<p>Seasonal forage scarcity is a persistent constraint in smallholder swamp buffalo (<i>Bubalus bubalis carabanensis</i>) production in tropical regions. However, quantitative evidence linking forage availability to farm-level economic performance in Thailand is limited. This study evaluated the association between forage resources and gross annual income from swamp buffalo production and introduced a Forage Security Index (FSI), defined as total forage land per buffalo. Data from 49 smallholder farms participating in the Buffalo Modern Farm project were analyzed using a General Linear Model and linear regression to assess the effects of herd size, forage resources, grazing access, labor efficiency, financial variables, and FSI on annual income. Year-round grass plots were the only significant predictor of annual income (<i>P</i> &lt; 0.05), with each additional rai associated with an increase of approximately + 14,600 Thai Baht (THB) per year. The FSI showed a positive association with annual income (<i>P</i> = 0.020), with each unit increase in FSI corresponding to approximately 47,000 THB/year. Farms with high FSI (&gt; 1 rai/buffalo) had, on average, 136,000 THB greater annual income than those with low FSI. In contrast, herd size, grazing area, rainy-season forage, loans, and labor efficiency were not significantly associated with income. These results indicate that forage availability, particularly year-round forage, is a factor associated with variation in annual income in smallholder swamp buffalo production systems. Strengthening forage development and feed resource management may improve production stability and economic outcomes under tropical smallholder conditions.</p>