<p>This study evaluates the cost-benefit of adopting Brachiaria grass (<i>Urochloa spp</i>) over Napier grass (<i>Pennisetum purpureum</i>) for dairy productivity in Tanzania. It aims to assess the economic advantages, productivity impacts, and opportunity costs associated with each forage type, providing insights into their potential for enhancing dairy farming profitability. A comparative analysis was conducted using data collected from 239 farmers in Lushoto District, Tanzania. The study employed econometric modeling to evaluate the gross margins per acre for both forages and determined the opportunity costs associated with the transition from Napier grass to Brachiaria. Additionally, the Average Treatment Effect on the Treated (ATT) and Average Treatment Effect on the Untreated (ATU) were calculated to assess the impact of switching forages on farmer profitability. The results indicate that Brachiaria grass yields significantly higher gross margins (USD 658.88 per acre) compared to Napier grass (USD 579.35 per acre), with a transitional heterogeneity of USD 79.53. However, variability in profitability was observed, indicating that Brachiaria’s success depends on factors such as farm size, input access, and management practices. This study provides new empirical evidence on the economic benefits of Brachiaria grass in Tanzania, emphasizing its potential to improve dairy farming profitability. The findings offer actionable insights for policymakers and farmers considering sustainable forage options in Tanzania.</p>