Contribution of Genetic and Agronomic Measures to Yield Gain of Wheat in Iran

World population will be increased up to 9 billion and food demand up to 50 percent by 2050. This exponential increase in population, without an associated increase in arable land, in recent years, significantly threaten crop production. Therefore, engineering crop plants in order to achieve greater yields has been a major focus of plant biologists and breeders with a view to ensuring food availability for an increasing world population under changing environmental conditions. Plant performance is strongly associated with, and dependent on, plant development and growth. Several developmental features of plants, such as overall plant architecture, leaf features and vasculature architecture, are major traits that determine the overall performance of crop plants. The importance of plant developmental features in increasing crop yield potential became evident during the ‘green revolution’, when an unprecedented increase in yield was achieved by breeding for semi-dwarf varieties of rice and wheat. Furthermore, due to rapid global environmental changes, restricted land and water resources,  increasing food production particularly for wheat should be achieved mainly by increased crop yield (Koning & van Ittersum, 2009). Yield could be increased by genetic or agronomic measures and understanding the share of each component is of great importance for designing future cropping systems. However, these issues are not fully studied and quantified. Therefore, in this research yield trend of irrigated wheat at national level is analyzed for 1971-2011 period and contribution of cultivar improvement and agronomic management to yield increment have been estimated. Trends of total production, cultivated area and yield of irrigated wheat were analyzed for the country for 40 years (1971-2011) using linear regression with slope as annual increment rate of each variable. Weather-adjusted yield trend was also estimated based on residuals of regression as described by Nassiri Mahallati & Koocheki (2014a). Share of cultivated area and yield in total production was calculated by using component analysis. Annual changes in yield of irrigated wheat (dY, kg ha-1 yr-1 i.e. the slope of yield trend model) described by Equ.(1): dY = dG + dN + dP + dO (1)
where dG, dN, dP and dO (all in kg ha-1 yr-1) are annual yield increment due to genetic, N-fertilizers, P-fertilizers and Other agronomic factors. Contribution of genetic factors (dG) was estimated based on the cultivar improvement index (Silvey, 1981). Share of chemical fertilizers (dN and dP) in remaining yield increase (dY - dG) was calculated for each year during the study period. Finally, contribution of other agronomic measures (dO) to yield was estimated by subtracting right hand terms of Equ. (1) from dY. Wheat production and yield was increased by 3.2 and 1.4 times over the studied period, respectively. However, cultivated area after a sharp increase at 2.8% per year until 2000 was decreased in the last decade by 1% per year. Annual weather variation showed significant effect on irrigated wheat yield so that averaged over the 40 years, estimated rate of yield increment was 24% lower than weather adjusted yield. During the 4 decades mean contribution of cultivated area and yield on total wheat production were 44 and 56%, respectively and it was estimated that share of yield will be increased up to 70% in the present decade. Genetic yield potential of irrigated wheat cultivars has increased at 57 kg ha-1 yr-1 (1.18% per year) for  the period 1968-2011. During the studied period wheat cultivar improvement contributed to annual rate of yield increment by 34.8% while in the same period average contribution of N and P fertilizers were estimated as 25.4 and 8.8%, respectively leaving 31% for the other agronomic measures.