Variability and trends in sowing dates across the Australian wheatbelt

Abstract

As sowing dates are critical for appropriate yield forecasting, a national survey of Australian wheat farmers was undertaken. This revealed that wheat sowing generally takes 2-4 weeks to complete between the middle of May and the middle of June. Distinct regional differences occur in the way sowing is completed and these are related to soil and climatic effects. In Western Australia, sowing follows a more distinct `break in the season" and the midpoint of farm sowing is fairly uniform across cropping areas. As one progresses into south-eastern and then north-eastern cropping areas the spatial variability in sowing increases. The combination of fallowing practices, unreliable autumn rainfall, and heavier soils (that delay operations when conditions are wet or dry), all add to the variability in sowing date and sowing duration in north-eastern areas. The range of midpoint in sowing (between years) generally decreases as the progression is made from a farm, to a State, to a national scale. Reduced variability at a national scale is enhanced by broad-scale weather patterns causing sowing opportunities to contrast markedly on different sides of the country. During the 1980s, sowing progressed a day earlier per year at a national scale. The most pronounced changes occurred in Queensland and Western Australia, where a 2-3-week shift to earlier sowing was recorded. Coinciding with this was a trend in all areas to reduced or minimum tillage techniques. Late opening rains in South Australia restricted early sowing opportunities during this time.