A simple method to estimate harvest index in grain crops

Several methods have been proposed to simulate yield in crop simulation models. In this work, we present a simple method to estimate harvest index (HI) of grain crops based on fractional post-anthesis phase growth (f_G = fraction of biomass accumulation that occurred in the post-anthesis phase). We propose that HI increases in a linear or curvilinear fashion in response to f_G. The linear model has two parameters, the intercept (HI_o) and the slope (s). The curvilinear model was assumed to be monotonic: HI = HI_x - (HI_x - HI_o) · exp(-k·f_G); where HI_x is the asymptote, HI_o is the intercept and k is a constant modulating the rate of HI increase. The models were tested for barley (Pullman, WA and Uruguay), wheat (Pullman, WA) and sorghum (Australia). A positive relationship between HI and f_G was in general evident. For barley, the linear model appropriately represented the response of HI to f_G, with both HI_o and s in the vicinity of 0.3. For wheat HI_o and s were 0.34 and 0.21, respectively, but the curvilinear model yielded a slightly better fitting than the linear model. For sorghum, both linear and linear-plateau models fitted data reasonably well. It is shown that the models work well in crops source-limited or source-sink co-limited during grain filling, but in sink-limited conditions the magnitude of the limitation needs to be characterized to compute HI. A major advantage of this method is that the parameters of the linear or curvilinear model are readily calibrated from yield data and biomass measurements at anthesis and harvest.