Integration of Biostimulants Alongside Various Advanced Nitrogen Fertilization Practices Improve the Yield, Quality, and Sustainability of Malting Barley in Mediterranean Conditions

Barley (Hordeum vulgare L.) is a key cereal crop for malting and brewing, where grain plumpness and optimal grain protein concentration (GPC) are essential quality traits. This study investigated the combined effects of nitrogen fertilization strategies and a seaweed-based biostimulant (Ascophyllum nodosum extract) on malting barley production across four environments in Thessaly, Greece, over two growing seasons. Treatments included urea (U), urea with biostimulant (U + B), urea with urease inhibitor (UI), urea with urease inhibitor and biostimulant (UI + B), and a control (no fertilization). Applications were tested on genotype G20 at mid-development (Z30–33) and genotype G45 at an earlier stage (Z24–30). UI + B treatment consistently enhanced yield by up to 71%, thousand-grain weight by 27%, and spikelets per square meter by 75% relative to the control, with responses influenced by genotype and environment. Grain fractions > 2.8 mm increased by up to 22% under UI + B, while GPC remained within the optimal malting range (9.5–11.5%). Early-stage applications produced strong benefits overall. Principal component analysis distinguished treatment effects, with UI + B samples clustering consistently apart from controls. These results demonstrate that combining biostimulants with urease inhibitors can simultaneously improve yield, quality, and sustainability in malting barley, supporting reduced nitrogen input in Mediterranean systems.