Sowing date, heat stress and frost risk in irrigated wheat crops.
Dale Grey and Chris Sounness
Keywords
Sowing date, frost, heat stress, wheat
Take home messages


Severe frost risk is minimal at Deniliquin
Severe heat stress is more likely and increases alarmingly sown in June.
As with all things in life there are tradeoffs. With sowing date, it’s a race between too early and belted by
frost and too late and getting smashed by heat; It’s an ongoing debate as to which has the greatest effect
on yield. What’s difficult to determine is what is the stress risk in your particular area.
We conducted a desktop study on historical climate data.
We used the Yield Prophet® APSIM derivative model to determine the median flowering dates for Janz
wheat sown at Deniliquin at five different sowing dates, two weeks apart starting at ANZAC day. Using the
DAFF Qld Patch Point dataset of daily temperatures for Deniliquin, we counted the number of years where
one or more extreme frost or heat events had occurred during flowering or grain fill over the last 125 years
and converted that number to a % chance of occurrence. The median (close to the average) value shows
how many frost or heat events are likely in the years that have one or more events and the range indicates
the largest number of events ever recorded. We decided to look at severe frosts of below 0oC (and lighter
frosts of < +2oC), 5 days before or 15 days after flowering and severe heat stress greater than 34oC (and
minor heat stress > +30oC) in the grain fill period 30 days after flowering.
The severe frost risk sowing on ANZAC day is 3% at Deniliquin at only three years in a 100, or put another
way, a 97% of chance of not getting severe frost. In those years where severe frost occurred, the average
number of frosts was one, but in the odd year there was up to three. If we look at all frosts less than 0oC
the risk is greater at 37%, with mainly one frost in those years, but sometimes up to six. Sowing at May 10
the severe and light frost risk is low, and at subsequent later sowing dates the risk is inconsequential or non
existent.
So to avoid frost, you say to yourself, “I will sow later”, but the frost figures pale into insignificance when
you look at the chance of heat. Those later sowings are nearly a monty for getting a touch up from heat.
Even sown on ANZAC day there is a 73% chance of a couple of days of low heat stress. At later sowing
dates it’s almost certain to get numerous heat stress event. The scary thing is the risk of extreme heat
stress when sown in June, which becomes almost certain and usually three events. Some drought years are
extreme and can have up to 15+ severe heat stress days during grain fill.
Deniliquin
%
Chance
of a frost
Flowering
near
date
flowering
0oC, 34oC
2oC, 30oC
17 Sept
17 Sept
3
37
Median
No.
%
days in
Chance
years
Range of heat
with
in No. at grain
frost
of days
fill
Sown 25 April
1
1-3
20
1
1-6
73
Sown 10 May
Median
No.
days in
years
Range
with
in No.
heat
of days
1
2
1-3
1-9
0oC, 34oC
2oC, 30oC
2 Oct
2 Oct
1
16
0oC, 34oC
2oC, 30oC
12 Oct
12 Oct
0
9
0oC, 34oC
2oC, 30oC
20 Oct
20 Oct
0
2
0oC, 34oC
2oC, 30oC
25 Oct
25 Oct
0
1
Dale Grey
DEPI Epsom
Email- dale.grey@depi.vic.gov.au
Phone- 0354 304 395
Twitter- @eladyerg
www.depi.vic.gov.au
Chris Sounness
DEPI Horsham
Email- chris.sounness@depi.vic.gov.au
Phone- 0353 620 741
Twitter- @quantonglanes
www.depi.vic.gov.au
1
1
1
1-3
Sown 25 May
0
0
1
1-3
Sown 10 June
0
0
1
1-2
Sown 25 June
0
0
1
1
49
93
2
4
1-6
1-15
65
98
2
6
1-9
1-17
80
99
3
8
1-15
1-23
90
99
3
8
1-16
1-22