Amor, thank you very much for your interesting question. Most plant growth and development models use knowledge of environmental factors as they influenced the plant during the course of growth. LAI is dependent upon leaf area development within a fixed area of land. LAI is dependent on the rate of leaf growth, a factor which is controlled primarily by temperature in the crop environment. Now, your observation that simulated LAI peaks are always earlier than observed values might be due to slow temperature changes in the crop environment while the model is simulating temperature changes following Julian day of the year. Thus, simulated temperature appears to be higher than the actual temperature observed in the field. You could check the changes in temperature during the period of growth of your wheat and see if they were congruent. Other causes could be the varietal differences, i.e. whether early or late maturing types. Check these factors to see if they could help you to unravel this problem.