VARIABILITY AND TRENDS IN SOWING DATES ACROSS THE AUSTRALIAN WHEAT-BELT

As sowing dates are critical for appropriate yield forecasting, a nati onal survey of Australian wheat farmers was undertaken. This revealed that wheat sowing generally takes 2-4 weeks to complete between the mi ddle of May and the middle of June. Distinct regional differences occu r in the way sowing is completed and these are related to soil and cli matic effects. Tn Western Australia, sowing follows a more distinct 'b reak in the season' and the midpoint of farm sowing is fairly uniform across cropping areas. As one progresses into south-eastern and then n orth-eastern cropping areas the spatial variability in sowing increase s. The combination of fallowing practices, unreliable autumn rainfall, and heavier soils (that delay operations when conditions are wet or d ry), all add to the variability in sowing date and sowing duration in north-eastern areas. The range of midpoint in sowing (between years) g enerally decreases as the progression is made from a farm, to a State, to a national scale. Reduced variability at a national scale is enhan ced by broad-scale weather patterns causing sowing opportunities to co ntrast markedly on different sides of the country. During the 1980s, s owing 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. La te opening rains in South Australia restricted early sowing opportunit ies during this time.