Abortion and Calf Mortality in Danish Cattle Herds

The aetiology of abortions and calf mortality in 65 Danish cattle herds consisting of both dairy and beef breeds during a 1-year period is described. All observed aborted foetuses, stillborn calves, and calves dying before 6 months of age were necropsied, and relevant microbiological examinations were performed. A total of 240 calves and 66 abortions were submitted corresponding to a calf mortality rate of 7%. The abortion frequency could not be calculated. 43% of the calves died at day 0, while 22% were aborted, 15% died during the first week of life, 9% died from 1 to 4 weeks of age, and 11% died at the age of 1 to 6 months. The most common cause was neonatal pulmonic atelectasis (stillbirth) followed by foetal infections, pneumonia, and septicaemia.     

Pre-weaning management of calves on commercial dairy farms and its influence on calf welfare and mortality

Welfare and management of calves is of increasing interest and also influences performance of these animals in later life. The aim of this study was to assess management and environmental conditions under which pre-weaned dairy calves are reared on commercial Irish dairy farms. We included 47 spring-calving, pasture-based herds in this study. Herd and animal-specific data, such as mortality rate, age and breed, were gathered from all participants via the HerdPlus® database. Information pertaining to management practices was collected by conducting an interview with the principal calf rearer, while an assessment of calf housing facilities was conducted to identify conditions calves were reared in. The environmental assessment included measurements of space allowance per calf, as well as feeding equipment hygiene. To assess calf behaviour video observations were used, while accounting for the number of calves present in a group and the space available per calf. Faecal samples were also collected to determine the presence of enteric pathogens among calves. To compare calf space allowance, group size and presence of enteric pathogens early and late in the calving season each farm was visited twice. Calf mortality was not associated with either herd size, space allowance per calf or post-colostrum feeding practices. Higher calf mortality was identified among herds which reported experiencing an on-set of calf pneumonia during weeks 8 to 10 of the calving season. This study demonstrates that factors associated with calf welfare on commercial Irish dairy farms (e.g. space allowance, mortality rate) are independent of herd size. Some management practices however, such as methods used for treating health issues can affect rates of calf mortality experienced. Calf mortality, for example, was lower in herds which treated diarrhoea cases by administering electrolytes, while continuing to offer milk. Behavioural observations indicate that smaller group sizes could promote expression of positive behaviours, potentially resulting from an overall improvement in welfare. Space allowance per calf was not associated with observed behaviour frequencies. We also identified that similar rates of calf mortality are experienced across herds of different sizes.     

Seroepidemiology of bovine viral diarrhoea virus (BVDV) in the Adamawa Region of Cameroon and use of the SPOT test to identify herds with PI calves

Bovine viral diarrhoea, caused by the bovine viral diarrhoea virus (BVDV) in the Pestivirus genus of the Flaviviridae, is one of the most important diseases of cattle world wide causing poor reproductive performance in adult cattle and mucosal disease in calves. In addition it causes immunosuppression and increased susceptibility to other infections, the impact of which is uncertain, particularly in sub-Saharan Africa where animals are exposed to a much wider range and higher intensity of infections compared to Europe. There are no previous estimates of the seroprevalence of BVDV in cattle in Cameroon. This paper describes the serological screening for antibodies to BVDV and antigen of BVDV in a cattle population in the Adamawa Region of Cameroon in 2000. The estimates of herd-level and within herd seroprevalences adjusted for test imperfections were 92% and 30% respectively and 16.5% of herds were classed as having a persistently infected calf (PI) in the herd within the last year based on the "spot" test approach. There was evidence of clustering of herds with PI calves across the north and west of the Region which corresponds with the higher cattle density areas and of self-clearance of infection from herds. A multivariable model was developed for the risk of having a PI calf in the herd; proximity to antelope, owning a goat, mixing with > 10 other herds at grazing and the catchment area of the veterinary centre the herd was registered at were all significant risk factors. Very little is known about BVDV in sub-Saharan Africa and these high seroprevalences suggest that there is a large problem which may be having both direct impacts on fertility and neonate mortality and morbidity and also indirect effects through immunosuppression and susceptibility to other infections. Understanding and accounting for BVDV should be an important component of epidemiological studies of other diseases in sub-Saharan Africa.     

Evaluation of productivity of sexually precocious Nelore heifers

The objectives of the present study were to investigate the influence of yearling weight on the occurrence of early pregnancy and to determine differences between precocious and non-precocious heifers in terms of pre- and postnatal calf mortality, calf weight and height, mature cow weight, and stayability of the cow in the herd. Data from 26 977 females of a Nelore herd that participated in the breeding season between 1986 and 2004 were analyzed. The influence of yearling weight on sexual precocity and differences between precocious and non-precocious heifers in pre- and postnatal calf mortality and stayability were analyzed using the GENMOD procedure of the SAS program. Differences in the growth traits between precocious and non-precocious animals were estimated by contrast analysis. Three groups were analyzed for postnatal mortality: first calving of the heifers, calves born from the third calving and all offspring of the cow. In order to have a standardized calving to conception period for all females (precocious and non-precocious), calves born from the second calving were not included in the analysis. This was necessary because the first calving to conception period (days open) of precocious heifers was longer than for non-precocious due to farm reproductive management. No differences in postnatal mortality rates, from the third calving, were observed between the two groups of heifers studied. Analysis of all offspring of the dams showed a 9% higher probability of death of calves born to precocious heifers compared with calves born to non-precocious heifers. With respect to stayability, precocious heifers presented 33% greater odds to remain in the herd until 5 and 6 years of age and 28% greater odds to remain in the herd until 7 years when compared with non-precocious heifers. Precocious heifers weaned calves (205 days) significantly heavier than non-precocious ones, 1.410 and 0.797 kg considering the weaning weights of all offspring and of the third calving, respectively. With respect to sexual precocity, the results suggest that the probability of heifers to become pregnant at 16 months of age increases with increasing weight. However, heifers weighing more than 240 kg present practically the same pregnancy probability. Nevertheless, exposure of heifers during the early breeding season is recommended for beef cattle herds raised in tropical regions in view of the numerous benefits demonstrated here.     

Emergency rabies control in a community of two high-density hosts

Background

Mortality and morbidity are hardly documented in the white veal industry, despite high levels of antimicrobial drug use and resistance. The objective of the present study was to determine the causes and epidemiology of morbidity and mortality in dairy, beef and crossbred white veal production. A total of 5853 calves, housed in 15 production cohorts, were followed during one production cycle. Causes of mortality were determined by necropsy. Morbidity was daily recorded by the producers.

Results

The total mortality risk was 5,3% and was significantly higher in beef veal production compared to dairy or crossbreds. The main causes of mortality were pneumonia (1.3% of the calves at risk), ruminal disorders (0.7%), idiopathic peritonitis (0.5%), enterotoxaemia (0.5%) and enteritis (0.4%). Belgian Blue beef calves were more likely to die from pneumonia, enterotoxaemia and arthritis. Detection of bovine viral diarrhea virus at necropsy was associated with chronic pneumonia and pleuritis. Of the calves, 25.4% was treated individually and the morbidity rate was 1.66 cases per 1000 calf days at risk. The incidence rate of respiratory disease, diarrhea, arthritis and otitis was 0.95, 0.30, 0.11 and 0.07 cases per 1000 calf days at risk respectively. Morbidity peaked in the first three weeks after arrival and gradually declined towards the end of the production cycle.

Conclusions

The present study provided insights into the causes and epidemiology of morbidity and mortality in white veal calves in Belgium, housed in the most frequent housing system in Europe. The necropsy findings, identified risk periods and differences between production systems can guide both veterinarians and producers towards the most profitable and ethical preventive and therapeutic protocols.     

Induced orphaning reveals post-weaning maternal care in reindeer

A common by-product of human harvesting is orphaning of calves in autumn. Despite this, there are few studies in northern and temperate ungulates evaluating the fate of orphaned calves and the potential benefits to offspring and costs to mothers of post-weaning maternal care. We manipulated orphaning and forage distribution during winter for two herds of reindeer (Rangifer tarandus): one nonfed and the other supplementally fed to increase level of interference competition. Both herds consisted of females with and without calves at heel and orphaned calves. We measured survival and somatic losses during winter and distances between mother–calves and adult females–orphans within the herds. All females survived the winter, and there was no evidence of post-weaning maternal cost in terms of female’s mass loss. The winter mortality among calves was negligible and did not differ between orphans as compared to nonorphans. However, nonorphaned calves lost less mass and stayed closer to their mothers than orphans to adult females, suggesting that increased mortality might occur in harsher winters. This tended to be more marked in the fed group where interference competition was more likely due to feed being concentrated both in space and time. Reduced mass loss in nonorphans is therefore most likely due to mothers sharing and defending feeding resources and protecting their offspring from harassment by other herd members during their first winter. We conclude that hunting practise of northern and temperate ungulates where females having calf/calves at heel are intentionally or non-intentionally harvested, may have demographic side effects at least in harsh winters.     

Genotype by environment interaction for livability of dairy calves from first parity cows

Death of calves around parturition is a matter of concern for dairy farmers. Relatively high stillbirth rates and unfavourable trends have been reported for Holstein heifers in the Netherlands and several other countries. In our study, we investigated herd differences, genetic parameters and genotype by environment interaction for heifer calf livability. A large dataset with data from calvings between 1993 and 2012 of Dutch dairy farms was used. There were considerable differences between herds in livability of calves from heifers, with averages ranging from 74% to 95%. Both herds with relatively high and low averages showed the same negative trend between 1993 and 2012, with largest declines in herds with the lowest averages. We found that heritability and genetic variation of first parity livability were substantially larger in herd environments where the likelihood of stillbirth was high v. environments where stillbirth was at a low level. The genetic correlations between herd environment levels were all very close to unity, indicating that ranking of sires was similar for all environments. However, for herds with a relatively high stillbirth incidence selecting sires with favourable breeding values is expected to be twice as profitable as in herds with a relatively low stillbirth incidence.     

Abnormal offspring following in vitro production of bovine preimplantation embryos: a field study

Data on 944 calves from 2228 in vitro-produced (IVP) bovine preimplantation embryos were compared with data on 2787 AI calves born in the same herds in 1995. Bovine preimplantation embryos were produced in vitro following ovum pick up (OPU) from donor cows and pregnant heifers in an open nucleus breeding program. After 7 d of in vitro culture on a BRL cell monolayer in the presence of 10% FCS, frozen-thawed expanded blastocysts and fresh morulae to expanded blastocysts were transferred into recipient heifers and cows at 119 contracted farms throughout the Netherlands. The pregnancy rate, as confirmed by palpation per rectum between 90 and 150 d after transfer was 43.5% for both fresh and frozen embryos. Data on IVP and AI calves were registered by the farmers. The percentage of calves with a congenital malformation and the percentage of male calves were related to the total number of calves born. Gestation length, birth weight (measured by a balance), perinatal mortality and ease of calving were analyzed in a subdataset (699 IVP and 2543 AI calves, respectively) by a comparative analysis of variance (ANOVA). The ANOVA model included herd, month of calving, sire nested within AI or IVP, parity and breed of the inseminated cow/embryo recipient, sex of calf, type of calf (AI or IVP) and two-way interactions between type of calf and sex, parity and breed. The percentage of calves with congenital malformations was 3.2% and 0.7% for IVP and AI calves, respectively. An increased incidence of hydro-allantois and abnormal spinal cords and limbs was observed in IVP calves. The percentage of male calves was significantly different between IVP and AI, 55.5% and 48.9%, respectively (Chi-square, 1 degree of freedom, P < 0.05). On the average, IVP calves showed a significant increase of birth weight by 10% (4-5 kg), a 3-d longer gestation period, 2.4% more perinatal mortality and a more difficult calving process compared to AI calves (P < 0.05). From these results it is concluded that calves produced by IVP deviate significantly from calves produced by AI.     

Factors associated with bovine neonatal pancytopenia (BNP) in calves: a case-control study

Bovine neonatal pancytopenia (BNP; previously known as idiopathic haemorrhagic diathesis and commonly known as bleeding calf syndrome) is a novel haemorrhagic disease of young calves which has emerged in a number of European countries during recent years. Data were retrospectively collected during June to November 2010 for 56 case calves diagnosed with BNP between 17 March and 7 June of the same year. These were compared with 58 control calves randomly recruited from herds with no history of BNP. Multivariable logistic regression analysis showed that increased odds of a calf being a BNP case were associated with its dam having received PregSure? BVD (Pfizer Animal Health) vaccination prior to the birth of the calf (odds ratio (OR) 40.78, p<0.001) and its herd of origin being located in Scotland (OR 9.71, p?=?0.006). Decreased odds of a calf being a BNP case were associated with the calf having been kept outside (OR 0.11, p?=?0.006). The longer that a cattle herd had been established on the farm was also associated with decreased odds of a calf in that herd being a BNP case (OR 0.97, p?=?0.011).     

Questionnaire identifying management practices surrounding calving on spring-calving dairy farms and their associations with herd size and herd expansion

Healthy calves are fundamental to any profitable dairy enterprise. Research to-date, has focused on year-round calving systems which experience many different challenges compared to spring-calving systems. The objective of the present study was to determine the on-farm dry cow, calving, and colostrum management practices of spring-calving dairy production systems, and quantify their associations with herd size and herd expansion status (i.e. expanding or not expanding). Information on these management practices was available from a survey of 262 Irish spring-calving dairy farmers, representative of the Irish national population. Herd expansion in the 2 years before, and the year that the survey was conducted was not associated with any of the management practices investigated. Fifty-three percent of respondents had an average calving season length of 10 to14 weeks with 35% of herds having a longer calving season. Previous research in cattle has documented that both colostrum source and feeding management are associated with the transmission of infectious disease from cow to calf. In the present study 60% of respondents fed calves colostrum from their own dam; however, 66% of those respondents allowed the calf to suckle the dam, 23% of survey respondents fed calves pooled colostrum. Larger herds were more likely (P<0.01) to use pooled colostrum supplies, while smaller herds were more likely (P<0.05) to allow the calf to suckle the dam. The majority (86%) of respondents had stored supplies of colostrum; average-sized herds had the greatest likelihood of storing colostrum (P<0.05), compared to other herd sizes; larger sized herds had a lesser likelihood (P<0.05) of storing colostrum in a freezer, compared to other herd sizes. Although freezing colostrum was the most common method used to store colostrum (54% of respondents), 17% of respondents stored colostrum at room temperature, 29% of which stored it at room temperature for greater than 4 days. The results from the present study indicate that a particular focus needs to be placed on calving and colostrum management because this study has highlighted a number of areas which are below international standards, and may have repercussions for calf health. Furthermore, management practices on larger farms could be improved and, as these represent the future of dairy farming, a focus needs to be placed on them. Expanding herds are not a particular concern as herd expansion, independent of herd size, does not seem to be associated with calving and colostrum management practices on Irish spring-calving dairy herds.     

Divergent estimates of herd‐wide caribou calf survival: Ecological factors and methodological biases

Population monitoring is a critical part of effective wildlife management, but methods are prone to biases that can hinder our ability to accurately track changes in populations through time. Calf survival plays an important role in ungulate population dynamics and can be monitored using telemetry and herd composition surveys. These methods, however, are susceptible to unrepresentative sampling and violations of the assumption of equal detectability, respectively. Here, we capitalized on 55 herd‐wide estimates of woodland caribou (Rangifer tarandus caribou) calf survival in Newfoundland, Canada, using telemetry (n = 1,175 calves) and 249 herd‐wide estimates of calf:cow ratios (C:C) using herd composition surveys to investigate these potential biases. These data included 17 herd‐wide estimates replicated from both methods concurrently (n = 448 calves and n = 17 surveys) which we used to understand which processes and sampling biases contributed to disagreement between estimates of herd‐wide calf survival. We used Cox proportional hazards models to determine whether estimates of calf mortality risk were biased by the date a calf was collared. We also used linear mixed‐effects models to determine whether estimates of C:C ratios were biased by survey date and herd size. We found that calves collared later in the calving season had a higher mortality risk and that C:C tended to be higher for surveys conducted later in the autumn. When we used these relationships to modify estimates of herd‐wide calf survival derived from telemetry and herd composition surveys concurrently, we found that formerly disparate estimates of woodland caribou calf survival now overlapped (within a 95% confidence interval) in a majority of cases. Our case study highlights the potential of under‐appreciated biases to impact our understanding of population dynamics and suggests ways that managers can limit the influence of these biases in the two widely applied methods for estimating herd‐wide survival.     

Hourly distribution of time of parturition in the domestic goat

Data on 2589 records collected during 12 years in one breed of dairy goats that were maintained in stables were analysed to examine the influence of various factors on the time of parturition. Parturitions were observed from mid-January to the end of April. Hourly frequencies of birth showed an unimodal distribution with maximum births at midday and minimum births around midnight with 90.6% of deliveries occurring between 0600 and 2000 h. The distribution showed a noticeable homogeneity when the following parameters were taken into account: litter-size, litters with live or stillborn kids, time of the year and length of gestation. In two other herds, 117 and 166 records obtained after 4 and 3 years of observation were studied. As in the first herd, births were more frequent between 0700 to 1900 h (day; 72.3 and 70.2 %, respectively) than between 1900 and 0700 h (night; P<0.001). These observations on the time of birth of the domestic goat suggest a regulation of the birth process by circadian events which themselves remain unknown.     

Housing, Feeding and Management of Calves and Replacement Heifers in Swedish Dairy Herds

A questionnaire was sent to 1500 randomly selected dairy herds in Sweden, asking for general information about the herds, including routines from birth to first calving and also routines at breeding, calving and during the grazing period. Fifty-eight percent of the questionnaires were returned. The preweaned calves were kept in individual calf pens in 68% and in group housing systems in 28% of the herds. Pens with slatted floors were the main housing system for replacement heifers from weaning to breeding, and tie stalls from breeding to first calving. Whole milk was used in 44% and milk replacements in 42% of the herds. The calves received, as a median, 2.5 litres of milk per meal and 2 meals per day. The median age at weaning was 8 weeks. Age was the single most common criteria used for deciding both weaning and breeding time. The median age when the heifers were first turned out to pasture was 6 months. Prophylactic anthelmintic treatment was used by 65% of the herds. The most common diet for replacement heifers before calving was a combination of grain, hay and silage.     

Male infanticide in captive plains zebra, Equus burchelli

On the assumption that infanticide exists in plains zebra, as reported for horses, Equus caballus, we tested the following hypothesis. Introducing a new zebra male into a herd of breeding females should increase foal mortality in comparison with herds in which the sire of the foals is still present. The younger the foal, the more likely infanticide should be. We collected data from five herds in two zoological gardens in the Czech Republic. We found nine records of infanticide in plains zebra and three cases of abortions that were probably induced by forced copulation. We analysed additional indirect data to investigate the possibility of introduced males causing other abortions. Abortions were three times more likely in herds with introduced males than with only fathers present. Postnatal mortality of the foals was four times greater with introduced males than with fathers. No indication of a sex preference was observed for infanticide by a new male for either abortions or postpartum deaths. When we combined all records involving introduced males, the probability of foal death was greatest when the new male joined the herd just after conception and decreased with increasing time between conception and date of the new male introduction (the chance of a foal surviving was less than 5% just after conception and more than 50% at the time of delivery). Mortality of foals did not depend on whether the new male was introduced before or after the foal was born. Survival increased to more than 60% after the foal reached 1 month of age. Our results suggest that captive plains zebra show the highest occurrence of infanticide reported among ungulates. Copyright 2000 The Association for the Study of Animal Behaviour.     

Group A Rotavirus as a Cause of Neonatal Calf Enteritis in Sweden

Faeces samples were collected during outbreaks of neonatal calf diarrhoea in 14 beef and dairy herds. Samples from 33 calves were taken at the onset of diarrhoea as well as from 30 calves with no signs of enteritis. No vaccines or medical treatment had previously been given. The mean age of the calves was 16.8 days (SD 8.2). The clinical evaluation of faeces consistency corresponded well to the dry matter content of the faeces (p<0.001). The samples were analyzed for rotavirus, Cryptosporidium species and Escherichia coli K99+. Group A rotavirus was detected by ELISA and RNA Polyacrylamide gel electrophoresis (RNA-PAGE) in 14/33 (43.8%) of the samples from scouring calves and 1/30 (3.7%) of the samples from non-scouring calves. The correlation between group A rotavirus and diarrhoea was statistically significant (p<0.001). No non-group A rotaviruses were found by RNA-PAGE. Cryptosporidium species were detected through demonstration of oocysts in smears from 12/63 (19.0%) of the faecal samples, but no statistically significant correlation between diarrhoea and detection of oocysts was demonstrated. Escherichia coli K99+ was not detected in any faeces sample. The clear association between group A rotavirus and diarrhoea is suggested to be due to low pathogenic load in the herds.     

Detection of <Emphasis Type="Italic">Coxiella burnetii</Emphasis> DNA and anti-<Emphasis Type="Italic">Coxiella burnetii</Emphasis> IgG antibodies in precolostral blood samples of stillborn calves in an endemically infected Holstein dairy herd

Coxiella burnetii (C. burnetii), an intracellular zoonotic bacterium causing Q fever, occurs widely in cattle herds. After invasion of the pregnant uterus and initial localization in the placenta, active C. burnetii infections may spread to the fetus hematogenously or by the amniotic-oral route and thus may cause abortion, premature delivery, stillbirth, and weak offspring (APSW) complex. In a case-control study, we investigated precolostral blood samples of 56 stillborn calves and 30 live births from a dairy herd endemically infected with C. burnetii “C-cluster” strains and an increased stillbirth rate in primiparous cows. Within the group of the stillborn calves, four precolostral blood samples (7.1%) were tested positive for C. burnetii DNA by PCR and one serum sample (1.8%) positive for anti-C. burnetii IgG antibodies by a commercial ELISA test, respectively. Neither C. burnetii DNA nor anti-C. burnetii IgG antibodies were detected in the samples of calves being born alive. In conclusion, we demonstrated that coxiellaemia and precolostral seroconversion occurred sporadically in stillborn calves from this endemically infected herd. Due to the low detection rates, C. burnetii could not be confirmed to be the cause of the increased stillbirth rate.     

The presence of black vultures at the calving sites and its effects on cows’ and calves’ behaviour immediately following parturition

Black vultures (Coragyps atratus) are often present near calving sites, and under this situation they may play a positive role by removing animal carcasses and afterbirth or a negative role by attacking neonate calves or disturbing cow–calf behaviours following parturition. Cow–calf behaviour was recorded over a 4-year study period from a total of 300 births involving 200 Nellore, 54 Guzerat, 20 Gyr and 26 Caracu cows. The calving site in relation to the location of the herd, considering cow–calf pairs within, close or distant to the herd, the presence of vultures and the behaviour of cows and calves were recorded instantaneously, at 5-min interval. On average, vultures were present at 80% of the calving sites. The frequency of vultures present at calving sites was dependent on the years for the Nellore herd, increasing from 1998 to 2003. When vultures were present, the time that the cow was in contact with its calf decreased, and the percentage of time that the cow was standing still increased. Vultures were observed pecking cows and their neonates during 34.1% of all recordings. However, in only two cases pecking injuries were actually observed on calves that were noted to be very weak. The preliminary results suggest that although black vultures cannot be characterized as a predator of neonate calves, they sometimes attack neonate calves and their presence near the calving sites alter the behaviours of cows and calves.     

An outbreak of fetal and neonatal losses associated with the diagnosis of bovine viral diarrhea virus

A severe outbreak of dairy herd pregnancy wastage was investigated. At the beginning of the outbreak, a total of 121 lactating cattle were pregnant and considered to be at risk. Overall, 33.1% of the population at risk aborted, while 25.6% gave birth to calves that either died during the early neonatal period or demonstrated signs compatible with congenital defects (abnormal births). A laboratory diagnosis of bovine viral diarrhea virus (BVDV) infection was made in two surviving neonatal calves with symptoms of cerebellar hypoplasia and blindness. An on-farm investigation was conducted to determine if the abortions and abnormal births were associated with BVDV infection. The rate of abortions versus abnormal births was biphasic when graphed by the date of occurrence. The cases of abortion occurred early in the outbreak and were followed by the neonatal losses. Within the population at risk, the mean values for gestational age at the beginning of the outbreak were different between the subpopulations described by gestational outcome. The outcome of each pregnancy that existed at the beginning of the outbreak was determined. Classifications included normal birth (birth of a normal calf), abnormal birth (a neonatal loss of the type described above), abortion, and continued gestation (normal, uncompleted pregnancy). The average gestational age at the time of the index case (the first cases of pregnancy wastage) for these four pregnancy outcome classfications was 142.0, 106.2, 86.7 and 31.3 days, respectively. Reasons for assuming that this outbreak was related to BVDV are discussed.     

Reproductive and economic impact following controlled introduction of cattle persistently infected with bovine viral diarrhea virus into a naive group of heifers

The reproductive impact following controlled introduction of animals persistently infected (PI) with bovine viral diarrhea virus (BVDV) was evaluated in BVDV-naive heifers. Heifers were randomly allocated into two groups: an unexposed control herd (n = 34) and a herd exposed to five persistently infected (PI) animals for 7 mo, beginning 50 days before the breeding season (n = 34). Initiation of the BVDV-challenge was timed to mimic either direct contact with PI calves born in the previous calving season or accidental introduction of PI herd additions prior to the breeding season. The PI animals represented BVDV Types 1a (n = 3), 1b (n = 1) and 2 (n = 1). Two BVDV-free, seropositive bulls were used in each group for 78 days breeding seasons. In both groups, 33 of 34 heifers became pregnant, with similar distribution of fetal ages. Two heifers in each group aborted (etiology undetermined). In addition, one calf was born dead and one calf died 3 days post-partum in the BVDV-exposed group. One calf in the unexposed group died 4 mo post-partum. No calves, including the stillborn calf and the two calves that died prior to weaning, were persistently infected with BVDV. In summary, introduction of PI cattle to a group of BVDV-naive heifers 50 days prior to the breeding season did not negatively impact reproductive performance. To the contrary, the active immunity that developed following field exposure to BVDV provided effective reproductive and fetal protection during the breeding season and subsequent gestations, despite continuous exposure to PI animals until approximately midgestation. Although BVDV can have potentially devastating reproductive effects, timing of infection is a critical determinant in the outcome of a BVDV infection. A controlled breeding season with introduction of herd additions at less critical reproductive time points can mitigate the negative reproductive health consequences of BVDV.