Reference | Inglis2005 (20001) (Excluded)

Three hundred crossbred steer calves (198 +/- 20 kg of initial body weight) were housed in 30 pens at the Lethbridge Research Centre experimental feedlot. Calves originated from a common location and did not receive antimicrobial agents subtherapeutically before the initiation of the experiment. The calves were arbitrarily assigned to one of the following six treatments: (i) no antimicrobial agents (control treatment), (ii) 350 mg head^-1 da^-1 chlortetracycline and 350 mg head^-1 da^-1 sulfamethazine (CS treatment) (Aure S-700 G; Alpharma Inc., N.J.), (iii) 11 ppm chlortetracycline (Ct treatment) (Aureomycin-100 G; Alpharma Inc.), (iv) 250 mg head^-1 da^-1 virginiamyci (Vi treatment) (V-Max; Pfizer Animal Health, N.Y.), (v) 25 ppm monensin (Mo treatment) (Rumensin; Elanco Animal Health, Alberta, Canada), and (vi) 11 ppm tylosin phosphate (Ty treatment) (Tylan; Elanco Animal Health). With the exception of virginiamycin, the antimicrobial agents were selected based on the commonality of their use in the Canadian feedlot industry and were fed at the concentrations recommended by the manufacturers. Each treatment was replicated five times, and the treatment groups were arranged in a randomized complete block design; each block consisted of a separate pen containing 10 steers. Water troughs were shared between adjacent pens, but treatments were arranged in a manner so that only cattle that received the same antimicrobial agent could drink from the same water troughs.Antimicrobial agents were first introduced into the diets 18 days after the cattle arrived at the feedlot, and they were included in the forage-based diet for 56 days thereafter (Fig. 1). Antimicrobial agents were subsequently removed from the diet for 91 days and then reintroduced for an additional 42-day period when the grain-based diet was used. The antimicrobial administration periods were chosen to coincide with the two feeding periods. To avoid cross-contamination, we mixed the antimicrobial agents with 5 kg of a supplement containing minerals and vitamins and spread the mixture manually over the surface of feed within each of the appropriate pens during the morning feeding. All animals in the pen were capable of feeding at the feed trough at the same time. Cattle assigned to the control treatment were provided with a supplement that contained no antimicrobial agents.

All cattle were restrained 11 times during the course of the feedlot period (Fig. 1). Although animals were processed in groups according to pen assignments, all animals were processed in the same constraint device. Once each animal was constrained, deep fecal samples were obtained from the rectum by the use of rectal swabs (Starswap; Starplex Scientific, Ontario, Canada). To obtain each fecal sample, we inserted the swab approximately 4 to 5 cm into the rectum and rotated it until it was covered with feces. Within 1 h of collection, the samples were transported to the laboratory on ice.

AST Method: None

Reference explicitly reports AST breakpoints: True

Reference reports using a MIC table: True