Growth stage specific optima for the green area index of winter wheat

• Based on data from two field trials with winter wheat we estimated optimal GAI (green area index) at different growth stages.
• The optimal growth curve should then as a guideline to adjust the canopy in the context of canopy management.
• Our results reveal several problems of this approach implicating an insufficient applicability in its proposed form.
• These obstacles divide into measurement be used errors for both, growth stage and canopy size and the variability in optimal GAI curves.
• However, optimal GAI may allow at least more detailed ex post analysis of the limitation during the yield formation of wheat.

High grain yields and high N use efficiency in cereals both rely on optimal canopy dynamics during the growth period. Target values of green area index (GAI) and, consequently, N uptake for specific growth stages (GS) may be a simple and practical approach for adjusting crop N management site and year specifically. Based on data from two field trials with widely differing N fertilization rates over a range of years and with a different number of varieties (Exp. 1: 2004–2007, one variety; Exp. 2: 2003, 2005–2008, four varieties), this study aims (i) to identify optimal GAI at different GS of winter wheat (estimated from the final grain yield using Linear-Plateau-response curves) and (ii) to test if these reference GAI values are sufficiently constant between years and varieties to be, in consequence, suitable to manage the wheat canopy.Optimal GAI increased during stem elongation, however, the correlation between GAI and yield often remained poor in early growth stages. In Exp. 1, optimal GAI at GS >31 differed between the years with values ≤3 in 2004 and 2007 and ≥4 in 2005 and 2006. In Exp. 2, considerable differences between the tested varieties could be observed. In general, it was not possible to consistently relate the optimal GAI course during spring to growth stages. An optimal GAI course for winter wheat, as a simple guideline for the N management, seems to be no promising approach for the conditions tested.