Bovine respiratory disease (BRD) is the most common and economically detrimental disease of cattle during the postweaning phase, causing approximately 75% of morbidity and over 50% of mortality in feedlots (Edwards, 1996; Smith, 1998). Respiratory disease costs the beef industry more than $690 million annually (National Agricultural Statistics Service, 2006). Literature clearly supports the negative impacts of BRD on cattle performance and economics (Martin et al., 1989; Gardner et al., 1999), yet research evaluating the effect of disease timing is rare. A recent study compared early and late BRD in 2 South African feedlots and found that cattle treated for BRD early in the feeding phase (<d 35) experienced a decrease of 0.22 kg in ADG compared with cattle without disease, whereas cattle treated late in the feeding phase (>d 35) experience no significant reduction in ADG relative to untreated cattle (Thompson et al., 2006). These findings suggest that the time of treatment for BRD affects cattle performance.
Our study is unique because we used individual animal data from a US feedlot to evaluate the effect of the timing of initial BRD treatment on cattle performance. Our hypothesis was that the time of identification and treatment of BRD during the feeding phase affects performance outcomes. This was tested through statistical analyses that considered time relative to arrival or slaughter. Multiple outcomes could be used to compare biological effects of disease timing, but we initially utilized a standardized net return measure as the outcome variable to incorporate all available individual animal performance and health data. The net returns figure is a screening metric to identify differences that could be due to multiple biological factors. Our objectives were to determine whether cattle performance and health were associated with the number of weeks from arrival to initial BRD treatment or the number of weeks from BRD treatment to slaughter.
MATERIALS AND METHODS
Animal Care and Use Committee approval was not obtained for this study because these data were obtained retrospectively from the feedyard.
Data were acquired from a Midwestern US feedlot and included cattle slaughtered between January 1, 2001, and February 2, 2006. The data set consisted of individual animal performance, health, and carcass records of 37,078 cattle treated for respiratory disease using the standard treatment protocol designed by the consulting veterinarian. Only cattle identified and treated for respiratory disease before 100 d after arrival were included in the data set (n = 31,131). Data for all cattle included individual identification number, lot, sex, pen, risk code (as assigned by feedlot), arrival date, arrival BW, morbidity data (number of times treated, date(s) treated, and diagnosis), mortality data (yes or no, date), slaughter date, HCW, quality grade, backfat, LM area, yield grade, as well as KPH. Sex, pen, and risk code were recorded on a pen basis, and all other variables were collected and reported on individual animals. Risk codes were assigned by feedlot personnel, which classified each incoming lot of cattle as to expected health risk based on cattle history, including weaning status, and types of preconditioning programs. Diagnosis at initial treatment was determined and recorded by feedlot personnel responsible for daily health management of the cattle. Cases were identified based on standard industry procedures including evaluation of animal appearance, demeanor, and body temperature.
Data were imported into Insightful Miner (Version 7, Insightful Corp., Seattle, WA), where each record was validated and new variables were created for the analysis. The validation procedure eliminated records if performance or carcass data were incomplete, days on feed at first treatment was past 100 d, or if diagnosis at initial treatment was not BRD. Data also were evaluated for unrealistic (i.e., negative number of days between treatment and slaughter) or indeterminable values (lot sex marked as mixed); data meeting these criteria were removed from the analysis. Heifers with arrival BW greater than 454 kg were removed from the data set because historical prices for these cattle were not available. Records for cattle that were treated for BRD and died before slaughter were removed from the economic analysis comparing cattle performance. The criteria above resulted in the elimination of 5,947 animals.
New variables were created for the analysis using the existing data. Calculated yield grade (CYG) was generated using the USDA yield grade equation (Busby and Loy, 2000), based on the reported HCW, backfat thickness over the 12th rib, KPH, and LM area of each animal. Individual final shrunk live pay BW was provided in the data set from the feedlot and used to calculate ADG over the feeding period. Arrival BW were utilized to place each animal into a categorical arrival BW variable. The variable was based on US feeder cattle BW price ranges consisting of 136 to 180 kg (3WT), 181 to 226 kg (4WT), 227 to 272 kg (5WT), 273 to 318 kg (6WT), 319 to 363 kg (7WT), 364 to 408 kg (8WT), and greater than 409 kg (9WT).
Two categorical variables were created to represent the amount of time between initial BRD treatment and either arrival or slaughter. The first variable used weeks on feed from arrival to initial BRD treatment to place each animal in categorical time variable (WKFA), which consisted of 14 categories (wk 1 through 14 post-arrival). The second variable measured weeks on feed from initial treatment for BRD to slaughter (WKTS) and was broken into 44 weekly categorical variables (wk 1 through 44 from treatment to slaughter). Descriptive statistics of the data were then generated using JMP (JMP Version 5.1, SAS Inst. Inc., Cary, NC).
An economic model was created to incorporate performance and health into a single figure allowing comparisons between cattle using similar methodology to previous studies of feedlot health (Gardner et al., 1998; White et al., 2007). The model calculated individual animal net returns, minimizing economic variation due to seasonal or long-term market conditions and reflecting differences related to cattle performance. Net return for each animal was calculated [Eq. 1] using the individual sex of the animal, arrival BW class, days on feed (arrival to slaughter), number of treatments, ADG, and carcass characteristics.
Base price was the average USDA boxed beef cutout value from November 1996 through April 2005 (Livestock Marketing Information Center, 2008a). The grid price was the average USDA monthly cattle premiums and discounts for slaughter steers and heifers over the same time period (Livestock Marketing Information Center, 2008b). Total expenditures represent the sum of health, processing, yardage, and feed costs. Treatment costs were fixed at $11.09/treatment (National Animal Health Monitoring System, 2000), processing at $20.00/animal, yardage $0.20/animal per day, and feed costs at $0.88/kg of gain. Feeder price was the average USDA Oklahoma City monthly feeder calf price aggregated by BW class from January 1996 through February 2005 (Livestock Marketing Information Center, 2008c). Using the entrance and slaughter price averages over this time period allowed us to account for feeder and fed price variability associated with multiple stages of the cattle cycle.
Before dead cattle were removed from the data set, logistic regression models using the GLIMMIX procedure (SAS Inst. Inc.) were used to determine if case fatality risks were (P < 0.05) different between WKFA categories for each BW class. Because the data consisted entirely of cattle treated for BRD, case fatality risks were considered as any mortality after initial treatment divided by total number of initial treatments for each weekly category. For each arrival BW class, models included the proportion of treated cattle that died (case fatality) for each weekly category as the dependent variable, WKFA as the independent variable, and a random effect to account for lack of independence among cattle within the same pen. Case-fatality risks were not assessed for WKTS because there were no slaughter dates for mortalities. Thus, no WKTS category could be assigned.
To assess the factors associated with differences in estimated net returns, generalized linear mixed models at the individual animal level were developed using the GLIMMIX procedure in SAS. A random effect for pen was used in all models to account for the lack of independence among individual animals within the same pen. Month, year, and the interaction between month and year were also included as random effects. Model-adjusted means were used to compare levels of effects that were significant at the P < 0.05 level. The first analysis investigated differences in net returns based on fixed effects representing sex, WKFA, risk code, arrival BW class, and the interaction between WKFA and all other fixed effects. We determined a priori that our objective was to assess effects related to the timing of initial treatment for BRD, and based on standard model building strategies for observational data we created 2-way interactions only involving the exposure variable of interest (Dohoo et al., 2003). Therefore, 2-way interactions between WKFA and all other fixed effects were investigated. The results from this initial model indicated a significant (P < 0.05) interaction between WKFA and BW class.
The same analysis described above was performed using the WKTS variable instead of WKFA. Results from the initial WKTS model indicated an interaction between WKTS and BW class (P < 0.05). However, due to sparse data in certain weeks for some BW classes, the model-adjusted mean values for the interaction between these effects could not be estimated. Therefore, we partitioned the data set by BW class strata (Jewell, 2004) and utilized 7 models, 1 for each BW class, for WKFA and WKTS to assess the effects of disease timing separately within each BW class.
To further investigate observed differences in estimated net returns, we assessed ADG, CYG, HCW, days on feed, and number of times treated across both WKFA and WKTS categories using the same models as for net return values. In brief, generalized linear mixed models were used to model each arrival BW class with random effects for pen, month, year, and the interaction between month and year. Potential differences in the respective performance or health variables were based on individual animal sex, WKFA or WKTS, risk code, and arrival BW class. Therefore, each performance model was estimated once with WKFA as an explanatory variable and once with WKTS as an explanatory variable. In addition, the proportions of cattle grading USDA Choice or above for each respective week category were analyzed with logistic regression models using the GLIMMIX procedure in SAS. The probability of grading Choice or above for each animal within each week category was modeled as the dependent variable, and either WKFA or WKTS was the independent variable. These models included a random effect to account for lack of independence among cattle within the same pen.
The number of cattle treated within each BW class are displayed in Table 1; descriptive statistics for performance measures, carcass traits, and each BW class are presented in Table 2. Days on feed at first treatment averaged 30 d, and the population distribution is plotted by WKFA in Figure 1. Days on feed from treatment to slaughter averaged 161 d with 71.9% of cattle treated between wk 31 and 16 from slaughter (Figure 2). The quality grades of the 31,131 cattle were 59% select, 35% choice, 4.9% standard, 0.2% prime, and 0.2% other (hard bone, dark cutter). The overall morbidity and mortality in the feedlot during the study period was 17.3 and 0.80%, respectively. Of the cattle in this study, 1,332 died, resulting in an overall case fatality risk of 4.27%. Case fatality risk was associated (P < 0.05) with WKFA for all BW classes but displayed no apparent ascending or descending pattern across the weekly categories.
Effects of WKFA on Estimated Net Returns
The initial analysis of estimated net returns indicated that sex, risk code, BW class, and the interaction of BW class and WKFA were all significant (Table 3). For the estimated net returns BW class-specific models, the 3WT cattle displayed a main effect (P < 0.05) of sex, whereas the 4WT, 5WT, and 6WT models indicated both sex and WKFA as main effects (P < 0.05). For the 7WT cattle, sex, risk code, and WKFA were all associated with net returns (P < 0.05), yet for 8WT cattle only WKFA was associated with net returns (P < 0.05). Only risk code (P < 0.05) was associated with estimated net returns for the 9WT cattle.
Our primary interest in assessing differences in estimated net returns was the measure of time relative to arrival (WKFA). Of the 7 BW classes examined, 5 (4WT, 5WT, 6WT, 7WT, and 8WT) displayed a WKFA effect (P < 0.05). The 4WT cattle had very few differences between WKFA categories, yet wk 1 and 2 were less (P < 0.05) than wk 5, 7, 10, and 13 (Figure 3). The 5WT and 6WT cattle exhibited decreased estimated net returns (P < 0.05) during wk 1 compared with any of the following week during the first 5 wk on feed (Figure 3). Estimated net returns for the 6WT cattle were (P < 0.05) less late in the feeding phase (wk 12 and 14) as compared with early in the feeding phase (wk 2, 3, 4, and 5; Figure 3). The other BW classes (7WT and 8WT) displayed the least net return figures late in the feeding phase with no differences in earlier weeks. The 7WT cattle had decreased estimated net returns (P < 0.05) during wk 14 compared with all previous weeks with the exception of 11, 12, and 13 (Figure 3). The 8WT cattle had decreased estimated net returns (P < 0.05) wk 12, 13, and 14 as compared with any proceeding weeks during the feeding phase (Figure 3).
Effects Associated with Individual Performance and Health Outcomes (WKFA)
For the models of ADG, the 3WT cattle displayed differences (P < 0.05) associated with sex, whereas the 4WT cattle displayed differences (P < 0.05) associated with sex and risk code. The 5WT, 6WT, 7WT, and 8WT cattle had differences (P < 0.01) based on sex, risk code, and WKFA. The 9WT cattle had differences (P < 0.05) in ADG due to WKFA. For all CYG models, there was no effect of WKFA (P > 0.05), but several other effects were significant at P < 0.05. For the 3WT and 5WT cattle, sex was an important effect (P < 0.05), whereas sex and risk code were important effects in the 4WT and 6WT models (P > 0.05). For the 7WT cattle only, risk code was the only effect associated with CYG (P < 0.05). The 8WT and 9WT had no effects associated with CYG (P > 0.05). For the HCW models, the 3WT, 4WT, and 9WT analyses showed difference associated with sex (P < 0.01). The model for 5WT, 6WT, 7WT, and 8WT cattle exhibited effects (P < 0.05) of sex and risk code. With days on feed as the dependent variable, risk code was significant (P < 0.05) for the 3WT cattle. The 4WT, 7WT, and 8WT cattle had effects (P < 0.05) of sex and risk code. Both 5WT and 6WT analysis indicated effects (P < 0.05) of sex, risk code, and WKFA. Finally, the 9WT analysis of days on feed exhibited an effect (P < 0.05) of sex. Number of times treated was considered a health outcome for each animal and differed by WKFA (P < 0.05) for all BW classes except 9WT. The times treated for 5WT cattle also had an effect (P < 0.01) of sex. The final performance outcome analyzed was the probability of grading Choice or above within each week category. For all BW classes, there were no differences (P > 0.05) between the probability of an animal grading Choice between WKFA weeks.
The effect of WKFA on performance and health outcomes that could be contributing to differences in estimated net returns was our primary interest. The 4WT cattle had a greater (P < 0.05) number of treatments when initially treated early in the feeding phase (wk 1 and 2) compared with cattle treated later in the feeding phase (Figure 4). For the 5WT cattle, ADG was greater (P < 0.05) wk 1 as compared with any subsequent week with the exception of wk 3, 9, 11, and 14. Hot carcass weight was greater (P < 0.05) for 5WT cattle treated wk 9 compared with other weeks with the exception of wk 10 and 11 (Figure 5). Days on feed were greater (P < 0.05) for cattle treated wk 2 and 9 compared with other weeks with the exception of wk 7, 10, 11, 12, and 13. In addition, number of times treated was greater (P < 0.05) early in the feeding phase (wk 1 to 3) compared with subsequent weeks (Figure 4).
The 6WT cattle had increased ADG (P < 0.05) if treated wk 1 compared with all subsequent weeks except wk 4 and 13. Cattle treated wk 14 displayed decreased HCW as compared with earlier weeks with the exception of wk 12 (Figure 5). These cattle also had a decrease (P < 0.05) in days on feed if treated wk 14, compared with others except wk 7, 9, 11, and 12. Finally, the number of times treated was different (P < 0.05) between the first 3 wk of the feeding phase (greater early and then decreasing), and the treatments for the first 3 wk were all greater (P < 0.05) than any subsequent weeks (Figure 4).
For the 7WT cattle, ADG was greater (P < 0.05) for cattle treated wk 1, with the exception of wk 8 and 13. These cattle also had less (P < 0.05) HCW when treated late in the feeding phase (wk 11 and 14) (Figure 5). The 7WT cattle also were treated more times (P < 0.05) wk 2 as compared with all other weeks, with the exception of wk 3, 13, and 14 (Figure 4).
The 8WT cattle were the final group in which WKFA was associated (P < 0.05) with estimated net returns. There was an effect (P < 0.05) of WKFA on ADG, but there was no apparent ascending or descending pattern across weeks (data not shown). The 8WT cattle had less (P < 0.05) HCW if treated late in the feeding phase (wk 12 and 13) compared with other weeks, with the exception of wk 8, 9, and 14 (Figure 5). The number of times treated also was associated (P < 0.05) with WKFA. Cattle that were treated wk 13 had more treatments than those treated other weeks except wk 1, 12, and 14 (Figure 4).
Effects of WKTS on Estimated Net Returns
In the initial analysis assessing effects on net returns related to weeks from treatment to slaughter, sex, WKTS, BW class, the interaction between BW class and WKTS, and the interaction between sex and WKTS were all significant. Data were then stratified and effects within BW classes assessed individually. The 3WT and 9WT cattle displayed a main effect (P < 0.05) of sex, whereas the 4WT and 7WT analyses exhibited effects (P < 0.05) of sex and WKTS. The 5WT, 6WT, and 8WT cattle all displayed effects (P < 0.05) of sex, risk code, and WKTS. All BW classes that had a WKTS effect (P < 0.05) and displayed a descending pattern in estimated net returns (Figure 6). When examining the 4WT cattle there were several WKTS (wk 1, 2, 3, 5, 6, 7, 43, and 44) in which no cattle were treated for BRD. Results indicated that cattle treated for respiratory disease wk 39 displayed greater estimated net returns (P < 0.05) compared with cattle that were treated wk 11 (Figure 6). Cattle in the 5WT class treated for respiratory disease during weeks further from slaughter also demonstrated greater estimated net returns (wk 44 to 42) compared with wk 5 (Figure 6). The 6WT cattle treated during wk 43 exhibited greater estimated net returns (P < 0.05) in comparison with cattle treated wk 12 to 4 (Figure 6). For 7WT cattle, there were weeks far from slaughter (wk 44, 42, 41, and 40) in which no treatments occurred, but data still indicated a very similar pattern in net returns to the 6WT cattle, with greater estimated net returns (P < 0.05) during wk 32 to 24 compared with wk 4 to 1 from slaughter (Figure 6). The final BW class that displayed a significant WKTS effect was the 8WT cattle, and these had several weeks that were not able to be assessed due to the fact that no cattle were treated for respiratory disease during wk 44 to 36 and 34 to 33. However, estimated net returns for the 8WT cattle still display a similar pattern as the BW classes previously discussed as wk 25, 24, 23, 21, and 20 were all greater (P < 0.05) than wk 4 and 1 (Figure 6).
Effects Associated with Individual Performance and Health Outcomes (WKTS)
Analyses on performance and health outcomes by each arrival BW category indicate several notable findings. When ADG was the dependent variable, the 3WT cattle displayed effects (P < 0.01) of sex and WKTS. For the 4WT, 5WT, 6WT, 7WT, and 8WT cattle, effects (P < 0.05) included sex, risk code, and WKTS. The 9WT cattle had no important effects (P > 0.05).
With CYG as the dependent variable, sex had an effect (P < 0.01) on the 3WT cattle. The sex effect plus WKTS were associated (P < 0.01) with CYG for 4WT cattle. Sex, risk code, and WKTS were all associated (P < 0.05) with CYG for 5WT cattle. For the 6WT and 7WT cattle, risk code and WKTS were both important effects (P < 0.05). The 8WT cattle had WKTS as an effect (P < 0.05), and the 9WT had no important effects (P > 0.05).
For the HCW outcome, the 3WT, 4WT, and 9WT cattle were affected (P < 0.01) by sex and WKTS. The 5WT, 6WT, and 7WT had those effects plus risk code (P < 0.01). When days on feed was the dependent variable, the 3WT and 5WT cattle had effects (P < 0.05) of risk code and WKTS. Weeks on feed from treatment to slaughter was associated (P < 0.05) with HCW for the 4WT and 9WT cattle. Finally, the 6WT, 7WT, and 8WT cattle had effects (P < 0.05) of sex, risk code, and WKTS. With number of times treated as a health outcome, the 3WT and 9WT cattle had no associated effects (P > 0.05). The 4WT, 5WT, 6WT, 7WT, and 8WT cattle all had WKTS as an effect (P < 0.05). Analysis of Choice or above grading within the category of each week could not be performed using WKTS as the independent variable because data were too sparse among many week categories. Therefore, differences in carcass performance across weeks on feed from treatment to slaughter were not evaluated.
The effect of time from treatment to slaughter (WKTS) was our primary interest for assessing differences in performance and health outcomes that could be contributing to differences in estimated net return. Models with ADG, CYG, HCW, days on feed, and number of times treated as dependent variables all had associations (P < 0.05) with WKTS. The 4WT to 8WT cattle all displayed an ascending pattern in ADG between WKTS categories with differences (P < 0.05) among weeks throughout the feeding phase (going from treatment to slaughter). For the performance models with CYG as the dependent variable, there was no apparent pattern between WKTS categories, although there are some differences (P < 0.05) among weeks for the, 4WT, 5WT, 6WT, 7WT, and 8WT cattle. In contrast to the ADG models, HCW were descending across WKTS categories (going from treatment to slaughter) for 4WT, 5WT, 6WT, 7WT, and 8WT cattle. Days on feed from entry to slaughter also displayed a descending pattern across WKTS categories, with differences (P < 0.05) among weeks throughout the feeding phase for the 4WT, 5WT, 6WT, 7WT, and 8WT cattle. Because the pattern of relationships between performance measures and WKTS were similar across all BW categories, we used results from the 5WT analysis for illustrative purposes (Figure 7).
Our research provides unique information on performance and health outcomes associated with the timing of BRD treatment during the feeding phase. Other research evaluating the impact of BRD on performance and health factors has focused on comparing healthy and sick cattle over the entire feeding phase (Gardner et al., 1999; Roeber et al., 2001). We found that performance and health measures differ based on when cattle are first treated from BRD relative to arrival and slaughter, and these associations depend on the arrival BW class. Our conclusions are based on retrospective individual animal data from over 5 yr that include multiple measures of performance and health, and analyses that use estimated individual animal net returns as a standardized screening metric to identify potential biological differences.
The temporal distribution of initial BRD treatments in our data set was similar to findings from other research. Thompson et al. (2006) found 87% of first treatments occurred within first 35 d and Faber et al. (1999) described 81% of first treatments within the first 42 d. Martin and Meek (1986) indicated that BRD cases peaked between d 7 and 14 on feed and then declined. In our data, 74% of cases occurred in the first 42 d and BRD cases peaked during wk 2 postarrival and then declined. When examining data from treatment to slaughter (WKTS), the cases appear to be normally distributed. This is not surprising given that our data set included cattle from a wide range of initial BW, resulting in variation in the total days on feed. Therefore, although most cattle were treated relatively close to arrival (WKFA), the WKTS varied greatly based on initial BW of cattle. Evaluations of the amount of time between treatments until slaughter have not been described in the literature. The feedlot that provided our data does not slaughter all cattle from the same arrival lot on the same date, but rather slaughters subsets of the cattle as they finish. Data for our study represent only cattle identified and treated for BRD within the first 100 d of arrival in a single commercial feedlot. Results should be applied carefully because factors influencing the outcomes may vary by feedlot. Also, our data were analyzed as a cross-sectional observational study, and thus no direct causal inferences can be drawn. Further research utilizing data from multiple different feedlots and evaluating cattle treated at different times and with different treatment protocols would provide additional insight into the effect of disease timing on health and performance.
We included placement month and year to adjust for variation in seasonal and management changes over the time period in the standardized economic model. Previous literature illustrates that cattle sex, season, and year affect cattle performance and should be accounted for in an analysis of this nature (Schake, 1996). Cattle that died after treatment did not have performance records, and the economic loss from death causes high variability in net returns. Therefore, we removed dead cattle and just compared data on slaughtered cattle, which allowed us to efficiently address our objectives, but does not allow us to assess the economic impact of mortality. We did assess the case fatality risks for studied cattle and found few differences between WKFA categories. However, visual evaluation of the data did not reveal a consistent ascending or descending pattern to the case fatality rate between WKFA categories; therefore, we believe our findings are still indicative of the population as a whole. We were unable to evaluate case fatality risks by WKTS because cattle that died did not have the slaughter date necessary to assign a WKTS category.
Assessment of Effects of WKFA
The analysis evaluating the timing of BRD treatment after arrival illustrated an interaction between WKFA and arrival BW class. Although WKFA was associated with net returns for the 4WT cattle, there were few major differences among weeks. For the 5WT cattle, net returns in wk 1 were less than any other week within the first 5 wk on feed. When comparing common performance measures (e.g., ADG, HCW) there were very few differences between WKFA categories for 5WT cattle and none that would explain the difference in estimated net returns. The fact that we found few significant differences in ADG in this BW category might be explained by the length of recovery time cattle had from disease identification to slaughter. Earlier work by Thompson et al. (2006) found that BRD treatment status affected ADG early in the feeding phase (<d 35) more than it affected overall ADG. Cattle entering the feedlot as 5WT had an average of 220 d from arrival to slaughter and displayed very few significant differences in performance variables (ADG and HCW) across WKFA categories. Days on feed did not differ across WKFA categories, which illustrates that all cattle in this BW class had the same potential recovery time from BRD treatment to slaughter.
A previous study by Faber et al. (1999) showed that the average number of times that steers with BRD were treated over the entire feeding phase was 1.7. In our data set, the average number of times treated over the feeding phase for both steers and heifers was similar at 1.6 times. However, in our study 5WT cattle treated for respiratory disease early in the feeding phase (wk 1, 2, and 3) were treated more times compared with those initially treated later weeks in the feeding phase. One potential explanation for the relationship is the number of days at risk for retreatment after the initial treatment of respiratory disease. Cattle identified and treated early in the feeding phase have more days at risk for further disease or treatment relative to cattle treated later in the feeding phase. This offers some explanation for the apparent negative association between the number of times treated and WKFA. The 5WT cattle treated for respiratory disease wk 1 were treated an average of 1.9 times over the feeding phase compared with cattle initially treated during wk 4, which were only treated 1.3 times on average. The difference in model estimated net returns between wk 1 and 4 was $7.54. The economic difference attributable to the average times treated between wk 1 and 4 equates to approximately $6.65, which would account for the majority of the difference in estimated net returns among these weeks.
The 6WT cattle had decreased estimated net returns early (wk 1) and late in the feeding phase (wk 14). Performance measures again did not differ among weeks in the first 5 wk of the feeding phase. The decreased estimated net returns for cattle in wk 1 could be explained again by a difference in the number of times treated as similar to the 5WT cattle. The 6WT cattle differed from the 5WT cattle in that they also experienced decreased estimated net returns when treated late in the feeding phase. Cattle treated during wk 14 experienced less HCW as compared with all other weeks with the exception of wk 12. This difference in HCW was apparently driving the difference in estimated net returns for this week. Yield and quality grades were not different among the weeks; therefore, if we assume $2.58/kg of carcass weight (which is the average base price in the net returns model), then an average carcass from wk 4 would be worth $30.84 more than an animal treated wk 14. The difference in HCW would explain the disparity in estimated net returns between these weeks.
Heavier cattle (7WT and 8WT) displayed a different relationship between estimated net returns and WKFA than other BW classes because they only exhibited decreased returns over the final few weeks of the evaluation period. There were few differences among number of times treated and ADG, with no differences between days on feed, or CYG with regard to the 7WT cattle. However, the 8WT cattle had differences in the number of times treated. The only factor that was less during the final few WKFA categories for the 7WT cattle was HCW. Thompson et al. (2006) found that the greatest effect of respiratory disease on growth occurred during the early finishing period with little effect in the later period, but they were looking at all BW classes and not specifically 7WT and 8WT cattle, where recovery time to finish could be an issue. Research by Gardner et al. (1999) found that cattle that were treated for respiratory disease averaged HCW 7.5 kg less than cattle that were not treated. However, we found substantial variability in HCW among cattle treated in different weeks; up to a 12-kg difference was identified among weeks for all treated cattle, which suggests there may be substantial variability among sick cattle. For the 7WT cattle treated during wk 14, the model adjusted mean HCW of 338 kg was 12 kg less than the wk 4 mean HCW of 350 kg. The cause for this variation is unknown but may be the result of different disease processes associated with days on feed at initial treatment or disease misclassification. Regardless, this finding indicates that the timing of initial treatment for BRD affects HCW in 7WT cattle. There were no other carcass characteristics that differed. Therefore, with the base price of $2.58/kg, the HCW difference would equate to a $30.96 difference in carcass value, nearly accounting for the $37.56 difference exhibited in the estimated net return model. For the 8WT cattle, differences in estimated net returns between cattle treated late, as compared with earlier, may be attributed to both decreased HCW and an increase in the number of treatments.
Assessment of Effects of WKTS
An interaction between WKTS and arrival BW class was identified, likely related to the biological differences among cattle in different arrival BW. The lighter BW classes had fewer cattle treated close to slaughter (e.g., 4WT wk 1 to 7), as compared with heavier cattle that had very few cattle treated far from slaughter (e.g., 7WT during wk 44 to 37). Sparseness of data for these weeks likely caused the relationship between estimated net returns and WKTS category to differ by BW class. This effect was evident when we truncated the data set to only wk 10 to 35 (93% of the data) and the interaction of WKTS and BW class was no longer present (data not shown). For all 5 BW classes that exhibited a significant WKTS effect, there was a similar pattern of decreased estimated net returns when cattle were slaughtered closer to their first BRD treatment date and greater net returns when slaughtered further from initial treatment.
Four performance factors were likely influencing differences in estimated net returns between WKTS for all BW classes: times treated, HCW, days on feed from arrival to slaughter, and ADG. Hot carcass weight was less when cattle were treated closer to slaughter. The reduction in total BW gain (as judged by HCW) may be related to the decreased time between treatment and slaughter that cattle have to regain BW lost after the disease event. Roeber et al. (2001) found that calves visiting the hospital 2 or more times had decreased HCW compared with healthy cattle and no statistical difference compared with cattle treated only once. Our data showed that differences in HCW among sick cattle were associated with how long the cattle were on feed from treatment to slaughter.
Days on feed tended to be less as cattle were treated closer to slaughter. Inferences from this should be carefully interpreted because treatment date could influence the slaughter date, thereby modifying the days on feed. Cattle in this feedlot were slaughtered in subsets within pens based on estimated level of maturity, and across all BW classes cattle treated late in the feeding phase were slaughtered sooner.
Average daily gain displayed an ascending pattern between WKTS categories as cattle were treated closer to slaughter. This result is somewhat surprising because cattle treated closer to slaughter displayed decreased HCW. However, combining this with the fact that these cattle were on feed fewer days, the overall affect was an increase in ADG over the entire feeding period. Because there were no differences in CYG among WKTS categories, it can be presumed that cattle were slaughtered at a similar degree of physiologic maturity. Thus, cattle treated farther from slaughter required more days on feed to finish; even though they finished with a heavier HCW, they were less efficient (in terms of ADG). Several authors (Wittum et al., 1996; Gardner et al., 1999; Thompson et al., 2006) have illustrated that a BRD event causes a depression in BW gain. Cattle treated early in the feeding phase (further from slaughter) have more days for a decreased rate of BW gain, causing them to require greater days on feed, yet decreased ADG compared with cattle treated in close proximity to slaughter. The number of times animals were treated displayed a descending pattern over WKTS categories. This finding could be attributed again to the days at risk for further treatment.
Evaluating differences in estimated net returns relative to treatment from arrival time (WKFA) and slaughter (WKTS) illustrated a significant interaction with arrival BW class. When analyzing differences in performance and health outcomes, the only variables that significantly differed between any of the weeks on feed categories were number of times treated (WKFA and WKTS), days on feed (WKTS only), ADG (WKTS only), and HCW (WKFA and WKTS). Both sets of analyses indicated that the timing of initial BRD treatment is associated with health and performance outcomes.
In conclusion, we found that disease timing, when measured relative to arrival and slaughter, affects performance and health outcomes. Although our data were derived from a single feedlot, we have demonstrated that the number of times cattle are treated and HCW appear associated with weeks from arrival to first treatment. Cattle treated further from slaughter had greater estimated net returns related to an increased HCW that appeared to offset increased costs due to more treatments, longer days on feed, and decreased ADG. Further insight into the relationship between BRD timing and performance and health variables could lead to management options that more effectively mitigate the economic impact of this extremely important disease syndrome in feedlot production systems.
|Receive BW, kg||168.2 (10.9)||209.1 (12.4)||250.8 (12.8)||293.1 (12.9)||336.4 (12.6)||380.4 (12.2)||429.8 (20.4)|
|Ending BW, kg||470.8 (42.4)||491.5 (49.6)||513.4 (52.5)||538.0 (53.7)||557.1 (52)||572.8 (46.4)||598.0 (47.1)|
|ADG, kg/d||1.26 (0.19)||1.29 (0.21)||1.32 (0.22)||1.36 (0.24)||1.38 (0.27)||1.39 (0.30)||1.37 (0.37)|
|HCW, kg||299.6 (30.9)||313.4 (35.7)||327.7 (37.4)||344.3 (38.5)||356.3 (37.5)||366.2 (34.08)||380.8 (33.13)|
|Fat thickness, cm||1.10 (0.44)||1.12 (0.41)||1.10 (0.401)||1.10 (0.40)||1.08 (0.404)||1.09 (0.41)||1.06 (0.45)|
|LM area (12th rib), cm2||77.4 (10.1)||80.2 (10.5)||82.5 (10.8)||84.5 (10.9)||85.4 (10.7)||86.2 (10.7)||87.6 (11.2)|
|Calculated yield grade||2.76 (0.75)||2.75 (0.72)||2.74 (0.73)||2.77 (0.73)||2.81 (0.73)||2.86 (0.73)||2.89 (0.81)|
|KPH, %||0.0076 (0.013)||0.0038 (0.0102)||0.0026 (0.0087)||0.0028 (0.009)||0.0035 (0.0326)||0.009 (0.122)||0.034 (0.258)|
|Times treated||1.57 (1.16)||1.67 (1.4)||1.64 (1.33)||1.57 (1.26)||1.50 (1.16)||1.44 (1.03)||1.46 (1.06)|
|Days on feed||241.5 (29.1)||221.0 (34.1)||200.6 (34.9)||182.5 (36.5)||161.1 (34.1)||140.6 (28.7)||125.4 (26.4)|
|Days on feed at first treatment||30.5 (24.2)||29.54 (24.9)||28.78 (24.7)||29.79 (24.7)||31.86 (24.9)||34.90 (24.6)||35.47 (26.5)|
|Days on feed from initial treatment date until slaughter date||211 (37.1)||191.5 (43.1)||171.8 (43.8)||152.7 (46.2)||129.2 (44.6)||105.7 (39.9)||89.9 (39.4)|
|Net return, $||11.77 (72.63)||3.11 (72.89)||2.51 (71.56)||9.82 (71.38)||10.10 (71.92)||9.24 (68.22)||0.94 (81.92)|