Effect of heifer-raising practices on E. coli antimicrobial resistance and Salmonella prevalence in heifer raisers.

Although cattle movement and commingling play an important role in the inter-herd transmission of pathogens, little is known about the effect of commingling of heifers at raising operations. The objective of this study was to compare the resistance of E. coli and prevalence of Salmonella from pooled faecal pats of heifers raised off-farm at multi-source raisers (MULTI) that raised heifers from at least two farms compared with on-farm raisers (HOME), with heifers from only that farm. MULTI faecal pat samples were collected from pens with animals that had arrived at the farm within the previous 2 months (AP) and from animals that would be departing the heifer raiser in 2-3 months (DP). Corresponding age sampling was conducted at HOME raisers. Odds of ampicillin resistance were 3·0 times greater in E. coli collected from MULTI compared to HOME raisers. E. coli from AP pens had significantly (P < 0·05) higher odds of resistance to ampicillin, neomycin, streptomycin, and tetracycline compared to DP pens. Salmonella recovery was not significantly different between heifer-raising systems (P = 0·3). Heifer-raising system did not have a major overall impact on selection of resistant E. coli, which was strongly affected by the age of the animals sampled.