A national study of 250 leading grain farms has revealed the size, variation and cause of yield gaps across Australia, and highlighted two options to ensure crops reach their full potential.
Lead researcher Roger Lawes from CSIRO Agriculture and Food told the Grain Research and Development Corporation’s (GRDC) Grain Research Update in Wagga Wagga today that there is potential to reduce the size of the yield gap through more targeted nitrogen (N) management and crop rotation, but multiple interacting factors can influence the final outcome.
CSIRO researcher Roger Lawes
His team’s GRDC-investment project, the National Paddock Survey – what causes the yield gap across Australian paddocks? - sought to identify the critical barriers to achieving full potential in the Northern, Southern and Western GRDC regions.
The yield gap is the difference between the actual yield achieved by the grower and the water limited yield potential, which is the maximum possible yield able to be grown with the optimal sowing date, current varieties and nutrients, and with limited effect from pests, diseases and weeds.
Dr Lawes says while a small yield gap indicates that management is near optimum, the National Paddock Survey showed that priorities for growers to improve yield and profit differ in each region.
“In the Northern region, growing season rainfall, applied nitrogen and root health score were the three most important variables. High yielding crops were challenging to grow because these crops have a high N demand that was sometimes not met.” he said.
“In the Southern region, the amount of applied N, the previous crop and root health score were the three most important variables. Weeds and growing season rainfall were secondary drivers.
“In the Western region, growing season rainfall was the most important variable to explain the yield gap, followed by the amount of N and the previous crops – wheat on wheat is still common in the region.
“This suggests that larger yield gaps occur in the high rainfall zone, possibly because these yields are harder and riskier to achieve. N does appear to be limiting the ability of growers to capture these higher yield potentials, while crop rotation is playing a role.”
The average amount of N fertiliser applied to wheat was 26kg N/ha, 43kg N/ha and 32kg N/ha in the Northern, Southern and Western regions, but this varied greatly among paddocks within each region.
“Growers are sometimes unable to capture the extra yield on offer in the high rainfall zones, but given the risks associated with targeting high yields, that’s understandable,” Dr Lawes said.
Previous research using shire-level data showed that the current yields growers are achieving across Australia are about half of that which is potentially possible (Hochman et al. 2016).
“Achieving yield potential is not uncommon though, and it’s possible for a broad range of growers with paddocks of all soil types and rainfalls.”
In the Northern region, 43% of paddocks fell with 80% to 100% of yield potential, while in the Southern region, 38% of wheat paddocks fell within this range. In the Western Region, 46% of wheat paddocks achieved between 80% and 100% of yield potential.
The National Paddock Survey concluded that:
* The yield gap, or the difference between actual and potential yield in wheat, was 1.1t/ha in the Northern region, 1.2t/ha in the Southern region, and 1.3t/ha in the Western region.
* No one factor (nitrogen (N), disease, weeds or rainfall) is a cause of the yield gap — it is a combination of factors.
* Growing season rainfall, the previous crop or crop sequence, N application, levels of disease and weeds were all important factors that impacted on the yield gap.
Research was carried out over the 2015-16 season, on farms owned by leading growers that represented the range of prevailing rainfall and soil conditions across the grain belt.
Average dry wheat yields, monitored by growers with grain harvesters, were higher in the Northern region (4.1t/ha) and Southern Region (3.7t/ha) and lower in the Western region (2.5t/ha).
The average weed density at Zadok’s stage 31 was 10 plants/m2 in both the Western and Southern regions with few paddocks with weeds detected in the Northern region. The incidences of diseases and insects were generally minor in most paddocks and on average, the root health score was low across all three regions.
Commercial wheat crops were monitored by collaborating consultants from pre-sowing to harvest at two transects (zones) selected within each paddock and correlated with plant water use, with yield maps and hand-cuts used to summarise actual crop yield.
To estimate water limited potential yield, researchers used the crop simulation model Agricultural Production Systems sIMulator (APSIM) combined with surveys of soil properties and agronomic practices.
While the potential exists to help growers increase on-farm yields by targeting the key factors that contribute to the yield gap, Dr Lawes says more work needs to be done to understand the constraints at an individual paddock level.