To achieve desirable - or optimum - wheat yields, nitrogen application rates should be adjusted to take into consideration sowing date, cultivar type and temperatures around flowering.
This recommendation is one of the key preliminary findings to stem from research conducted by South Australian Research and Development Institute (SARDI) researchers Dr Mariano Cossani and Dr Victor Sadras, investigating the combined effect of elevated temperatures and nitrogen supply on wheat yield.
"We are looking at the combined effects of temperature and nitrogen, because nitrogen is one of the main drivers of yield but yield can also be reduced by elevated temperature ," Dr Cossani says.
"We don't know much about the interaction between these two factors when it comes to impacting yield."
Dr Cossani says it is well known high temperatures during critical stages, such as flowering and grain fill, can cause yield penalties.
He says nitrogen management is an important decision for wheat growers and knowing the relationship between nitrogen and temperature could be beneficial for decision making and risk reduction.
We are looking at the combined effects of temperature and nitrogen, because nitrogen is one of the main drivers of yield but yield can also be reduced by elevated temperature.
Sowing date trials looked to answer the question of whether crops need more or less nitrogen when sown late and grown under elevated temperatures, compared to early sown crops.
Trials were carried out at four locations in SA's Mid North over two contrasting seasons, 2017 with an average yield of 4.1 tonnes per hectare and 2018 with an average yield of 1.4t/ha.
Six cultivars - Axe (PBR), Cobra (PBR), Mace (PBR), Scout (PBR), Spitfire (PBR) and Trojan (PBR) - were sown at four dates (from 12 May to 10 July), with four nitrogen rates (nil, 50, 100 and 200kg/ha).
Across all cultivars, yield declined at a rate of 26kg/ha per day of delay in sowing which correlated to higher temperatures during the critical flowering and grain filling period.
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"Sowing date trials showed a maximum yield loss of 0.67 t/ha for each degree Celsius increase in mean temperature during the critical period," Dr Cossani says.
"For farmers, who are more familiar with maximum temperature, the loss was about 0.5t/ha for each 1 degrees Celsius of maximum temperature."
Results showed early-sown long season cultivars had high yields and that an increase in temperature of 1.7 degrees above ambient before flowering reduced the grain number and yield in unfertilised crops.
However, grain number and yield were not reduced in the fertilised crops.
It was shown that the response of specific cultivars to nitrogen depended largely upon sowing time and location.
The early maturing Axe (PBR) variety and the mid-to-long maturing Trojan (PBR) variety both had increased yield responses to nitrogen, while mid-season cultivars Mace (PBR), Cobra (PBR) and Spitfire (PBR) did not.
"Late-sown crops grow under high temperature, but also under higher radiation and evaporative demand, compared to early sown crops," Dr Cossani says.
"To untangle these factors, we also did experiments using a novel open-top heating system."
In the heating trials, three times of sowing, two cultivars - Mace (PBR) and Spitfire (PBR) - and two nitrogen rates (nil and 100kg/ha) were examined under two temperature conditions:
- Heating before or after flowering
- Unheated controls.
"As expected, yield decreased when the temperature was higher during the critical period for yield determination from booting to 10 days after flowering," Dr Cossani says.
The grain yield of both fertilised and unfertilised cultivars was affected by temperature, sowing time and nitrogen.
Nitrogen applications increased yield by 1.4t/ha in the first time of sowing and had no effect on the second time of sowing, while delayed sowing reduced grain yield of fertilised crops but not of the unfertilised crops.
Warming before anthesis reduced yield in unfertilised crops but not in fertilised crops.
Increasing the temperature before anthesis and a lack of nitrogen application reduced both grain number and kernel weight in both sowing times.
"The maintenance of yield in fertilised crops despite the elevated temperature leads us to believe that nitrogen application can be tailored to mitigate the impact of higher temperatures," Dr Cossani says.
Dr Cossani says results stemming from two contrasting years highlight the importance of temperature in dry and wet seasons.
Trials will continue in 2019.
GRDC Research Code: DAS00166_BA
More information: Mariano Cossani, SARDI, 0437 828 185, email@example.com