Genetic Variation for Autumn–Winter Forage Yield in a Segregating Tetraploid F1 Population of Paspalum notatum

Autumn–winter forage scarcity limits subtropical livestock systems. This study aimed to: (1) develop a segregating F1 population from parents contrasting in autumn–winter biomass yield (WBY) in tetraploid Paspalum notatum; (2) estimate phenotypic and genetic variability for WBY across environments; and (3) evaluate the relationship between WBY and spring–summer biomass yield (SBY), and the feasibility of unmanned aerial vehicle (UAV)-derived vegetation indices as non-destructive estimators of WBY. A population of 182 tetraploid F1 hybrids was evaluated at two sites in Corrientes Province, Argentina (2022–2024). WBY exhibited wide genotypic variability across locations and years (p < 0.001), with significant genotype, location, and genotype × location effects. Broad-sense heritability (H2) ranged from 0.41 to 0.64, reflecting sensitivity to the thermal and moisture conditions of each environment. WBY showed a positive, moderate association with SBY (R2 = 0.20–0.26), indicating that selection for cool-season yield does not compromise summer productivity. The Normalized Difference Red Edge Index (NDRE) was the most robust WBY predictor (R2 up to 0.67 at MES-2022 vs. 0.58–0.59 for ARVI, GNDVI and NDVI at the same site–year), though predictive accuracy varied with environmental conditions. The results demonstrate substantial and exploitable genetic variation for cool-season forage yield in P. notatum.