THE KEY to increasing grain yields on Mallee farms is primarily linked to making more efficient use of rainfall rather than being tied to the actual amount of rain that falls in a given year.
This direction is encouraged by a three-year study by CSIRO Land and Water scientists Victor Sadras, David Roget and Darry O'Leary working as part of the GRDCsupported Mallee Sustainable Farming Project. They analysed yields from 75 wheat crops grown in NSW, Victoria and SA for three seasons from 1998.
According to the researchers the results clearly demonstrate that:
- water-use efficiency (WUE) is generally low across the Mallee
- more attention needs to be paid to nitrogen as a factor in the efficient use of rainfall, particularly as cropping intensity increases
- there is enormous potential not only for individual Mallee farmers to increase crop yields, but also for overall production in the region.
Measurements included those relating to soil water levels, soil nutrient status and soil-borne root diseases while subsoil constraints such as high salinity, sodicity or soil boron levels were also identified. As well, the researchers had access to information about each crop including paddock history, crop preparation, sowing dates and the type and amount of fertilisers applied.
Yield far from potential
"Over the three years we found that attainable yield per unit growing season rainfall was around 21 kg/ha/mm," Dr Sadras said. "Yet 75 per cent of crops produced less than 12 kg/ha/mm of seasonal rainfall. "Our analysis shows that rainfall accounted for less than a third in the variation in yields. It is clear that efficiency in the use of rain is a much more important factor to deal with. Low efficiency was related to both crop management and subsoil constraints."
Management: look at time of sowing and nitrogen
Mr Roget said that Mallee farmers would need to pay more attention to the amount of nitrogen available to crops, and to the time of sowing. "There are some farms where medics are fixing nitrogen well and others where they are not. In general it appears that pulse crops are not providing following wheat crops with much nitrogen;' he said.
"Because of these variations it is critical that farmers, particularly those involved in intensive cropping systems, monitor soil nitrogen levels and apply nitrogen if soil levels are down, and if seasonal yield potential is reasonable to high. If plants are short of nitrogen you get reduced water-use efficiency and, at the end of the season, grain protein levels are lower too."
The link between early sowing and high WUE was also clearly demonstrated in the study. Sowing dates ranged from 24 April to 21 July and yields fell with delayed sowings at an average rate of 17 kg/ha/day, or about 0.5 t/month.
"Of course, farmers have no control over when rain falls but it is critical that paddocks are ready to be sown as soon as it does," Mr Roget said. "That means having weeds and crop residues under control and the equipment to sow quickly and efficiently.
"If you are getting, say, 200 mm of growing season rainfall, then there is the potential to achieve yields of 2.8 t/ha, or more, where there is stored soil water and no serious subsoil problem.
"The study showed, however, that 50 per cent of the crops yielded less than 1.85 t/ha."
Subsoil problems bar roots from waters
The researchers found that, in some of the crops studied, root diseases and chemical soil constraints, including salinity, alkalinity, sodicity and boron toxicity, reduced crop yields as plant roots had limited access to moisture at depth (generally below 50 cm).
These constraints, and practical ways they can be identified and managed, are the subject of a separate GRDC-supported study by Dr O'Leary. (See Ground Cover 36.)
Program 4 Contact: Dr Victor Sadra. 08 8303 8543 Mr David Roget 08 8303 8528
This graph shows grain yield of wheat crops in grower-managed paddocks in the Mallee between 1998 and 2000. Half the crops yielded less than 1.85 t/ha.
This graph shows grain yield per unit seasonal rainfall; 75 per cent of crops produced less than 12 kg grain/ha/mm rain