could increase the risk of stillbirth (Lombard et al., 2007; Gundelach et al., 2009).

Suckling calves had an increased risk of death during the first week of life. Although the presence of the dam has been found to increase the amount of IgG absorbed (Quigley et al., 1995), calves left to suckle their dams often fail to ingest adequate volumes of colostrum (Franklin et al., 2003). Svensson et al. (2003) found an increased risk of diarrhea in calves that received colostrum from suckling compared with calves that were given their first meal by the farmer. Wells et al. (1996) found that leaving the calves with the dam for more than 24 h increased the odds of mortality during the following 21 d.

This study indicates that housing calves in a group pen during the first month of life is disadvantageous compared with housing calves in individual pens, as has been reported by Svensson and Liberg (2006). Calves in group pens are likely to experience greater exposure to higher levels of infectious agents earlier in life than calves housed individually, which may result in increased infection and death rates. Svensson et al. (2006) and Svensson and Liberg (2006) found that being housed in group pens was associated with an increased risk of respiratory disease, lower age of onset of diarrhea, and more severe cases of diarrhea compared with calves housed in individual pens. However, group housing enables social interactions between calves and fulfills their needs for motion and play, and is therefore considered preferable from a welfare point of view. Our results indicate that infection status among calves in a herd should be included in the animal welfare debate on optimal housing systems for calves.

Calving Difficulties and Congenital Defects.
Calving difficulties and congenital defects were found to be, by far, the most important factors influencing the risk of stillbirth and death during the first week of life, and other studies have reported similar findings (Wells et al., 1996). A calf dying before delivery could also be a reason for calving difficulties. Calves that have undergone a prolonged and difficult birth often experience respiratory problems and weakness, which may result in the period from birth to first colostrum intake being prolonged and colostrum intake being reduced. These factors could result in insufficient ingestion of IgG, thereby increasing disease susceptibility and mortality risk. Calving difficulty and stillbirth have been part of the total merit index used for selection of Norwegian Red sires since 1978 (Geno, 2008). According to Heringstad et al. (2007), the mean frequency of stillbirth has remained unchanged from 1978 to 2004, as has the proportion of calvings reported as "difficult." The results of the present study indicate that the reasons for calving difficulties are complex, but improved management practices around calving and increased calving supervision would probably be beneficial.

Twins and Triplets.
According to the current study, twinning is undesirable in dairy cattle production because of increased calf mortality rates during the first month of life. Mee et al. (2008) also found significantly increased risk of perinatal mortality in twins compared with singletons (OR = 13.4 in first-parity cows), and Silva del Río et al. (2007) concluded that cows with twins had a higher incidence of abortion, calving difficulties, retained placenta, and metritis. The influence of twinning on abortion risk could not be included in the current study because for the majority of aborted calves in the NDHRS there is an absence of registration on whether they were singletons, twins, or triplets.

Calving Season.
The abortion rate showed both seasonal and geographical variation, being highest during summer and most pronounced in the western part of Norway. The tick Ixodes ricinus is more widespread in this region (Stuen et al., 2002) and could cause infections such as anaplasmosis during summer. Anaplasmosis could be a significant cause of abortion in these areas and this possibility should be followed up by further studies. Calving during summer or autumn, however, was found to reduce the risk of death during the first week and the first month of life. Similar results were reported by Svensson et al. (2006) and Silva del Río et al. (2007), who found that calf mortality rates were lower during warm months compared with cold months. Gulliksen et al. (2008) found that Norwegian dairy cows calving during winter months produced colostrum with a significantly lower IgG content than cows calving during any other season of the year, and colostrum of the highest quality was produced by cows calving during autumn. Thus, there seems to be a relationship between seasonal variation in colostrum quality and calf mortality.

Herd Size.
Calf mortality rate increased with increasing herd size in all age groups, as has previously been reported in numerous other studies (Lance et al., 1992; Silva del Río et al., 2007). However, this contrasts with results from the NAHMS (USDA, 2002, 2007) in which large operations (>500 cows) had a lower percentage of unweaned heifer deaths compared with small (<100 cows) and medium-sized (100 to 499 cows) herds, although large herds had a higher percentage of abortions. When comparing these studies, it should be noted that Norwegian dairy herds are generally considerably smaller than the herds included in both USDA studies. As herd size increases and there is more mechanization and technological developments used in the cattle industry, the time physically spent in the barn decreases and the daily inspection of each individual animal is reduced. These factors could contribute toward increased mortality rates.

Parity.
Studies from Sweden (Berglund et al., 2003), Ireland (Silva del Río et al., 2007), and North America (Meyer et al., 2000) indicate that bovine perinatal mortality rates are increasing, particularly in Holstein heifers. Feto-pelvic disproportion has an effect on stillbirths, and the increased risk of stillbirth in primiparous cows could be due in part to a higher frequency of calving difficulties. Nevertheless, the risk of stillbirth was still significantly higher in primiparous cows when only those calvings reported as normal were included in the analyses.

Other Risk Factors.
The lower risk of stillbirth in the Norwegian Red breed compared with other breeds or crossbreeds is likely to be a result of numerous years of genetic selection for sires with low stillbirth rates (Heringstad et al., 2007). In addition, the use of beef cattle sires in crossbreeding might lead to feto-pelvic disproportion, and thereby a higher risk of calving difficulties.

No difference in stillbirth risk between male and female calves was shown in our study, which is in contrast to the results of the studies of Chassagne et al. (1999) and Meyer et al. (2001), who found that bull calves were more likely to be stillborn than heifer calves. Sex is not recorded for the majority of aborted and stillborn calves in the NDHRS, and therefore interpretation of the results concerning sex of calf as a potential risk factor for abortion and stillbirth is difficult when using data from this database.

Significant cluster effects within herd were found in all age groups. The OR varied between 2.13 for stillbirth and 3.59 for death between 180 and 365 d of life. This implies that, in addition to management factors, the occurrence of infectious agents within each herd is important in the variation in risk of calf mortality.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 
Calves diagnosed with diarrhea, respiratory disease, or arthritis had a significantly increased risk of death. This study indicates that individual housing and manual feeding of colostrum are preferable to group housing and suckling during the first 4 wk of life. To reduce calf mortality in Norwegian dairy herds, attention should be focused on those risk factors that are under management control. Efforts should be made to increase calving supervision, improve management of newborn calves, and prevent respiratory diseases and diarrhea.

	ACKNOWLEDGMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 
The authors thank all the participating farmers, veterinarians, and laboratory personnel for their involvement in the current research. The study was funded by TINE Norwegian Dairies BA (Ås, Norway), Animalia (Løren, Norway), and the Research Council of Norway (Oslo). Access to production and health data was given by the Norwegian Dairy Herd Recording System and the Norwegian Cattle Health Services in agreement number 8/2002.

Received for publication October 14, 2008. Accepted for publication February 10, 2009.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 


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