Sowing date, heat stress and frost risk in irrigated wheat crops

Dale Grey and Chris Sounness,

DEPI, Vic

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.

Introduction

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 0^oC (and lighter frosts of < +2^oC), 5 days before or 15 days after flowering and severe heat stress greater than 34^oC (and minor heat stress > +30^oC) in the grain fill period 30 days after flowering.

The severe frost risk sowing on ANZAC day is 3% at Deniliquin and 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 were up to three. If we look at all frosts less than 0^oC the risk is greater at 37%, with mainly one frost in those years, but sometimes up to six. In comparison, sowing at May 10 the risk of a severe and light frost 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. If sowing late you are nearly guaranteed of 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 a certainty to get numerous heat stress events. The scary thing is the risk of extreme heat stress if sowing occurs in June.  Exposure to extreme heat stress becomes almost certain and usually there are at least three events. Some drought years are extreme and can have up to 15+ severe heat stress days during grain fill.

Table 1. Possibility of frost and heat stress events at Deniliquin for different sowing dates.

Contact details

Dale Grey

DEPI Epsom

0354 304 395

@eladyerg

www.depi.vic.gov.au

dale.grey@depi.vic.gov.au

Chris Sounness

DEPI Horsham

0353 620 741

@quantonglanes

chris.sounness@depi.vic.gov.au