Fiber Accumulation Is Associated with Forage Quality Decline in Grass Pea (Lathyrus sativus L.): Genotypic and Harvest Stage Effects

Optimizing harvest timing and genotype selection is essential for forage quality in grass pea (Lathyrus sativus L.). Twelve genotypes were evaluated at five weekly harvest timings in a one-site, one-season (2019) field trial in Rize, on the humid Black Sea coast of Türkiye. Crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), acid detergent protein (ADP), P, K, Ca, and Mg were predicted by near-infrared reflectance spectroscopy (NIRS) on one sample per genotype × harvest combination, and digestibility and energy indices were calculated from ADF and NDF. Harvest timing affected all variables (p < 0.01). With later harvests, ADF and NDF generally increased, CP declined after peaking at the third harvest (29.07%), and calculated digestible dry matter and relative feed value decreased. The first three harvest timings (vegetative growth to early-to-mid flowering) combined higher CP with lower fiber. Genotypes differed in CP, P, K, Ca, and mineral ratios (p < 0.05) but not in fiber fractions or calculated indices. Because the genotype × harvest interaction could not be tested and biomass yield and β-ODAP were not measured, genotype comparisons are provisional. Fiber accumulation was associated with declining forage quality, highlighting the importance of early harvest timing.