Growers in higher rainfall environments who can regularly grow wheat that attains high yields have a good understanding of crop development, wheat growth stages and the influence of climate and cultivar on grain yield.
The longer the growing season the more time the crop has to capture available resources including sunlight and water, and convert these into dry matter and ultimately grain. Consequently, cultivars that take longer to mature tend to have higher grain yield potential than cultivars that mature quickly. Crop growth before flowering allows the crop to build the physical structure (e.g. stems and roots) that hold the plant upright. It also sets the potential number of grains that fill from dry matter produced after flowering.
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Growth stages need to be timed to ensure the dry matter produced is partitioned efficiently into roots, leaf, stem, ear and grain and that flowering occurs during the period of least climatic risk. Climatic risks that cause crop stress are temperatures less than 0°C (frost risk) and greater than 30°C (heat stress). In some regions, reaching the optimum flowering time is a compromise between flowering early enough to avoid heat stress and late enough to avoid frost.
Controls for crop development
The timing of growth stages is determined by how quickly or slowly the crop develops. Crop development is controlled by temperature and day length.
Accumulated temperature or day degrees (°Cd)
The accumulation of mean daily temperature or day degrees (°Cd) dictates crop development. Various terms have been used to describe this including thermal time. A crop will develop rapidly under warm conditions. For example sowing to emergence generally takes 150°Cd. If the average temperature per day during this period is 10°C, then sowing to emergence will take 15 days (150/10=15). If the average temperature is 15°C, then it will take only 10 days.
Photoperiod or day length
Day length can influence the timing of the different growth stages. Crops sown at lower latitudes (closer to the equator) do not experience the same extremes in day length as those at higher latitudes. A cultivar sown in Queensland or New South Wales is likely to have a very different flowering date and thermal times to the same cultivar sown in Victoria because of differences in day length. For wheat crops the longer the day length the shorter the thermal time needed for flowering.
Winter cultivars (such as Mackellar and EGA Wedgetail) need a period of cold temperature (vernalisation) to trigger the onset of the reproductive phase. This is a survival mechanism delaying development to avoid flowering during high risk periods in winter. Vernalisation is unlikely to occur, or be seriously reduced when a winter cultivar is sown in spring, resulting either in flowering being erratic or not occurring at all.