One of the main problems in crop production in Iran is the difference between actual yield and attainable yield which is called yield gap. Thus, identifying yield constraints and yield gap are important. To do this, a study was conducted in 95 farms Gorgan of in five villages, during two years 2007 and 2008. In this study, all of the information about management operations, soil and crop characteristics were recorded and measured (including 200 variables). Then, relationships between variables and yield were considered (using stepwise regression). The results showed there is a 2348 kg/ha difference (gap) between actual yield (average farmers' yield) and attainable yield (maximum farmers' yield). It was identified that rate of potassium fertilizer before planting, rate of nitrogen fertilizer after planting, leaf area index at anthesis, leaf chlorophyll index at anthesis, total nitrogen uptake at maturity and the length of vegetative growth period were responsible in creation of yield gap with contribution of about 20, 18, 19, 10, 14 and 19%, respectively. Data investigations revealed that leaf area index at anthesis, leaf chlorophyll index at anthesis and total nitrogen uptake at maturity were controlled by nitrogen fertilizer management. Also, the length of vegetative growth period was controlled by planting date. Thus, it was then concluded that rate of applied potassium, nitrogen nutrition management and planting date are the most important responsible factors in creating yield gap with contribution of about 20, 61 and 19%, respectively. Optimizing of these factors could be increase wheat yield in Gorgan up to 2348 kg/ha.
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