TABLE 5.

Percentages of calf fecal isolates with tetracycline MICs, before and after the start of intervention, with the Cochran-Mantel-Haenszel statistic test for differences in distributions of MICs between pre- and postintervention isolates

Agent	MIC (μg/ml)	Control herds				Intervention herds
		% of isolatesa		Cochran-Mantel-Haenszel result		% of isolates		Cochran-Mantel-Haenszel result
		Pre-Intb	Post-Intc	χ2	P	Pre-Intd	Post-Inte	χ2	P
E. colif				0.6	0.4494			10.1	0.0015
	4	1.3	2.1			2.0	9.3
	8	0	0.4			0	1.4
	16	0	0.7			2.0	1.7
	32	14.1	13.6			5.6	5.0
	>32	84.6	83.2			90.4	81.7
Salmonellaf								1.1	0.2889
	4	0	0			21.4	45.4
	8	0	0			0	0
	16	0	0			0	0
	32	0	0			7.1	0
	>32	0	100.0			71.4	54.5
Campylobacterg				0.1	0.7238			1.6	0.2083
	2	0	8.3			0	7.7
	4	0	0			0	0
	8	0	0			0	0
	16	0	0			0	0
	32	0	0			0	0
	64	50.0	16.7			0	15.4
	>64	50.0	75.0			100	76.9

^aPre-Int, preintervention; Post-Int, postintervention.

^bNumbers of isolates: E. coli, 156; Salmonella, 0; Campylobacter, 2.

^cNumbers of isolates: E. coli, 834; Salmonella, 144; Campylobacter, 12.

^dNumbers of isolates: E. coli, 197; Salmonella, 14; Campylobacter, 7.

^eNumbers of isolates: E. coli, 578; Salmonella, 11; Campylobacter, 13.

^fMIC: susceptible, <8 μg/ml; resistant, ≥32 μg/ml.

^gMIC: susceptible, <8 μg/ml; resistant, ≥16 μg/ml.