Time and the Productivity of Agronomic Crops and Cropping Systems

Economic yield of a crop is always a function of a growth rate expressed over time, but historically plant processes associated with rates probably received more attention than those associated with time. Time influences crop productivity in two ways; the time devoted to the growth of the crop can affect yield, and the time available for plant growth in any environment determines the potential resource capture by the crop. These relationships were investigated using data describing the length of the vegetative and reproductive growth phases of major grain crops. The total growth duration (TGD) of six crop species (n = 83) varied from 62 to 185 d. Vegetative growth averaged 67% of TGD while the reproductive phase accounted for 33%. Total biomass was related to the TGD, and seed yield was related to the length of the reproductive phase. There was a curvilinear relationship between TGD and reproductive phase duration with the maximum reproductive phase occurring at a TGD of similar to 110 d. Consequently, selecting a longer TGD cultivar does not necessarily increase seed yield, making it difficult to convert the extra resources available in long growing seasons into seed yield. The length of the vegetative phase can be manipulated to enhance yield or production efficiency by, for example, shortening it to place reproductive growth in a more favorable environment or to reduce the irrigation requirement. Involving time as well as rate in efforts to improve yield provides more opportunities to increase food supplies in the future.