VARIABILITY IN HARVEST INDEX OF GRAIN CROPS AND POTENTIAL SIGNIFICANCE FOR CARBON ACCOUNTING: EXAMPLES FROM AUSTRALIAN AGRICULTURE

For grain crops, harvest index (HI) is the ratio of harvested grain to total shoot dry matter, and this can be used as a measure of reproductive efficiency. The index can also be used to estimate crop carbon (C) balances by applying it to grain yield statistics to determine total shoot dry matter and then calculating crop residues as the difference between shoot C and grain C. Such an approach is widely used in C-accounting systems. Such a C-accounting practice is sensitive to changes in HI. In Australia, measured variations in H I are large enough to alter C balance calculations for some crops. Much of this variation results from the diverse range of climates and soils, which are a feature of the Australian cereal cropping region. Factors that influence crop HI include the energy and protein content of seeds, long-term breeding achievements, and extreme (either hot or cold) temperatures during crop reproductive development. Crop husbandry can also influence HI, especially delayed sowing, which shortens the length of the vegetative phase and increases HI. For wheat, and perhaps some other C-3 cereals, excess nitrogen can enhance the allocation of photosynthate to structural carbon, which cannot be mobilized to grain later, resulting in a decrease in HI. Evidence for the balance between pre and postanthesis water use of field-grown crops having a significant influence on crop HI is equivocal. A dataset containing more than 3000 estimates of HI in Australia has been assembled and used to summarize observed HI variations for each of the principal field crops grown in Australia. There remains a need for more reliable field HI data to be used in C-accounting systems and to aid the development of models to simulate likely regional and seasonal differences in HI for C-accounting purposes.