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Frost risk and site variation therein

D.R. Woodruff

Queensland Wheat Research Institute, PO Box 2282, TOOW000MBA Q 4350

Introduction

Frost causes large yield reductions in winter cereals in the Northern Wheat belt. This is due not only through direct damage to the crop but more particularly because the risk of such damage requires crops to flower and mature later when the likelihood of water stress is rapidly rising. The yield loss associated with this later flowering is of the order of 1.0 to 1.5% per days delay after early August in Queensland.

Frost risk and site variation therein

The original work of Foley has been updated to provide the full frequency data on minimum temperature expectancy for many Meteorological Bureau sites in Queensland. This data has shown that a 1C difference in minimum temperature translates into a 10 day change in flowering time for the same frost risk and this in turn translates into up to 3 weeks difference in planting time for many common varieties. Table 1 presents the variation in minimum ear temperature at the top and bottom of the slope at 2 farms and a screen and ground temperature at a remote (20 km) meteorological station (B) from the ground temperature at a control Meteorological station (A).

This data shows consistent large differences in daily minimum ear temperature due to sites on a farm. The relationship between these sites and the minimum ground temperatures at the reference meteorological sites is however relatively poor. Without satellite information on minimum temperature it is thus difficult to relate frost measurements on farm to the long term data available from Met. Bureau sites.

Critical ear temperatures for frost damage

Extensive testing of wheat and barley genotypes under both controlled and field freezing conditions shows the critical ear temperature to he -3.8 -0.6C in wheat and -4.4 -0.7C in barley. No substantial or consistent differences were found within genotypes. The lower temperatures which caused severe yield damage in the field were associated with low seeding rates. wide row spacings and/or severe crop water stress conditions

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