Optimizing legume green manure applications for enhanced forage sorghum-sudangrass performance and soil property improvements.

Background: Legume-based green manuring is an environmentally sustainable and economically viable approach that enhances soil fertility by improving organic matter content, facilitating biological nitrogen fixation, and stimulating microbial activity. These benefits collectively reduce reliance on mineral fertilizers, which are associated with environmental degradation. While the soil-enhancing properties of legumes are well-established, limited information is available regarding how the timing of green manure incorporation affects the growth performance and forage quality of subsequent crops. This study aimed to assess the effects of incorporating different legume species at various phenological stages on the growth, yield, and forage quality of sorghum × sudangrass under Mediterranean climatic conditions. Methods: A field experiment was conducted from 2022 to 2024 at Aydın Adnan Menderes University, Türkiye, using a randomized complete block design with three replicates. Eleven treatments were implemented, consisting of three legume species (common vetch, narbon vetch, and forage pea) incorporated at three distinct phenological stages (pre-flowering, 10% flowering, and full flowering), along with unfertilized and fertilized controls. Following incorporation, sorghum × sudangrass was sown without additional fertilization, except in the fertilized control. Key response variables included fresh forage yield (FFY), hay yield (HY), plant height, leaf number, leaf area index (LAI), chlorophyll content (SPAD), and crude protein yield (CPY). Soil organic matter and total nitrogen levels were also measured before and after treatments. Results: Green manure application significantly enhanced the growth, biomass accumulation, and forage quality of sorghum × sudangrass. The incorporation of common vetch and narbon vetch at the 10% flowering stage led to up to a 50% increase in fresh forage yield compared to the unfertilized control. Forage pea maintained consistent yield performance across all incorporation stages. Improvements in soil organic matter and nitrogen content were particularly notable in legume-amended plots. Furthermore, the highest SPAD and LAI values were recorded at early flowering stages, suggesting enhanced photosynthetic efficiency. These findings underscore the effectiveness of legume green manuring, particularly with optimal incorporation timing, as a sustainable strategy to improve soil health, forage productivity, and resource-use efficiency in Mediterranean agroecosystems.