The acquisition of antibiotic resistance due to the consumption of food contaminated with resistant strains is a public health problem that has been increasing in the last decades. Mathematical modeling is contributing to the solution of this problem. In this article we performed the qualitative analysis of a mathematical model that explores the competition dynamics in vivo of ceftiofur-resistant and sensitive commensal enteric Escherichia coli (E. coli) in the absence and during parenteral ceftiofur therapy within the gut of cattle, considering the therapeutic effects (pharmacokinetics (PK)/pharmacodynamics (PD)) in the outcome of infection. Through this analysis, empirical properties obtained through in vivo experimentation were verified, and it also evidenced other properties of bacterial dynamics that had not been previously shown. In addition, the impact of PD and PK has been evaluated.