An evaluation of the relative efficacy of tulathromycin for the treatment of undifferentiated fever in feedlot calves in Nebraska

Abstract

A field trial was performed under commercial feedlot conditions in central Nebraska to assess the relative efficacy of tulathromycin (TULA) to florfenicol (FLOR) for the treatment of undifferentiated fever (UF) in feedlot calves that did not receive a metaphylactic antimicrobial or vaccines/bacterins containing Mannheimia haemolytica or Histophilus somni at feedlot arrival by comparing animal health, feedlot performance, and carcass characteristic variables. Two hundred recently weaned, auction market derived, crossbred beef calves that met the study-specific case definition of UF were randomly allocated in a 1:1 ratio to 1 of 2 experimental groups as follows: TULA, which received tulathromycin administered subcutaneously at the rate of 2.5 mg/kg body weight (BW) once at the time of allocation; or FLOR, which received florfenicol administered subcutaneously at the rate of 40 mg/kg BW once at the time of allocation.

In terms of animal health, the first UF relapse (RR = 0.65), overall mortality (RR = 0.33), and BRD mortality (RR = 0.29) rates in the TULA group were significantly (P < 0.05) lower than in the FLOR group. There were no significant (P = 0.05) differences between the TULA and FLOR groups for the other animal health variables measured. There was no significant (P ≥ 0.05) difference in average daily gain between the TULA and FLOR groups. There were no significant (P ≥ 0.05) differences in the overall distributions of quality grade and yield grade between the experimental groups; however, a significantly (P < 0.05) higher proportion of carcasses in the TULA group graded yield grade USDA-4 as compared with the FLOR group.

In the economic analysis, the benefits observed resulted in an economic advantage of $52.50 USD/animal in the TULA group due to lower first UF relapse and overall mortality rates, even though the occurrence of yield grade USDA-4 carcasses increased and the initial UF treatment cost was higher.

Introduction

Undifferentiated fever (UF), also referred to as the bovine respiratory disease (BRD) complex or shipping fever, continues to be one of the most economically significant health problems in calves entering beef feedlots (1–5). Management of this disease complex involves both metaphylactic and therapeutic administration of parenteral antimicrobials. Several studies have demonstrated the therapeutic efficacy of various antimicrobials, including florfenicol, tilmicosin, trimethoprim sulfadoxine, ceftiofur, and oxytetracycline, for the treatment of UF/BRD (6–13). Tulathromycin is a triamilide member of the macrolide antimicrobial class that was specifically developed for the treatment and prevention of UF/BRD. A recent series of articles describe the pharmacology, microbiology, and clinical safety and efficacy of this novel antimicrobial, developed specifically for preventing and treating respiratory disease of cattle and swine (14–25). From an animal health perspective, the primary measure of clinical efficacy in the studies reported in these articles was “treatment success” rate and very little information is presented on what ultimately happened to the treatment failures, animals designated to have chronic respiratory disease, and animals that were removed from the study during the course of each field study. While treatment success rate information is useful for assessing clinical efficacy as it pertains to pharmaceutical licensing, a void of information describing the final clinical outcome of treatment failures, animals designated to have chronic disease, and animals that were removed from the study, makes it very difficult to properly model the economic impact of improved treatment success, because all the various clinical outcomes (prolonged convalescence followed by recovery, sale for salvage slaughter, and death loss) have substantially different economic values. As a result, additional research is required to more completely describe the relative efficacy of tulathromycin compared with other commonly used antimicrobials for the treatment of UF in feedlot calves, so that appropriate economic models can be constructed to determine the relative cost-effectiveness of various antimicrobial treatment options.

The purpose of the investigation reported herein was to compare the relative therapeutic efficacy of tulathromycin (Draxxin; Pfizer, Animal Health Group, New York, New York, USA), administered at 2.5 mg/kg body weight (BW), to florfenicol (Nuflor; Schering Plough Animal Health Corporation, Union, New Jersey, USA), administered at 40 mg/kg BW, for the treatment of UF in feedlot calves.

Materials and methods

Trial facilities

The study was conducted in a commercial feedlot near Broken Bow, Nebraska, which has a capacity of 85 000 animals. The basic design of this feedlot is representative of the standard designs used in Nebraska. The animals are housed in open-air, dirt-floor pens arranged side by side with central feed alleys. There are 176 large pens in the feedlot with capacities of 200 to 600 animals/pen. The remaining 102 pens are smaller in dimension and have animal capacities ranging from 60 to 200 animals/pen.

There is 1 permanent hospital facility and 2 mobile hospital facilities at the feedlot. Each hospital facility has a hydraulic chute equipped with an individual animal scale, a chute-side computer for animal health data, and separation alleys to facilitate the return of animals to designated pens. The permanent hospital has 6 recovery pens with a total capacity of 250 animals. Adjacent to the permanent hospital, there are 3 pens designated to house animals with chronic disease, with a total capacity of 400 animals, and 8 receiving pens, with a capacity of 150 animals each. Also, an enclosed processing facility and several shipping pens are located at the feedlot.

Trial animals

The candidate animals were recently weaned crossbred beef calves, purchased from auction markets throughout Minnesota, North Dakota, and Kansas. Four hundred and ninety-eight candidate animals were assembled at the auction markets and transported to the feedlot by truck within 48 h of purchase. The candidate animals were approximately 5 to 9 mo of age and the average individual animal weight of the purchase groups used in the study (purchase group weight ÷ number of animals in the group) ranged from 199 kg to 246 kg.

Upon arrival at the feedlot, the animals were moved through a hydraulic chute for a group of study-specific procedures known collectively as processing. All animals were weighed, ear tagged (to provide unique, individual animal identification), implanted with a zeranol growth implant (Ralgro; Schering Plough Animal Health Corporation), and vaccinated against Infectious bovine rhinotracheitis virus (IBRV), Bovine parainfluenza virus 3 (BPIV-3), Bovine viral diarrhea virus types 1 and 2, and Bovine respiratory syncytial virus (Bovishield 4; Pfizer). In addition, each animal received a multivalent clostridial bacterintoxoid (Ultrabac 7; Pfizer), and topical doramectin (Dectomax Pour-On; Pfizer) at a rate of 1.0 mL/10 kg BW. All animals identified as bulls, sick on arrival (rectal temperature ≥ 40°C at processing), or lame on arrival were removed from the candidate pool for this study.

Experimental design

The candidate animals were processed on October 19, 2001, and October 20, 2001, and housed in 3 designated feedlot pens prior to allocation. On a daily basis, beginning on October 21, 2001, and ending on October 23, 2001, animal health personnel examined the candidate animals for clinical signs of UF. Animals exhibiting signs of UF, based on subjective criteria, including general appearance and attitude, gauntness, and reluctance to move, and having an absence of abnormal clinical signs referable to organ systems other than the respiratory system were moved from the candidate pens and presented to the hospital facility for assignment of a clinical attitude score (CAS) by the study investigator. The CAS system used in this study was 0 = normal, bright, alert, and responsive; 1 = mild depression as characterized by mild lethargy, slight weakness, mild reduction in gut fill, and/ or a lack of attentiveness to surroundings or willingness to move until disturbed or aroused; 2 = moderate to marked depression as characterized by moderate lethargy, notable weakness, moderate reduction in gut fill, and/or a lack of attentiveness to surroundings and willingness to move that is not generally responsive to disturbance or arousal; 3 = severe depression as characterized by severe lethargy, severe weakness, severe reduction in gut fill, and/or recumbency and lack of willingness to rise unless disturbed or aroused; and 4 = moribund. Animals fulfilling the CAS entrance criteria 1, 2, or 3 were moved through the hospital facility for rectal temperature measurement. Those animals that had a rectal temperature of ≥ 40.0°C were weighed and randomly allocated, using a computer generated randomization table, to 1 of 2 experimental groups as follows: TULA, which received tulathromycin, SC, at the rate of 2.5 mg/kg BW at allocation; or FLOR, which received florfenicol, SC, at the rate of 40 mg/kg BW at allocation. One hundred and two animals were allocated to the study on October 22, 2001, and 98 animals were allocated to the study on October 23, 2001. Subsequent to allocation, the animals from both experimental groups were commingled and moved to 1 of 2 designated postal-location pens (1 pen for each allocation day). Animals that did not meet the study entrance criteria of a CAS of 1, 2, or 3 and a rectal temperature of ≥ 40.0°C were returned the designated candidate pen without treatment and were eligible for selection on subsequent days.

Subsequent to allocation and initial UF therapy, experienced animal health personnel or the study investigator, all of which were blinded as to the experimental status of each animal, observed the experimental animals once daily for evidence of recurrent disease and managed them as described below. Animals with a CAS of ≥ 1 were moved from the pen and presented to the hospital facility for rectal temperature measurement. Animals with a CAS of 1 or 2 and a rectal temperature of ≥ 40.0°C were defined as nonresponders. In addition, animals with a CAS of 3 or 4 were defined as nonresponders, regardless of rectal temperature. From day 3 to day 28, animals that met the described nonresponder criteria for the 1st time were treated with long-acting oxytetracycline (Liquamycin LA 200; Pfizer), 20 mg/kg BW, IM, and returned to the study pen. Animals that met the described nonresponder criteria for a 2nd time between day 3 and day 28, and at least 48 h after the initial nonresponder therapy, were treated with enrofloxacin (Baytril 100; Bayer, Animal Health Division, Shawnee Mission, Kansas), 11 mg/kg BW, SC, and returned to the study pen. From day 29 to 28 d prior to slaughter (S-28), animals that met the described non-responder criteria for the 1st time were treated with long-acting oxytetracycline (Liquamycin LA 200), 20 mg/kg BW, IM, and returned to the study pen. Animals that met the described non-responder criteria for a 2nd time between day 29 and S-28, and at least 48 h after the initial nonresponder therapy, were treated with enrofloxacin (Baytril 100), 11 mg/kg BW, SC, and returned to the study pen. Animals that met the described nonresponder criteria for the 1st or 2nd time between day S-27 and slaughter were treated with ceftiofur (Naxcel; Pharmacia and Upjohn, Kalamazoo, Michigan), 1.0 mg/kg BW, IM, for 3 consecutive days and returned to the study pen after the 3rd injection. In each of the day 3 to day 28 and the day 29 to slaughter intervals, the maximum number of times that an animal could be treated as a nonresponder was 2. Animals that met the described nonresponder criteria for a 3rd time in an interval were treated as per the standard feedlot treatment protocol for UF, removed from the study pens, and housed separately until shipment for salvage slaughter following fulfillment of all prescribed withdrawal periods or death (refer to Table 3 of the results for additional clarification). In all intervals, a time period of at least 48 h was required between nonresponder episodes. Finally, all other diseases were treated as per a standard feedlot protocol provided by the consulting veterinarians. All treatment events, including treatment date, presumptive diagnosis, drug(s) used, and dose(s), were recorded on the chute-side computer system.

Table 3.

Undifferentiated fever (UF) relapse therapy summary

Day 3 to 28a		Day 29 to slaughtera		Experimental group
First UF relapse	Second UF relapse	First UF relapse	Second UF relapse	FLOR	TULA
N/Ab	N/A	N/A	N/A	25	51
N/A	N/A	LA	N/A	2	7
N/A	N/A	LA	BAY	5	15
LA	N/A	N/A	N/A	11	4
LA	N/A	LA	N/A	3	3
LA	N/A	LA	BAY	8	7
LA	BAY	N/A	N/A	29	7
LA	BAY	LA	N/A	5	1
LA	BAY	LA	BAY	12	5

FLOR — Florfenicol experimental group; TULA — Tulathromycin experimental group; LA — Treatment with long-acting oxytetracycline; BAY — Treatment with enrofloxacin

^aAnimals that met the nonresponder criteria described above for a 3rd time in an interval were treated as per the standard feedlot treatment protocol for UF, removed from the study pens, and housed separately until shipment for salvage slaughter following fulfillment of all prescribed withdrawal periods or death

^bN/A indicates that an animal was not diagnosed as a UF relapse in the specified interval

On January 22, 2002, the animals from the 2 designated postallocation pens were commingled in a single feedlot pen and remained there for the duration of the feeding period. On March 11, 2002, all remaining study animals were implanted with an estradiol benzoate/trenbolone acetate implant (Synovex Plus; Fort Dodge Animal Health, Division of Wyeth, Fort Dodge, Iowa, USA) and vaccinated against IBRV (Pyramid IBR; Fort Dodge Animal Health, Division of Wyeth). On September 5, 2002, all remaining study animals were transported to a commercial packing plant for slaughter [Tyson Fresh Meats, A Division of Tyson Foods (formerly IBP), Lexington, Nebraska, USA].

The animal health events of each animal were followed from allocation to slaughter. All animals that died during the study were prosected by designated feedlot personnel, using a standardized method, and the appropriate digital images were captured, as outlined in the written necropsy protocol provided by the study veterinarians. Subsequently, the captured digital images were transferred electronically to the study veterinarians, and the cause of death was determined for each animal based on the recorded findings of the gross postmortem examination (14).

Data collection and management

The computerized animal health data from allocation to slaughter were verified and summarized. Over the entire feeding period, the 1st time that an animal fulfilled the nonresponder criteria was defined as a 1st UF relapse, the 2nd time that an animal fulfilled the nonresponder criteria was defined as a 2nd UF relapse, and animals that were removed from the study pens and shipped for salvage slaughter were defined as wastage. From these data, risk rates for 1st UF relapse, 2nd UF relapse, wastage, overall mortality (mortality due to all causes), BRD mortality (mortality due to bronchointerstitial pneumonia, chronic pneumonia, bronchopneumonia, or fibrinous pneumonia), and miscellaneous mortality (mortality due to musculoskeletal injury) were calculated for each experimental group (Table 1).

Table 1.

Formulas for the calculation of morbidity and mortality rates

First UF relapse	= (number of 1st UF relapses divided by the number of animals initially treated for UF)	× 100%
Second UF relapse	= (number of 2nd UF relapses divided by the number of 1st UF relapses)	× 100%
Wastage	= (number of animals marketed for salvage slaughter divided by the number of animals initially treated for UF)	× 100%
Overall mortality	= (number of mortalities due to all causes divided by the number of animals initially treated for UF)	× 100%
BRD mortality	= (number of mortalities due to BRD divided by the number of animals initially treated for UF)	× 100%
Miscellaneous mortality	= (number of mortalities due to causes other than BRD by the number of animals initially treated for UF)	× 100%
Relative risk	= (risk for the TULA group divided by the risk for the FLOR group)

UF — Undifferentiated fever; BRD — Bovine respiratory disease; TULA — Tulathromycin experimental group; FLOR — Florfenicol experimental group

Each animal was weighed at allocation and the weight was recorded. Each animal that was slaughtered at the end of the study was weighed on the day immediately prior to slaughter (Day S-1) and the weight was recorded. In addition, the hot weight of each carcass was measured and recorded at slaughter. Average daily gain was calculated for animals surviving to slaughter, using 2 methods: the live weight basis method utilized the live weights obtained on Day S-1 of the study; and the carcass weight basis method utilized the hot carcass weights obtained from the packing plant divided by a fixed dressing percentage of 63%. Actual dressing percentage was calculated by dividing the carcass weight obtained at slaughter by the live weight obtained on Day S-1.

The USDA yield grade (YG) and quality grade (QG) of each carcass was collected at slaughter by representatives from the Beef Carcass Research Center, West Texas A&M University, Canyon, Texas. With respect to YG, the proportions of animals that graded YG USDA-1, USDA-2, USDA-3, USDA-4, or USDA-5 were calculated for each experimental group. With respect to QG, the proportions of animals that graded QG USDA Prime, USDA Choice, USDA Select, or USDA Standard were calculated for each experimental group.

Statistical analysis

The data were analyzed by using an analytical software program (The SAS^TM System for Windows, Release 9.1; SAS Institute, Cary, North Carolina, USA). The animal health variables were compared between the experimental groups by using generalized linear modeling techniques with individual animals as the unit of analysis, as described by McDermott, Schukken, and Shoukri (27,28). Calculation of confidence intervals was done by using the partially maximized likelihood function (likelihood ratio based confidence intervals) (29). The ancillary and performance variables were compared between the experimental groups by using least squares analysis of variance for experimental group effects (30). The baseline variables were tested as covariates of the feedlot performance variables and included in the final model used for comparison of each variable between the experimental groups when significant (P < 0.05) effects were detected (29). The carcass characteristic variables were compared between the experimental groups by using rectangular contingency tables to assess the overall distributions of QG and YG and 2 × 2 tables to compare the frequency of each QG and YG level, using likelihood ratio chi-square tests (29).

Economic analysis

The relative cost-effectiveness of the experimental groups was calculated by using a proprietary computer spreadsheet program (Microsoft Excel 2000; Microsoft Corporation, Redmond, Washington, USA) that simulates all economic aspects of feedlot production, as previously described (1–3,8,9,19). The actual morbidity and mortality rates, average daily gain carcass weight basis, and carcass characteristics of each experimental group were included in the economic model when the rates were significantly (P < 0.05) different between the groups. When there were no significant (P ≥ 0.05) differences between the experimental groups, the morbidity and mortality rates, average daily gain carcass weight basis, and carcass characteristics of the FLOR group were used for both experimental groups. All other factors were fixed in the economic simulations. The costs of initial UF treatment used in the analysis were $13.70 USD/animal and $11.90 USD/animal for the TULA and FLOR groups, respectively. The therapeutic costs used in the economic analysis for relapse therapy were $2.03 USD and $12.64 USD for each Liquamycin LA 200 and Baytril 100-treatment regime, respectively. The purchase price used in the analysis was US$220.46/100 kg (US$100.00/100 lb) BW and the discount for YG USDA-4 carcasses was − US$24.25/100 kg (− US$11.00/100 lb) BW. The interest rate used in the analysis was 4.0%/annum. The value of a dead animal was $0.00 USD. Feed consumed by animals prior to death was not estimated.

Results

One hundred animals were allocated to the TULA group and 100 animals were allocated to the FLOR group. The final categorization of allocated animals in the TULA group was wastage — 18, mortality — 7, and slaughter — 75 and in the FLOR group was wastage — 27, mortality — 21, and slaughter — 52. Based on clinical observations before (0.5 h) and after (3.0 h) tulathromycin or florfenicol administration, there were no adverse clinical reactions to either antimicrobial.

The morbidity and mortality variables for the TULA and FLOR groups are summarized in Table 2. The 1st UF relapse (RR = 0.65), overall mortality (RR = 0.33), and BRD mortality (RR = 0.29) rates of the TULA group were significantly (P < 0.05) lower than those of the FLOR group. There were no significant (P ≥ 0.05) differences in 2nd UF relapse, wastage, or miscellaneous mortality rates between the TULA and FLOR groups. A summary of the UF relapse therapy for the intervals day 3 to 28 and day 29 to slaughter is presented in Table 3.

Table 2.

Animal health data summary

	Experimental group
Animal health variable	TULA	FLOR	Relative risk	95% CI	P-value
Number of casesa	100	100
First UF relapseb	49 (49.00)c	75 (75.00)	0.65	0.53–0.81	< 0.001
Second UF relapseb	39 (79.59)	62 (82.67)	0.96	0.82–1.12	0.668
Wastageb	18 (18.00)	27 (27.00)	0.67	0.39–1.12	0.127
Overall mortalityb	7 (7.00)	21 (21.00)	0.33	0.14–0.71	0.004
BRD mortalityb	6 (6.00)	21 (21.00)	0.29	0.11–0.63	0.002
Miscellaneous mortalityb	1 (1.00)	0 (0.00)	N/A	N/A	0.238

95% CI — the 95% confidence interval calculated for each relative risk; BRD — Bovine respiratory disease; TULA — Tulathromycin experimental group; FLOR — Florenicol experimental group; UF — Undifferentiated fever

^a75 animals in the TULA group (100 cases minus 18 wastage minus 7 mortality) and 52 animals in the FLOR group (100 cases minus 27 wastage minus 21 mortality) remained on the study and were followed to slaughter

^bRefer to Table 1 for formulas and definitions of terms

^cNumbers in parentheses are percentages

The ancillary and performance variables for the TULA and FLOR groups are summarized in Tables 4 and 5, respectively. There were no significant (P ≥ 0.05) differences in allocation weight, slaughter weight, carcass weight, dressing percentage, average daily gain (ADG) live weight basis, or ADG carcass weight basis between the experimental groups.

Table 4.

Ancillary data summary

	Experimental group
Ancillary variable	TULA	FLOR	P-value
Allocation weight (kg)	199.7 (3.0)a	200.8 (3.6)	0.807
Slaughter weight (kg)	620.0 (5.9)	614.4 (7.0)	0.544
Carcass weight (kg)	406.6 (4.2)	404.2 (5.0)	0.716
Dress (%)	65.56 (0.21)	65.74 (0.25)	0.593

TULA — Tulathromycin experimental group; FLOR — Florfenicol experimental group

^aThe values presented are least squares means, with standard errors of each least squares mean in parentheses

Allocation weight — calculated using the individual animal live weight obtained at the time of allocation; Slaughter weight — calculated using the individual animal live weight obtained prior to regular slaughter, corrected for gut fill using a 4% pencil shrink; Carcass weight — calculated using the individual animal carcass data obtained at regular slaughter; Dress — calculated for each animal using the carcass weight obtained at regular slaughter divided by the shrunk live weight obtained prior to slaughter and represents the average dressing percentage of animals sold for regular slaughter

Table 5.

Performance data summary based on live weight

	Experimental group
Performance variable	TULA	FLOR	P-value
Average daily gain
ADG live (kg/d)a	1.33 (0.02)	1.31 (0.02)	0.531
ADG carcass (kg/d)a	1.41 (0.02)	1.40 (0.03)	0.716

TULA — Tulathromycin experimental group; FLOR — Florfenicol experimental group

^aThe values presented are least squares means, with standard errors of each least squares mean in parentheses

ADG live — live weight basis values were calculated using the live weights obtained prior to slaughter, correcting for gut fill by using a 4% pencil shrink; ADG carcass — carcass weight basis values were calculated using carcass weights obtained at slaughter, converted to live weights by using a fixed dressing percentage of 63.0%

The carcass characteristic variables for the TULA and FLOR groups are summarized in Table 6. There were no significant (P ≥ 0.05) differences in the overall distributions of QG and YG between the experimental groups. However, there were significantly (P < 0.05) more YG USDA-4 carcasses in the TULA group compared with the FLOR group. There were no significant (P ≥ 0.05) differences in the other YG or QG variables assessed in the study between the experimental groups.

Table 6.

Carcass characteristic data summary

	Experimental group
Carcass characteristic variable	TULA	FLOR	P-value
Yield grade (overall effect)			0.164
USDA-1	8 (10.67)a	5 (9.62)	0.847
USDA-2	19 (25.33)	16 (30.77)	0.502
USDA-3	26 (34.67)	25 (48.08)	0.130
USDA-4	15 (20.00)	4 (7.69)	0.047
USDA-5	7 (9.33)	2 (3.85)	0.219
Quality grade (overall effect)			0.397
USDA prime	2 (2.67)	0 (0.00)	0.145
USDA choice	49 (65.33)	38 (73.08)	0.353
USDA select	22 (29.33)	12 (23.08)	0.431
USDA standard	2 (2.67)	2 (3.85)	0.711

^aNumbers in parentheses are percentages

TULA — Tulathromycin experimental group; FLOR — Florfenicol experimental group; USDA-1 — proportion of animals that graded USDA 1; USDA-2 — proportion of animals that graded USDA 2; USDA-3 — proportion of animals that graded USDA 3; USDA-4 — proportion of animals that graded USDA 4; USDA-5 — proportion of animals that graded USDA 5; USDA Prime — proportion of animals that graded USDA Prime; USDA Choice — proportion of animals that graded USDA Choice; USDA Select — proportion of animals that graded USDA Select; USDA Standard — proportion of animals that graded USDA Standard

In the economic analysis, there was an advantage of US$52.50/animal in the TULA group due to lower 1st UF relapse and overall mortality rates, even though the occurrence of YG USDA-4 carcasses was increased and the initial UF treatment cost was higher.

Discussion

The results of this study demonstrate that tulathromycin, administered once, SC, at the rate of 2.5 mg/kg BW, is more effective for the treatment of UF in feedlot calves than florfenicol, administered once, SC, at the rate of 40 mg/kg BW, due to its lower 1st UF relapse and overall mortality rates. Interpretation of these observations must take into consideration that the calves in this study received neither a metaphylactic antimicrobial nor vaccines/bacterins containing Mannheimia haemolytica or Histophilus somni upon arrival at the feedlot, both of which are standard, cost-effective, management procedures utilized in high-risk, auction market derived calves in North America (1–3,8,9). In addition, the results of this study are specific to the case definition for UF utilized in this study. As a result, direct extrapolation of these results to commercial feedlot production scenarios may not be appropriate.

The economic advantage of US$52.50/animal in the TULA group was due to a US$3.30/animal lower 1st UF relapse treatment cost, a US$63.09/animal lower overall mortality cost, a reduced carcass value of −$12.06US/animal due to a higher proportion of YG USDA-4 carcasses, and an increased initial UF treatment cost of US$1.83/animal. If the purchase price of the feeder animal was reduced to US$176.37/100 kg (US$80.00/100 lb), the benefit of the lower overall mortality rate would be reduced to US$50.47/animal. Conversely, if the purchase price of the feeder animal was increased to US$264.56/100 kg (US$120.00/100 lb), the benefit of the lower overall mortality rate would be increased to US$75.71.

In this study, there were no significant (P ≥ 0.05) differences in the overall distributions of QG and YG between the experimental groups. However, due to the fact that many carcass-based marketing arrangements are based on the distribution of individual levels of QG and YG rather than the overall distribution of each, it is necessary to “drill down” to the individual levels of QG and YG to provide the necessary information for making data-based decisions. In terms of YG USDA-4 in this case, if the YG USDA-4 discount was decreased to −$19.84US/100 kg carcass weight [−$9.00US/100 lb carcass weight (CW)] then the reduced carcass value would be decreased to −$9.87US/animal. Conversely if the YG USDA-4 discount was increased to −$28.66US/100 kg CW (−$13.00US/100 lb CW) then the reduced carcass value would be increased to −$14.26US/animal.

In summary, the results of this study indicate that it is more effective to use tulathromycin, SC, than florfenicol, SC, for the treatment of UF in feedlot calves and that the benefits observed result in a economic advantage of US$52.50/animal. Additional studies are necessary to more accurately characterize the relative efficacy of tulathromycin to other commonly used antimicrobials for the treatment of UF in feedlot calves under commercial feedlot conditions, in animals that have received a metaphylactic antimicrobial upon arrival at the feedlot, and using commercial feedlot case definitions for UF.

Tulathromycin (Draxxin, Pfizer) was approved for use in Canada on August 29, 2006, for the treatment of BRD and for the reduction of morbidity associated with BRD in feedlot calves when administered at arrival. The withdrawal period that has been assigned to tulathromycin in Canada is 44 d. In this study, animals that were treated with tulathromycin were observed 3 h post treatment. As noted in the results, no adverse clinical reactions were observed in any of the animals treated with tulathromycin. The evaluation of injection sites was not required by the study specific protocol approved by the FDA-CVM and was therefore not evaluated in this study. Additional information about injection site reactions that occur following the administration of tulathromycin are included on the product label accompanying each vial of product.