Biological risk of using waste milk as a source of liquid diet for suckling Holstein calves

Waste milk (WM) is widely used as the primary source of liquid diet in calf-rearing systems, despite its high microbial load and antimicrobial residues. This study evaluated the health history of donor cows and the composition of waste milk, as well as the antimicrobial resistance of Escherichia coli strains isolated from the feces of calves fed milk collected from antibiotic-treated cows. Milk batches used to prepare the liquid diet were collected biweekly and analyzed for bacteriological culture, aerobic bacterial resistance, somatic cell count (SCC), standard plate count (SPC), and qualitative analysis of antimicrobial residues. This milk bank served as the liquid diet source for 30 male Holstein calves, randomly assigned to three experimental groups: saleable milk (SM, n=10), pasteurized waste milk (PWM, n=10), and raw waste milk (WM, n=10). Fecal and nasal swab samples were collected on days 7, 21, 35, 49, and 63 for culture and identification of target bacteria. Antimicrobial resistance testing was performed on E. coli strains isolated from feces. Results showed that WM presented the highest SCC and SPC values, as well as the greatest detection rate of antimicrobial residues (83.08%). The prevalence of cephalexin-resistant E. coli was higher in the WM (55.56%) and PWM (100%) groups compared to the SM group (9.09%). While pasteurization reduced microbial load, it did not eliminate antimicrobial residues nor prevent the selection of resistant strains. Resistance to antimicrobials such as cephalexin and oxytetracycline was more pronounced in WM and PWM groups than in SM, and pathogenic bacteria such as Salmonella enterica were detected exclusively in WM. Thus, WM poses a biological risk by contributing to the transmission of antimicrobial-resistant microorganisms and potential pathogens to calves—even after pasteurization. The use of waste milk as a liquid diet should be carefully assessed due to its impact on calf microbiota and its role in the dissemination of antimicrobial resistance.