GRDC Grains Research and Development : Diversity the key to balancing frost heat risks

Dr James Hunt, pictured, says planting a range of cultivars at their optimal sowing time can improve profitability for wheat growers.

Sowing a range of cultivars in their ideal sowing windows will give wheat growers the best chance of balancing the increasing risks of heat and frost damage.

Former CSIRO senior research scientist James Hunt says weather conditions in recent years have shown why it is so important to sow a diversified wheat program.

'We had unprecedented frosts in 2014 and we had very early heat conditions in 2015,' he said. 'It is more important than ever to optimise the sowing window so that, as much as possible, all wheat flowers in its ideal window to minimise the risk of frost or heat damage.'

A hot topic

The direct effects of heat stress are estimated to cost grain growers in south-east Australia almost $600 million per year and about $1.1 billion nation-wide. Frost is estimated to cost south-east Australia at least $100 million a year in unfulfilled or lost yield potential.

Due to the effects of climate change, both heat stress and frost are likely to play an increasing role in the future and will require growers to take steps to manage the risks.

Growers who plant the majority of their wheat program using a single high-performing cultivar struggle to plant their whole wheat program in a time close to the ideal sowing window. This can result in flowering occurring earlier or later than desired. This then leads to a higher heat stress risk if sowing is delayed or higher frost risk if planting too early.

For example, if the ideal sowing window is considered to be about five days either side of the target date, growers who sow a single cultivar over three weeks will have sown at least half of their crop (11 days out of 21) outside of this window. By comparison, if the wheat program was split up into two cultivars, almost 100 per cent of the crop can be sown in its ideal window.

Time of sowing

Dr Hunt says it would be impossible to choose a combination of sowing time and cultivars that would prevent exposure to heat and frost risk. However, time of sowing trials in South Australia and Victoria have shown that certain strategies will give crops the best chance.

'Depending on the local climate and duration of the wheat sowing program, growers can take a few different approaches to optimise time of sowing,' Dr Hunt said.

'In many regions of Victoria, growers can start with a winter wheat after a rain in April, then move onto slow-spring wheats and then mid-fast cultivars in May. The different maturity drivers of the cultivars mean that they still flower in the ideal window despite being sown at different times, meaning that overall yield is optimised and risk is minimised.'

A time of sowing trial at Berriwillock in Victoria showed that where there is soil moisture, sowing early can provide higher yields than traditional sowing dates (Table 1). In this trial, early rains were simulated with 8mm of irrigation; winter wheats should not be sown dry.

Yield (t/Ha)	Time of Sowing
Cultivar	9-Apr	8-May
Bolac	1.51	0.87
Yitpi	1.59	1.08
Kiora	1.68	0.98
Lancer	1.44	0.99
RAC2341	1.57	0.99
Scout	1.78	1.09
Wedgetail	1.40	0.70
P-Value	0.004
LSD (P=0.05)	0.14

Currently, winter wheats do not perform particularly well in South Australia. However, three years of trials have shown that incorporating different cultivars improves overall results. 'Trojan has an unusual photoperiod sensitivity which is rare in Australian cultivars. This seems to delay flowering from an April sowing relative to Mace quite successfully,' Dr Hunt said. Trojan sown in its ideal sowing window outperformed Mace sown in its ideal window in all time of sowing trials sites across SA, with an average benefit of 0.6t/ha, as shown in Table 2.

Yield (t/Ha)	Time of Sowing
Location	Cultivar	10-Apr	29-Apr	14-May
Cummins	Wedgetail	3.3	2.5	3.1
	Rosella	2.8	2.6	2.8
	Eaglehawk	3.3	2.7	2.8
	Cutlass	4.3	3.8	4.0
	Trojan	4.7	4.1	3.9
	Mace	4.0	4.1	4.0
	P-Value	0.012
	LSD (P=0.05)	0.5
Minnipa	Wedgetail	2.8	2.5	1.9
	Eaglehawk	2.9	2.3	1.8
	RAC2341	3.4	3.6	2.7
	Cutlass	3.5	3.6	2.9
	Trojan	3.3	3.9	3.0
	Mace	2.9	3.8	3.2
	P-Value	<0.001>
	LSD (P=0.05)	0.3
Port Germain	Wedgetail	2.7	2.3	1.6
	Rosella	2.7	2.3	1.4
	Eagelhawk	2.9	2.6	1.4
	Cutlass	4.1	3.8	2.3
	Trojan	4.5	4.0	2.6
	Mace	4.4	4.3	3.1
	P-Value	<0.001>
	LSD (P=0.05)	0.2
Hart	Wedgetail	3.6	3.4	2.8
	Trojan	3.7	4.0	3.3
	Mace	2.9	3.4	3.5
	P-Value	<0.001>
	LSD (P=0.05)	0.3

Dr Hunt has outlined a suggested wheat program for South Australian growers, shown in Table 3. Despite winter wheats not being well adapted to SA climate, EGA-Wedgetail can be used for a long sowing program in conjunction with grazing to delay its development. This strategy assumes average frost and heat risks but where risks vary, growers may want to adapt the plan. For instance, in Port Germein, where the frost risk is low, growers could choose to sow earlier.

Duration of wheat sowing program	Cultivars (or equivalent maturity types) required to maximise average yield	Sowing window if seed-bed moisture available
10 days or less	Mace	5-15 May
10-20 days	Trojan, Mace	25 April - 15 May
20-25 days	Cutlass, Trojan, Mace	20 April - 15 May
25 days or more	Wedgetail, Cutlass, Trojan, Mace	10 April - 15 May

'Three years of trials across multiple environments in SA have shown that yields decline at a rate of 28 kg/ha per day once sowing extends past the end of the first week in May,' Dr Hunt said. 'In order to maximize average yields, growers should therefore aim to finish seeding wheat by mid-May.'

Diversity is the key

The best strategy to manage heat and frost risk is diversity. By choosing a range of crops, cultivars with different maturity drivers and optimum sowing dates, growers will have the highest percentage of their program flowering in its ideal window.

'The opportunities to take advantage of early sowing have never been better,' Dr Hunt said. 'Previous barriers have been overcome through no-till technologies, summer fallow management and cheaper chemistries to control early pests and diseases.

'Researchers are working on developing new cultivars that are better suited for sowing early, including a new winter wheat for South Australia. But there is no reason most growers can't spread out their wheat sowing by incorporating a few different cultivars with different maturity drivers.'

Dr Hunt says that while aiming for all of a farm's wheat crop to flower at the same time runs against conventional wisdom of spreading flowering dates over a broad period to minimise exposure to single extreme frost or heat events, studies have shown the conventional logic is not the best approach.

'Spreading flowering dates out so they are before or after the optimal period is a bad way of managing frost and heat risk, because the really extreme frost and heat events will affect crops at a very broad range of growth stages,' he says. 'All our modelling clearly shows that yields are maximised and variability minimised by getting as much crop to flower during the optimal window as possible.'

Dr Hunt suggests growers are better off managing risk by including a variety of crops into their program, including frost tolerant crops like barley or oats, and considering further diversification such as the inclusion of hay or livestock into the business.

Management of early sown crops

Dr Hunt's tips for management of early sown crops:

• Don't dry-sow slow developing cultivars (EGA Wedgetail, Cutlass). They will flower too late if not established early. There needs to be seed-bed moisture and ideally some stored soil water to get them through to winter.

• If growing winter wheat (EGA Wedgetail) and not grazing, defer nitrogen inputs until after growth stage Z30.

• Pick clean paddocks - winter wheat is not competitive with ryegrass and common root diseases are exacerbated by early sowing.

• Protect against diseases associated with early sowing - barley yellow dwarf virus (imidicloprid on seed backed up with in-crop insecticides at the start of tillering if aphid pressure high), Zymoseptoria tritici in some areas (flutriafol on fertilizer and timely foliar epoxiconazole applications at growth stages Z30 and Z39). Many slow developing cultivars also have poor resistance to stripe rust (flutriafol on fertilizer and timely foliar fungicide application at Z39)

More Information

James Hunt, 02 6246 5066,[e-mail]

Useful resources

GRDC Project Code CSP00178, CSP00160

Region South