Climatic differences, especially minimum temperature and frost incidence, can be great over a given paddock with elevation variances of as little as 20 to 45 metres.
Such differences can impact greatly on best sowing time, variety choice and yield expectations that can result in gains or losses of several hundred dollars a hectare.
Agricultural Marketing and Production Systems (AMPS) research agronomist Matt Gardner, delivered results from three years of research at Gurley (northern NSW) and the Liverpool plains, highlighting these findings at recent GRDC farmer and agronomists' updates.
Average minimum temperatures over the three years were 2.4 and 2.9 degrees lower at the bottom slope compared to the top slope at the Liverpool Plains and Gurley sites, respectively.
That doesn't sound much, but it adds greatly to frost risk as well as to slower crop development. However, average maximum temperatures were similar for both top and bottom slopes sites.
At the Liverpool Plains and Gurley trial sites, the bottom slope experienced on average an additional 30 and 32 frost events respectively compared to the top slope.
There were not only more frosts at the bottom slope site, but frost duration was greater. On average, the time that temperatures were at or below 0 degrees at the top slope was only 36, and 7 per cent of that measured for the bottom slope sites at Liverpool Plains and Gurley, respectively.
Matt Gardner points out that frost length can be a major determining factor of crop damage. On average, length of frost at the top slope sites were 3.3 and 2.5 hours at Liverpool Plains and Gurley, respectively. This is compared to the bottom slope sites, where frosts on average lasted 4.3 and 4.6 hours respectively for Liverpool Plains and Gurley.
Lower average minimum temperatures and more frost events, both contributed to slower accumulation of thermal time throughout the season at the bottom slope compared to the top slope. At both locations the difference in accumulated thermal time Growing Degree Days (GDD) was in excess of 150 GDD higher at the top slope sites.
Slower accumulation of GDD at Liverpool Plains resulted in bottom slope sites on average, across six varieties assessed, being much slower to reach mid-flowering. On average the lower site was 13, nine and seven days longer flowering than the top slope from the three sowing times, late April, mid-May and early June.
Similarly, delays occurred at Gurley, although in this warmer area all flowerings were on average 17 days earlier. Varieties assessed were (slowest to fastest) Eaglehawk, Gregory, Lancer, Suntop, Spitfire and Dart.
Both the Liverpool Plains and Gurley sites showed yield benefit of early planting in both dry and good spring seasons. For example delaying planting from late April to early June at Liverpool Plains top slope site on average yielded 1.53 tonne a hectare less across six varieties. Based on a wheat price of $250 a tonne, this is equivalent to an average net return of $383 a hectare.
Frost damage did occur at the Liverpool Plains bottom slope site in all three years, particularly in Dart and Spitfire. For example Dart in 2015 yielded 6.17 t/ha at the top of the slope but only 1.23 t/ha at the bottom. Frost damage at the bottom reduced average grain yield of the six varieties by 1.91 t/ha across the three years.
There was minimal frost damage on the two later planting dates and grain yields were similar between the top and bottom sites for Liverpool Plains and Gurley in all three years. Unlike the Liverpool Plains in 2016 (a good spring), there was no yield penalty in delaying planting date from late April until early June at Gurley, but in the years with a dry spring (the other two years) delayed planting resulted on average in a 1.88 t/ha reduction in grain yield.
Matt Gardner notes that optimum flowering window at Gurley (plotting grain yield against flowering date to achieve maximum yield) was generally 12-14 days from mid to late August in dry spring years for the top of slope site, and in a good spring was much wider (24 days) and began in early September. Optimum window for the bottom was similar but generally started nine to 13 days later. At Liverpool Plains optimum flowering window for the bottom slope started 10-22 days later than the top slope. Optimum window for the top generally started around the beginning of September while for the bottom around mid-September.
Mr Gardner stresses that these trials show it is profitable to sow as early as possible to maximise optimal grain filling conditions, while avoiding risk of frost damage. Because lower parts of landscapes generally have a narrower and later optimum flowering window, variety choice and sowing time especially, need to be assess the interaction of yield and frost risk.
For a complete copy of this research paper, visit the GRDC website or email Matt Gardner at email@example.com
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact (0428) 752 149.