Study of Changes in Long-term Wheat Production Trend and Factors Affecting it in North Khorasan Province: I- Irrigated Wheat

One of the main challenges of modern agriculture in ensuring food security is development of strategies to deal with potential negative impacts and adapt to climate change. To address this challenge, it is crucial to investigate the effects of climatic factors on agricultural production at a spatiotemporal dimension, develop and utilize crop management decision-support tools, and support targeted agronomic research and policy. These endeavors necessitate the availability of accurate and standardized meteorological data.Studying growth degree days and wheat phenology can significantly enhance our understanding of how wheat growth responds to climate change and aid farmers in adapting to and effectively mitigating its influence.

To determine the environmental and management factors affecting the yield of irrigated and rainfed wheat in different regions of North Khorasan province, we investigated the trend of yield changes from 1980 to 2009. Subsequently, we simulated the wheat plant growth stages using the DSSAT model and analyzed the impact of temperature and rainfall changes on yield through panel data analysis. Panel data analysis is a widely used statistical method in social science, epidemiology, and econometrics for analyzing two-dimensional (typically cross-sectional and longitudinal) panel data. This method involves collecting data over time from the same individuals and conducting regression analysis across these two dimensions.

According to the results of this study, 63% of the changes in irrigated wheat yield between the years 1980-2009 can be attributed to environmental factors (temperature and precipitation), while 37% can be attributed to management factors. When comparing environmental parameters, it was observed that the number of temperatures above 30°C (N30TMAX), mean temperature (GSTMEAN), interaction of amount and frequency of precipitation (TPRAT * NPRAT) significantly affect yield (p ≤ 0.05). Bojnord, Shirvan, and Esfarayen regions exhibited significant positive cross-sectional effects in terms of environmental parameters, whereas Farooj, Raz-Jargalan, Maneh Semelghan, and Jajarm regions displayed negative cross-sectional fixed effects.A study examining the critical stages of wheat growth during good years (with high wheat grain yield) and poor years (with low wheat grain yield) revealed that in all weak years, the minimum temperatures fell below the critical level (-11°C). The occurrence of very low temperatures during the early stages of growth and primary leaf production, which is the plant establishment stage, resulted in reduced photosynthesis levels and subsequently severe yield reduction.In all regions and for 100% of the studied years, irrigated wheat in the grain-filling stage experienced temperatures above 30°C, leading to negative cross-sectional effects in Farooj, Raz-Jargalan, Maneh-Semelghan, and Jajarm. The frequency of temperatures above 30°C during the hard dough stage of irrigated wheat was higher than that during the soft dough stage in all regions. Therefore, delaying the planting date from October (the common planting date in the studied areas) would result in conflicts with high temperatures during the soft dough stage and negative temperatures during the primary leaf production stage and plant establishment at the beginning of the growing season, severely reducing yield.

In general, the results of this study demonstrated that implementing effective management methods, particularly selecting the appropriate planting date, can lead to better adaptation of wheat's phenological stages to environmental conditions. This, in turn, has the potential to enhance wheat yield.