Abstract: Field experiment was conducted to determine the effect of individual plant vigour on the final grain yield in sunflower.Time of emergence and distances to the neighbouring plants within each of the three natural rows were recorded in the field.Head dry weight was measured for each individual plant after harvest.and grain dry weights of individual plants were measured on 50 head subsample for each of the three rows,to draw relationship between head and grain dry weight for prediction of the latter from the former.Results show that it in higii'y reliable to predict grain dry weight from head dry weight of a plant according to the regression between the two (R²=0.97-0.99).For quantifying grain yield loss due to poor plant stand, although the regression technique of Wade et al (1988) has proved valuable, adding individual piant vigour for time of emergence into the equation can greatly improve the accountability of the regression.K R² of the multiple regression increase from 0.25, when only area per plant and coefficient of variation in plant spacing were included, to 0.44, when time of emergence was added.Moreover the interactions between area per plant and variation in plant spacing,area per plant and individual plant vigour are also very important.Among all the factors influencing individual plant yield,area per plant and time of emergence seem the most important.

Key words: Sunflower, Seedling stage, Mathematical model, Computation of yield, Individual plant vigour