Managing sowing time and frost risk

Managing sowing time and frost risk                              

 

 

Neil Fettell

Industry & Investment NSW

Condobolin Agricultural Research & Advisory Station

 

 

Background

 

Sowing time is a major determinant of crop yield because, in partnership with the variety chosen, it determines the timing of key development stages and the environmental stresses at these key stages. Crop yields are particularly sensitive to stresses in the period from about 3-4 weeks prior to flowering through to the start of grain fill as this is when grain number is determined and stem reserves are accumulated.

 

Changes in crop management have also given farmers more flexibility in sowing time. The availability of herbicides for fallow weed control and the swing to direct drilling and no-till means that crops can be established on the autumn break without the delays caused by pre-sowing cultivation. Improved machinery including higher breakout tines, presswheels, and ground following seeding boots have allowed crops to be established under a wider range of soil conditions, including dry sowing. These abilities plus a run of dry seasons has prompted many growers to establish crops earlier in autumn, but this may increase the risk from frost damage.

 

Crop development

 

Flowering time is therefore a key factor in adapting a crop to an environment. It is determined by sowing time, variety, environmental conditions and the interactions of these three factors. Temperature is a major environmental driver as the rate of development increases with increases in temperature across a broad range. Secondly, many varieties respond to low temperatures during the seedling stage in a process called vernalisation, and flowering is delayed if this requirement is not met. Photoperiod or day length can also influence development, with short days delaying flowering, and varieties differ in their sensitivity to this factor. Finally, varieties differ in intrinsic earliness, a measure of how quickly they flower under optimum conditions.

 

Variation for these factors means that there is a wide range of development patterns available in wheat varieties in NSW. This allows growers to choose a variety which best suits the timing of the autumn break for their location and farming system. For example, above average May-June temperatures such as in 2010 will hasten development in varieties with no vernalisation response (e.g. Ventura, Axe, Gladius) but delay development in varieties such as Wedgetail and Marombi. 

 

An example of the interaction of sowing time and variety on flowering date is shown in Figure 1, which is for some older varieties grown at Condobolin. From early April sowing, the quickest variety (H45) flowered in late July and the slowest (Sunbrook) in mid-September, the same time as H45 sown in mid June. This clearly shows the need to match variety and sowing time if flowering is to occur in the desired window. 

 

 

 

Stresses such as frost, high temperature, high evaporative demand and low soil moisture can be very detrimental to yield in the period around flowering. The typical timing of frosts and high temperatures for Condobolin is shown in Figure 2. Frost frequency drops substantially between September 14th and 24th, but average maximum temperature and evaporative demand (not shown) also increase rapidly from the 19th to the 30th. While there can be considerable year-to-year and topographic variation, the third week of September appears on average to be the optimum flowering time at Condobolin.

 

 

 

The GRDC-supported Barley Agronomy and Variety Specific Agronomy Projects are conducting sowing time trials for wheat at Cowra, Condobolin, Trangie and Wagga Wagga and for barley at Condobolin and Trangie. Up to 40 varieties, including all recent releases and advanced lines have been sown at three or four sowing dates, and measurements taken include flowering time, yield and grain quality.

 

A typical response to sowing is shown in Figure 3, for wheat at Condobolin in 2006. Early varieties (including H46, Ventura and Drysdale) were lowest yielding at the first sowing date but highest at the last sowing date. Mid season varieties (including Gregory, Lang and Sentinel) performed well at the first two dates but poorly sown late. Late varieties (including Wedgetail and Sunzell) yielded reasonably when sown in late April but yield declined with later sowings; these varieties may have done better sown earlier in April. The rate of yield decline for each group is also shown and averaged 0.17 t/ha or almost 10% per week.

 

 

 

 

 

 

 

These responses can be understood by examining the relationship between yield and flowering time rather than sowing time and this is shown in Figure 4. For all maturity groups, highest yield tended to occur for varieties which flowered in a very narrow window, between September 18th and 23rd. Flowering before this period gave lower yields. This may have been partly due to frosts, but short season varieties can develop rapidly when sown early, restricting the number and size of the ears, the leaf area at flowering and in some cases limiting dry matter production. Low solar radiation around early flowering times can also restrict growth and yield.

 

New varieties

 

Information on variety by sowing time interactions is limited for newer varieties. This was addressed at Condobolin in 2009 when 36 varieties were sown on three dates, and the results are shown in Table 1.

 

The warm, dry conditions in 2009 hastened development, particularly for varieties established at the later sowing dates. Axe, followed by H46, were the quickest commercial varieties to flower from all sowing dates. As expected, the spread of anthesis times across varieties declined with later sowing but there were few changes in the ranking of varieties across the sowing times. The ideal anthesis date will vary with season, being a balance between achieving sufficient biomass (but not excessive water use) by flowering, the occurrence of frost and the timing of rainfall events. The dry spring conditions and lack of frost at the Condobolin and Trangie sites in 2009 resulted in the highest yields being achieved by early-mid flowering varieties sown in late April.

 

Flowering dates were measured on 17 barley varieties at Condobolin in 2009 (Table 2). There were few changes in rankings with sowing time, Hindmarsh being the quickest at all sowing dates. Urambie is unique in having a vernalisation requirement which delays flowering when sown early.

 

 

 

Contact details

Dr Neil Fettell

Industry & Investment NSW

Condobolin Agricultural Research Station

PO Box 300, Condobolin NSW 2877

Ph: 02 6895 2099

Fax; 02 68 952688

Email: neil.fettell@industry.nsw.gov.au