Harvest timing is a critical factor in viticulture, as it directly influences grape composition and, consequently, wine quality. This study evaluated the effects of harvest timing on the physical and chemical traits of two widely cultivated grape varieties from the Trebinje vineyard area, Bosnia and Herzegovina. Basic chemical parameters (total soluble solids, pH, titratable acidity) were measured alongside cluster and berry traits, including length, width, and weight of clusters; the number and weight of berries; the skin and flesh weight of 100 berries; and the number and weigh of seeds. Early-harvested grapes showed greater cluster weight but lower total soluble solids, higher titratable acidity, and lower pH compared to later harvests in both cultivars. Changes in cluster and berry traits were more pronounced in ‘Vranac’, where berry weight, skin and flesh mass increased significantly at later harvests, however, in ‘Žilavka’ only the seed weight was notably affected. Across all harvests period, ‘Žilavka’ consistently exhibited higher total soluble solids and lower pH values than ‘Vranac’, indicating a strong varietal effect on grape quality. These findings highlight that delaying harvest within the recommended window can enhance grape composition, while varietal differences determine the extent of morphological and chemical changes during ripening.

The aim of this study was to study the impact of crop load on the yield and grape quality of ‘Merlot’ and ‘Vranac’ (Vitis vinifera L.) in Trebinje vineyard. The crop load levels studied in this trial were 9 buds (V1) and 12 buds (V2) per vine at each variety trained on Lenz-Moser bilateral cordon system. The impact was determined by measurements of yield per vine and grape quality characterized by the contents of total soluble solids, titratable acidity, total phenolics, total flavonoids and by total antioxidant capacity. The measured parameters of grape quality of ‘Merlot’ and ‘Vranac’ were not influenced significantly by crop load levels. V2, compared to the V1, showed the potential for increasing grape yield only for Merlot variety under experimental conditions. The results of this study also showed a positive correlation between total phenolics/flavonoids and total antioxidant capacity of grape berries in both varieties, regardless of crop loads applied.