Table 3.

Prevalence of M. haemolytica isolates not susceptible to antimicrobials on arrival at the feedlot (first test) and later in the feeding period (second test) (isolates identified as intermediate or resistant are classified as not susceptible) and calves matched (first test to second test).

Auction market calves	M. haemolytica isolates not susceptible on first test (%) (n = 116)	M. haemolytica isolates not susceptible on second test (%) (n = 44)	Prevalence of matched calves with M. haemolytica isolates not susceptible on first test (%) (n = 217)	Prevalence of matched calves with M. haemolytica isolates not susceptible on second test (%) (n = 217)	P-value for matched calvesa
Ampicillin	0%	0%	0%	0%	—
Ceftiofur	0%	0%	0%	0%	—
Danofloxacin	0%	0%	0%	0%	—
Enrofloxacin	0%	0%	0%	0%	—
Florfenicol	0%	0%	0%	0%	—
Oxytetracycline	26.7%	52.3%	10.6%	10.6%	0.99
Penicillin	1.7%	2.3%	0%	0.5%	0.99
Tilmicosin	27.6%	50%	11.1%	10.1%	0.76
Tulathromycin	4.3%	36.4%	1.4%	7.4%	0.006
Ranch-raised calves	M. haemolytica isolates not susceptible on first test (%) (n = 79)	M. haemolytica isolates not susceptible on second test (%) (n = 57)	Prevalence of matched calves with M. haemolytica isolates not susceptible on first test (%) (n = 279)	Prevalence of matched calves with M. haemolytica isolates not susceptible on second test (%) (n = 279)	P-value for matched calvesa
Ampicillin	0%	0%	0%	0%	—
Ceftiofur	0%	0%	0%	0%	—
Danofloxacin	0%	0%	0%	0%	—
Enrofloxacin	1.3%	0%	0.4%	0%	0.99
Florfenicol	0%	0%	0%	0%	—
Oxytetracycline	20.3%	1.8%	5.7%	0.4%	0.005
Penicillin	0%	8.8%	0%	1.8%	0.06
Tilmicosin	19.0%	0%	5.4%	0%	0.001
Tulathromycin	19.0%	0%	5.4%	0%	0.001

^aCalves with both a first test and a second test sample collected were designated as test matched.