Optimising wheat production through management in the semi-arid tropics of Central Queensland

Optimisation of wheat productivity encompasses the improvement of total aboveground dry matter (AGDM) for hay and grain yield, harvest index, and grain quality, through improved efficiencies of water and nutrient use (WUE and NUE s leads to increasing profitability and sustainability of farming systems. To achieve this objective, defoliation experiments over three seasons (1997, 1998 and 1999) and fertiliser treatments on wheat during the same period were conducted at Biloela, Jambin and Alton Downs in Central Queensland (CQ).

Overall, defoliation at the middle tillering (DMT) and late tillering (DLT) growth stages increased not only wheat grain yield (by c. 7.4%) through the increase of harvest index by 5.9-12.5%, but also increased WUE by 16.2-18.1%. Nitrogen uptake efficiency was also increased by 6.5% when 76 kg of N was applied. Conditions for the positive effects of defoliation were defined, especially in crop -rain, and the probabilities for success of defoliation were also estimated. In addition, to secure the positive effect of defoliation, cultivars should be sown early enough yet without the risk of frost attack during pre -flowering to grain filling. Wheat growers will be able to decide the appropriate sowing time based on their own experience and the sowing guideline annually produced by the Queensland Department of Primary Industries.

Analyses of physiological mechanism indicated that the following four likely interlinked aspects contributed to the grain yield increase with defoliation (DLT in 1998and DMT in 1999): it conserved 32mm and 15mm of soil water from the time of treatment imposed until anthesis in 1998 and the early boot in 1999; respectively; it suppressed the vegetative growth and optimised the ratio of AGDM at anthesis over that at final harvest (0.5-0.8, peak 0.66) for maximal grain production; it increased leaf area ratio which was positively related to grain yield; and it enhanced overall photosynthetic capacity at anthesis and during milk -grain filling.

The results of fertiliser trials indicated that application of basal N (40 and 80 kg ha 1) alone to the water -limited soil showing alkalinity and salinity/sodicity did not help the wheat produce the expected higher grain yield or grain protein. Therefore, superphosphate was applied (600 kg ha -1) with and without basal N to attempt to ameliorate the soil properties with an anticipated positive effect in following seasons as well. As a result, the mean grain yield was increased by 21.9%, WUE by 21%, and grain N by 21.3%. Minor benefits of soil amelioration were also initially evident through reduction of soil pH (0-10 cm) and increase of Na+ and Cl- uptake in the plant AGDM.

At another site without the problems of alkalinity and salinity/sodicity, the application of basal N (49 kg ha 1)to rainfed and water -limited wheat did not increase grain yield, but increased grain protein from 9.7% to 13%.

Irrespective of soil and follow-up rainfall conditions, the application of foliar N increased grain protein from 12.6% to 13.4% at Jambin, and from 10.6% to 11.7% at Alton Downs, provided that the solution is applied in cool weather with low concentration (c. 5% urea equivalent).

We conclude that, as a part of sustainable farming systems in CQ, crop canopy modification and integrated fertiliser strategies are two important field management practices to improve grain productivity along with increase of grain WUE and NUE. However, it is proposed that these two packages could be integrated for maximum benefits.