This study aimed to evaluate the economic benefits of using the in-line milk analyzer “Brolis HerdLine” (Brolis Sensor Technology, Vilnius, Lithuania) for the early detection of negative energy balance (NEB) and to assess the value of preventive treatment in dairy cows at high risk of NEB. A total of 52 Holstein cows were selected and paired based on lactation number, days in milk, and fat-to-protein ratio. The pairs were randomly allocated into two treatment groups: Control group (CON, n = 26) and Test group (TE, n = 26). Cows in the TE group received a single 32.4 g monensin (Kexxtone®) controlled-release capsule, while CON cows received no treatment. Milk composition was monitored using the in-line analyzer, and energy-corrected milk (ECM) was used to evaluate performance. Over a 200-day period, the TE group produced 9,916 kg more ECM than the CON group, resulting in an additional €3,056 profit after subtracting treatment costs. On day 20 post-treatment, TE cows showed significantly higher lactose levels (4.64%) than CON cows (p = 0.03). The successful insemination rate was 8.11% higher in the CON group. These results imply that early intervention through the use of an in-line milk analyzer to identify cows at high risk of NEB improves economic performance. The study emphasizes how milk analyzers can be used to make decisions in real time when managing dairy herds.