Determination of Marginal Effects of Climatic and Phenotypic Factors on Risk and Average Yield of Irrigated Wheat Lines in Breeding Research

One of the important and effective economic activities in the field of agricultural economics is to release varieties or to adopt crop improvement technology. This can result in increased yield and production of various crops, enhanced food security, self-reliance and reduced dependency (Aghaei & Asadi, 2010 ). Historically, breeders have focused more on increasing yield of varieties. However, reducing variability and instability in yield of varieties at different locations and times is considered as an important breeding goal (Byerlee & Moya, 1993).The objectives of this research were to determine the direct and indirect marginal effects of climatic and phenotypic factors, variety release year as breeders’ efforts in genetic improvement, location and time trend factors on the mean and variance of grain yield of irrigated wheat lines. Materials and .

In this study, the required panel data were collected from 23 agricultural locations method and the marginal effects of the variables on the mean and variance of wheat grain yield were assessed using stochastic production function of Just and Pope and spatial durbin models (SDMs). Just and Pope production function was estimated with linear functions in three steps as follows.(Just & Pope, 1979). Where; Yht: mean yield of bread wheat lines (kg/ha) in year t, trend: time trend, Ln (ei ) 2 : yield risk for wheat lines in year t,, DHE: days to heading in year t, DMA: days to physiological maturity, GFD: mean of grain filling duration in year t, TGW: thousand grain weight (TGW) (g), MTem: mean of annual temperature ( MTem ) (0 C), S9Rain: sum of rainfall during cropping season ( S9Rain ) (mm), pH , DLoch : Dummy variables of locations, RLyear(h=1, 2,..23): variety release year, Φ.β: δ, ɑ: models parameters and ϵht:, residuals

According to the results, the study of spatial spillover effects using spatial durbin models (SDM) showed that the direct marginal effects of all the phenotypic and climatic factors on the mean and variance of grain yield of irrigated bread wheat lines were statistically significant. The direct marginal effect of time trend and variety release year on the mean and variance of grain yield of irrigated bread wheat lines was positive but not statistically significant .

In general, the direct marginal effects of all the phenotypic factors on the mean of grain yield of irrigated wheat lines was satistically significant. The direct marginal effect of time trend and variety release year on on the mean of grain yield of irrigated wheat lines was positive.