The aim of this study was to demonstrate the noninferiority of a novel teat disinfectant based on copper and zinc (ZkinCu; Copper Andino, Santiago de Chile, Chile) compared with a previously proven glycolic acid active disinfectant (OceanBlu; DeLaval, Kansas City, MO) as a positive control, with respect to the incidence of new intramammary infections under natural challenge conditions on a commercial robotic dairy farm. This study was conducted in 6 robotic pens of approximately 60 milking cows each. The pens were randomly assigned to 1 of the 2 studied disinfectants. Throughout the 8 wk study, the same pre- and post-milking teat disinfectant was used in each pen. The same milking procedures were used in each robot throughout the study. Pre-milking hygiene consisted of applying the disinfectant (OceanBlu or ZkinCu) with the robotic arm. The same product was applied on the teats after milking. At the beginning of the study, all quarters of all study cows were sampled. In successive samplings (wk 2, 4, 6, and 8), composite milk samples were collected on farm to determine SCC. Once composite SCC results were available (2 d) and based on an SCC of >= 100,000 cells/mL, quarter milk samples underwent bacteriological culture. Clinical mastitis was identified by study personnel. Intramammary infection in biweekly quarter milk samples was determined based on composite SCC levels (>= 100,000 cells/mL) and the presence of bacteria. A new IMI was defined as a quarter in which the organism isolated was not present in the previous bacteriological sample, or the previous composite SCC sample was <100,000 cells/mL. Clinical mastitis samples were also considered to be new IMI. The trial was designed as a positive control field trial, in which the objective was to show noninferiority of ZkinCu versus the control (OceanBlu). The overall crude incidences of new IMI for 2 wk at risk were 4.9 and 7.3% for the ZkinCu and OceanBlu groups, respectively. The predominant organisms recovered from quarters with new IMI were Streptococcus uberis, Corynebacterium spp., and coagulase-negative staphylococci in both the ZkinCu and OceanBlu groups. The risk of infection in the OceanBlu group was higher (beta = 0.644; 95% confidence interval = 0.05-1.22). The interaction of treatment by week was not significant. The new IMI rate estimates (95% confidence interval) for ZkinCu and OceanBlu were 1.7% (0.8-2.5) and 3.2% (1.7-4.7), respectively. One novel aspect of this study is that it was one of the first commercial noninferiority trials to evaluate a new pre- and post-milking teat disinfectant in a dairy herd with an automatic milking system. The experimental teat disinfectant ZkinCu, evaluated in this field trial with naturally occurring IMI, showed noninferiority relative to the positive control for the prevention of new IMI. This study was conducted in a herd with an automatic milking system, and the results are applicable to herds with similar characteristics. Additional studies are needed to ensure reproducibility under different management conditions.