Serial and Parallel Testing Strategies for Johne's Disease: Effects on Milk Production Traits and Milk Income on New Zealand Dairy Cattle

The suboptimal accuracy of current diagnostic tests necessitates alternative testing strategies to improve the detection of Johne's disease. We studied parallel and serial testing strategies to quantify the association between milk production and Johne's disease status. For this, two commercial Johne's disease Enzyme‐Linked Immunosorbent Assay, ELISAs, (Kit A and Kit B) were used with milk and serum samples. A linear mixed model that included 2388 lactational records from 878 cows across three milk production seasons (2021/22, 2022/23 and 2023/24) was considered as a model. There was an interaction between production season and ELISA status in animals across seasons. When serial testing approach of ELISA assay (Kit B) was used with milk and serum samples, ELISA‐positive cows had 13.65 (95% CI: −4.69 to −22.62) fewer days in milk, produced 277.35 (95% CI: −37.98 to −516.73) kg lower milk yield and 36.35 (95% CI: −14.53 to −58.18) kg lower milk solids and had 366.09 (95% CI: −147.86 to −584.32) NZD lower milk income than negative cows during the 2023/24 production season. In contrast, when serial testing approach of ELISA assay (Kit A) was used with milk and serum samples, ELISA‐positive cows produced 355.61 (95% CI: 129.84–581.38) kg more milk yield and 24.58 (95% CI: 4.11–45.06) kg more milk solids and had 243.40 (95% CI: 38.73–448.06) NZD more milk income than negative cows during the 2021/22 production season. Despite the point estimate differences, the results of this study could not demonstrate the usefulness of different combinational testing strategies with confidence.