The factors that leave feedlot calves at high risk for bovine respiratory disease complex, or BRDC, have been studied and are fairly well understood. However, no large-scale studies have been reported that investigate the management factors that could be associated with higher rates of BRDC in preweaned calves in cow/calf operations. So, Kansas researchers drew from the extensive real-world data collected during USDA’s regular National Animal Health Monitoring System surveys of beef producers. USDA’s Beef 2007–2008 study generated data both on management factors and on the rate of disease occurrences for  cow/calf herds. The Kansas State study used those data to provide a unique assessment of potential statistical associations between cow/calf herd management practices and the rate of BRDC in preweaned calves.

The study, published in the May 1 issue of the Journal of the American Veterinary Medical Association, surveyed a total of 443 beef operations from an initial pool off 4,001 producers contacted to participate, based on their willingness to respond and whether they had any calves on the operation any longer and could give them feedback on BRDC. The median number of beef cows on the 443 operations was 128, with a range of from two to 5,847. The study authors used a sophisticated statistical model to try to account for the impact of several variables at once and to predict the chances of BRDC occurring based on those variables.

They found several management factors did not significantly predict whether pre-weaning calves on an operation would have BRDC--some of which do not square with conventional wisdom. They were:

  • Herd size.
  • The proportion of cows under 5 years old.
  • How often sick cows were housed in the calving pen.
  • Vaccination of calves between 22 days old and weaning with a 4-way viral vaccine.
  • Whether a veterinarian had been consultation with the herd manager about disease prevention.
  • The number of specifically defined breeding seasons.
  • Separating cow/calf pairs from pregnant cows.
  • Average number of days from birth to dehorning.
  • Creep feeding.
  • AI-ing cows.
  • Body condition scoring.

The final multivariable model included 6 independent variables that were found to be significantly associated with herd-level BRDC rates. They were:

Feeding antibiotic feed additives to calves to prevent respiratory disease was associated with higher rates of BRDC. You have to careful about evaluating what this association means, the study authors cautioned. Because we can’t know the timing of the BRDC vs. the antibiotic administration, it’s entirely possible the antibiotics were given in response to present or previous BRDC outbreaks. Or, the association they observed could be caused by other unmeasured factors. However, it is possible that feeding antimicrobials to preweaned calves is somehow a true risk factor for higher BRDC.

Bringing bred heifers into a herd was associated lower BRDC rates, while bringing in weaned steers was associated with higher rates. The researchers theorized that producers marketing relatively higher-value bred heifers may be more likely to put them through a complete health program that includes vaccination and management of genetic factors like dystocia and udder conformation, which other studies have associated with BRDC in young calves. At the same time, it’s logical to believe producers buying bred heifers are stricter about health histories and biosecurity than they are when buying weaned steers, which could explain why bringing in steers might increase risk of BRDC. It’s also believable that purchased steers are more likely to be assembled from multiple sources and market channels where they get exposed to several disease organisms. Higher BRDC rates among herds in which steers were imported could be because the preweaned calves were exposed to new BRDC pathogens not already present on the operation or because they were exposed to higher levels of them.

The presence of visitors on the operation was associated with increased BRDC risk, although there was some confusion surrounding the number of visitors. Among herds that received at least one visit per month, they found that herds that received both one or two visits per month or more than 30 visits per month had higher BRDC rates than those receiveing between three and 29 visits a month. It’s possible these effects show personnel at operations with few visitors are less concerned with biosecurity than personnel at operations with moderate numbers of visitors. Or they could indicate large numbers of visits may overwhelm your biosecurity programs. Because the questionnaire didn’t ask the producers to differentiate between visits involving the cattle vs. visits that never left the kitchen table, it’s hard to say exactly why the effect showed up. Plus, they warn, the number of visitors could simply be a proxy for some other factor that increases BRDC risk.

Operations that had calves that were two- or three-breed crosses or composites had higher BRDC rates compared with operations that had single-breed calves. Because the Kansas State study drew data from across the nation, the authors note, it’s not likely this effect was related to genetic traits of the single-breed herds. More likely: Other management practices affecting the incidence rate were present that somehow differed along single- vs. multiple-breed operations.

Operations that said beef cattle provided only a supplemental source of income had lower BRDC rates than those where cattle were the primary income source or only a hobby. The authors speculate that the reason is logical for the hobby operations:  Because of the potential added importance of calf sales in supplemental-income herds, it’s possible owners were on the lookout for BRDC and more likely to catch and prevent it when compared against pure hobby cattlemen. However, the reason why herds relying primarily on cattle sales for their living were more likely to experience BRDC is less clear. It could be simply that having cattle as a primary income source was also associated with simply being a bigger herd, and increased herd size could be associatied with greater within-herd exposure to BRDC-causing organisms.

Although the study gives some interesting insights into the factors that might be leading to BRDC in cow/calf operations, the study’s own authors caution you to take them with a grain of salt. Because we know nothing about whether the risk factors occur before, after or during BRDC episodes, you can’t assume any of them actually cause BRDC. However, understanding the association of risk factors may help identify future opportunities to control or prevent the disease.