Assessing the relative importance of species choice, irrigation, and nitrogen management for sustainable tropical forage legume systems

Abstract

Abstract Water scarcity and declining soil fertility are placing increasing pressure on forage-based livestock systems in the tropics. Although irrigation and nitrogen inputs are widely used to improve forage accumulation, their relative importance compared with species choice remains unclear for tropical forage legumes. This field study examined the effects of irrigation level (60% (I60), 80% (I80), and 100% (I100) of crop water requirement), fertilizer management (organic manure 30 t ha − 1 , 100 kg ha⁻¹ urea, and 300 kg ha⁻¹ urea), and three forage legume species: Desmodium uncinatum (Jacq.) DC. (ILRI 6765), Stylosanthes scabra Vogel (ILRI 140), and Stylosanthes hamata (L.) Taub. (ILRI 75) on forage accumulation, water use efficiency (WUE), and nutritive value. The experiment followed a split–split plot design with three replications. Data was collected over two years (2021–2022). No significant two or three-way interactions on forage accumulation and WUE were found, indicating that the effects of irrigation, fertilizer, and species acted largely independently. Increasing irrigation raised forage accumulation from 2.8 t ha⁻¹ under I60 to 4.1 t ha⁻¹ under full irrigation (I100), but no significant effect on WUE or nutritive value. Fertilizer application did not significantly influence forage accumulation, WUE, or nutritive value, suggesting limited response of legumes to added nitrogen. In contrast, significant differences were observed among species. D. uncinatum ILRI 6765 consistently showed the highest WUE (9.5 kg ha⁻¹ mm⁻¹) and nutrient-based water productivity, while S. hamata ILRI 75 showed lower efficiency despite relatively high crude protein content. Irrigation at 80% of crop water requirement provided the best balance between forage accumulation, water use efficiency, and nutritive value. These results show that, under sub-humid tropical conditions, species choice plays a more important role than irrigation or nitrogen input in determining forage legume performance. Focusing on well-adapted, efficient legume species may contribute to improving forage sustainability under increasing water constraints.