The number of calves in the enclosure was dichotomized into group or individual housing type

The distribution of storage time for colostrum before feeding in hours underwent a square root transformation to achieve normality. If colostrum was stored for less than 2 h, zero hours were recorded for colostrum storage time. Storage temperature for colostrum was recorded as either room temperature , refrigerated, frozen, or first refrigerated then frozen. A variable for colostrum storage temperature had indicator variables for each of the 4 storage temperatures. The type of storage container for colostrum was dichotomized as solid or bags, as thawing frozen colostrum stored in bags may achieve a more uniform increase in colostrum temperature compared with colostrum stored in plastic bottles and may result in less heat damage to immunoglobulins. The variable for percent of colostrum fed that is from firstcalf heifers was dichotomized into the categories any or no colostrum from first calf heifers. The amount of colostrum fed in the first 12 h was dichotomized into <2.84 L or ≥2.84 L , as feeding 3 L of colostrum was considered a relevant biological cutpoint . Four dairies let calves nurse from their dams to meet the calves’ colostrum needs; hence, estimates for the amount of colostrum fed in the first 12 h on these dairies could not be made. In addition, a dichotomous variable for whether or not colostrum was tested for immunoglobulin content and a dichotomous variable for whether or not calves were assessed for failure of transfer of passive immunity based on serum total protein were explored in the model.Data to create variables concerning housing of calves were drawn from the questionnaire sections that captured herd-level data as well as from data collected at the individual calf level.

Questionnaire Data. Proportions of calves housed individually or in groups,rolling flood tables average group size of grouphoused calves, housing types, weaning age, age when moved from individual hutches to group housing, percent sick preweaned calves moved to a hospital pen, clinical signs to detect BRD, and treatment choices for BRD were used only for descriptive statistics purposes. This was done because these variables provide information about the whole cohort of preweaned calves and are not suitable for analyzing the association of management practices and BRD in individual calves. Data on the percent of calves raised on the premises from other dairies was dichotomized into any or no calves from other dairies on the premises. The variable feeding order by age was created with indicator variables for the answer choices youngest typically fed first , calves fed in no particular order, and oldest typically fed first. Only 1 dairy in the sample used a BRD scoring system; hence, the variable describing use of such systems was dropped from analysis. As some dairies gave multiple answers to the question about what surfaces, driven on daily, cover the roads adjacent to the calf-raising area, dichotomized variables for paved, gravel, dirt, or other surfaces were created and analyzed as separate variables. Similarly, dichotomized variables for dust-abatement procedures were created describing whether or not water, magnesium chloride, or other dust-abatement measures were implemented . For the question on how often dust was a problem in the area where preweaned calves were housed, a categorical variable was created with never as the reference and the indicator variables rarely , no week passes by without dust, daily during the nonrainy season, and daily all year round. Calf-Level Data.

Whether calves had calf-to-calf contact or not was recorded as a dichotomous variable. Hutch space was defined as the product of the width and depth, as measured during the visit. A categorical variable was created, categorizing the data as less than , greater than, or within 1 standard deviation around the mean space for wooden hutches in the sample population, and a separate category for group pens. Hutch elevation from the ground was dichotomized into elevated or not . Dichotomous variables for each of the reported floor materials were specified where absence of the specific floor material was the R [concrete, rubber, sand, Tenderfoot , dirt, grass, wood, and bedding]. In addition, dichotomous variables for slatted floors and floors that allow airflow were specified. Wall materials were categorized with indicator variables for only wood , only metal, only plastic, and a category for a mix of materials or group pens. Fourteen dairies had at least some hutches that were made of a combination of materials, such as wood and metal or metal and plastic. Calves housed in groups were added to the latter category to be able to assess the association of a single hutch material on an individually housed calf without losing group-housed calves or calves in hutches made from a mix of materials in the analysis. In addition, a dichotomous variable for solid walls or nonsolid walls was specified to explore an association with air flow in hutches and BRD. A categorical variable describing the presence and type of flush system under the calf hutches was specified with 3 levels: no flush , fresh water flush, or flush with lagoon water. A categorical variable describing the presence and type of extra shade structures above calf hutches was specified with levels no cover , partial roof, complete roof without side walls, complete roof with 1 to 3 side walls, and fully indoors.

The calving space in square meters per cow was calculated by dividing the average number of cows per pen by the dimensions of the maternity pen. Calving space was normalized using the natural log transformation. The percent of calving area that is pasture was dichotomized into any or no calving on pasture . The number of times the bedding is changed in the calving pens was dichotomized into less than or equal to 5 times and more than 5 times per month. Typical maternity pen bedding type was summarized into categories of plant fiber , recycled manure , gypsum or lime, dirt , and pasture. Those dairies that answered no maternity pen exists or no bedding in maternity pen were assigned to the category sand or dirt. Dichotomous variables were created for the presence or absence of each type of bedding,flood and drain tray as multiple dairies used more than 1 type of bedding. The data for the percent of calves removed in 1 h from the dam was dichotomized into ≥95 and <95% . Vaccine timing and frequency for cows and calves was dichotomized into whether a type of vaccine was given or not . The time since vaccination for each calf was estimated as the time between the reported age when calves receive a certain type of vaccine as stated by the interviewee in the questionnaire and the age of the calf at the time of visit. Dichotomous variables were created describing whether a certain type of vaccine was given at least 7 d before the visit to account for the fact that there is a variable lag time between vaccine administration and an immune reaction. Hygiene scores for up to 15 cows present in the maternity pen during the dairy visit were transformed into cumulative hygiene scores, which facilitated comparisons of percent of cows scoring 1, 1 and 2, or 1 and 2 and 3 versus the percent of cows scoring the respective remaining scores.Questions that could be answered with yes or no resulted in dichotomous variables including if milk was ever tested for bacterial content both before pasteurization and after pasteurization and if any medication was added to milk or milk replacer . The source of milk for each diet fed on the study dairies was recorded as a percentage of the total diet during the farm visit. For the analysis, these percentages were converted into dichotomous variables per milk source describing whether a calf’s diet consisted of >90% of one of the sources saleable milk, waste or hospital milk, unmedicated milk replacer, or medicated milk replacer for at least 7 d before the farm visit or not . No distinction was made between waste milk from fresh cows and milk from cows in a milk-withdrawal period due to medication. One dairy fed waste from a yogurt producer, which consisted of a water-yogurt mixture created during flushing of the production lines. A separate dichotomous variable for feeding of the yogurt waste product was created. The variable describing whether milk fed had been pasteurized contained a category for weaned calves to avoid exclusion of weaned calves from the model. For the number of times calves were fed milk per day, a categorical variable was created with twice feeding as the reference and levels of once daily as well as a level including 3 or 4 times or free choice feeding.

Two calves in the study were suckling from the dam based on their age and management of neonatal calves described by the producer and had no amount of milk fed or number of times fed assigned. The total volume of milk fed per day, which was calculated from the number of times calves were fed milk and the volume offered at each feeding, was categorized into ≤2.84 L , between 2.84 L and up to 5.68 L , and >5.68 L. Finally, dichotomous variables for whether or not antimicrobial drugs, vitamins, or electrolytes were added to the milk were explored. The type of medication used to treat BRD was used for descriptive statistics only.The current study is the first to evaluate BRD in preweaned calves housed on dairies throughout California and to associate BRD in these calves with management practices. Our results suggest that management relative to both housing and calf feeding practices may be the most important areas associated with the prevalence of BRD in young dairy calves on California dairies. Using lagoon water for flushing under hutches, the use of metal hutches, as well as calf-to-calf contact in older calves, and feeding Holstein calves ≤2.84 L of milk or replacer per day were all positively associated with BRD. Pasteurizing milk, feeding saleable milk, feedingJersey calves more than 5.68 L of milk or replacer per day, and providing extra shelter were all negatively associated with BRD. Interestingly, no associations were found between vaccinating dams or calves or the amount or quality of colostrum fed and BRD. Management factors significantly associated with BRD in our study may reflect some of the areas where the California dairy industry is less uniform and where changes may have the biggest effect on BRD prevalence. Current knowledge based on numerous studies underscore colostrum management and vaccinations as important components of BRD prevention; however, the current study’s dairies may have exhibited uniform colostrum and vaccination practices. The principal component analysis of a California survey on management practices related to bovine respiratory disease in preweaned dairy calves performed in 2013 identified calf housing as the component construct with the largest variation proportion . The component construct contained the variables hutch style, calf contact, flush used, and waste milk fed to heifers. Three of these variables were significantly associated with BRD in our study, supporting the idea that these are areas of highly variable management on California dairies. Although we did not observe an association between the fourth variable in the construct and BRD in our study, we did observe a negative association between BRD and feeding saleable milk for at least 7 d, which is one of the alternatives to feeding waste milk.The state-average dairy herd size in California in 2015 was 1,215 milking cows , which is less than the average herd size of 1,718 milking cows but close to the median herd size of 1,100 milking cows in our study. Herd sizes are expected to increase , so the calves enrolled in our study likely represent a growing proportion of dairy calves raised in California. Nationally, Holsteins make up 86.0% , Jerseys 7.8% , and other breeds 6.2% of US dairy cows , compared with 81.6% Holsteins, , 13.1% Jerseys , and 5.3% crossbred calves in our study. Jersey cows have increased in popularity over the last years in California due to their higher milk fat content and potential for higher feed-to-milk conversion ratios among other health related reasons . Although no official statistics for California dairy breeds are available, it may not be unreasonable to assume that Jersey cows are more common in California than nationally.