TABLE 2.

Percentages of the E. coli isolates collected from pens of steers fed antimicrobial agents with silage- or grain-based diets and that were capable of growth on selective media containing antibiotics^a

Selective medium and parameter	% of E. coli isolates from antimicrobial treatment groupb:						SE	Pc
	Control	TET-SUL	TET	VIR	MON	TYL
MAC+TET
Silage-based diet	1.4 b	15.8 a	4.1 b	1.6 b	3.5 b	1.7 b	0.7	0.0011
Grain-based diet	11.6 bc	25.9 a	20.3 ab	5.2 c	9.4 bc	12.4 bc	2.5	0.0001
SE	3.72	4.50	3.37	1.49	2.15	1.30
Silage vs graind	0.055	0.087	0.008	0.07	0.05	0.001
MAC+AMP
Silage-based diet	0.23 b	4.14 a	0.43 b	0.19 b	0.03 b	0.58 b	0.69	0.004
Grain-based diet	0.28 b	0.94 a	0.13 b	0.01 b	0.28 b	0.80 b	0.27	0.145
SE	0.38	1.64	0.42	0.625	0.235	0.39
Silage vs grain	0.89	0.122	0.51	0.28	0.35	0.60
MAC+GEN
Silage-based diet	0.004 ab	0.003 b	0.011 a	0.003 b	0.004 ab	0.002 b	0.002	0.018
Grain-based diet	0.023	0.007	0.011	0.006	0.006	0.005	0.004	0.149
SE	0.01	0.001	0.008	0.001	NEe	0.001
Silage vs grain	0.164	0.148	0.987	0.27	NE	0.08

^aResistance was determined by plating onto MAC+TET, MAC+AMP, or MAC+GEN as defined in the text. Percentages were calculated as follows: (number of colonies on the antibiotic-supplemented MAC plates)/(total number of colonies counted on nonselective MAC). The E. coli isolate numbers on MAC plates are indicated in Table 1.

^bControl, no added antimicrobial agents; the TET-SUL, TET, VIR, MON, and TYL treatments are described in detail in Materials and Methods.

^cWithin a row, values followed by different letters differ.

^dThat is, the effect of diet formulation (P).

^eNE, not executable.