Megathyrsus maximus (Guinea grass) has a high potential for use as a raw material for organic fertilizer production. In the present study, leaf nutrient content and dry matter yield of naturally grown M. maximus under a coconut plantation were measured when harvested 4, 6, 8, 10, and 12 weeks after initial cutting to evaluate its feasibility. All the Guinea grass in the experiment field was cut at a height of 15 cm and allowed to regrow before harvesting. The percentage of leaf nutrients, nitrogen, phosphorus, potassium, calcium, and magnesium were determined on a dry weight basis, and the dry matter yield per hectare was measured at each harvesting interval. Leaf nutrient content and dry matter yield of M. maximus varied significantly (p < 0.05) when harvested at different growth stages. The highest nutrient levels were recorded when harvested 4 weeks after initial cutting, and the lowest was 12 weeks after initial cutting. The highest dry matter yield was obtained 12 weeks after initial cutting, and the lowest was 4 weeks after initial cutting. The highest leaf nutrient levels recorded for nitrogen, phosphorus, potassium, calcium, and magnesium were 1.49%, 0.29%, 3.16%, 0.57%, and 0.45%, respectively. The highest dry matter yield was 1.89 t ha−1, and the lowest was 0.84 t ha−1. Considering leaf nutrient levels and the dry matter yield, it is suggested that M. maximus can be harvested six weeks after initial cutting to use as a raw material to produce organic fertilizer or as green manure for coconut plantations.