Early sown wheat and frost
Author: James Hunt | Date: 27 Apr 2018
Over the past couple of decades, growers have been sowing wheat crops increasingly early. This has resulted in substantial increases in farm yield, and narrowing of the yield gap between farm yield and water limited potential yield. Over the same period of time, frosts in south eastern Australia have increased in frequency, severity and lateness of occurrence. As a result of these management and climatic changes, growers are more frequently encountering losses due to frost. Unfortunately, frost is a fact of life in our modern high yielding farming systems, but losses can be minimised by balancing the risks of frost damage with losses due to drought and heat stress. Because of the nature of the climate of southern Australia (cool, frosty winters and hot, dry summers), there is a relatively narrow time period of the year during which wheat must flower in order for yields to be maximised. This period is referred to as the ‘Optimal Flowering Period’ (Fig.1). This is not a ‘safe’ flowering window, but it is the time of year when the combined losses of frost, drought and heat are on average minimised, and thus yield maximised. The timing of the optimal flowering period varies between environments, and to a lesser extent seasons. In very low rainfall Mediterranean environments the optimal flowering period is usually in late August to early September. In medium rainfall districts it is usually mid to late September, and in high rainfall environments it is in late October.
Figure 1. The optimal flowering period of wheat determined by APSIM simulation for Temora, NSW. Grey lines represent the standard deviation of the frost and heat reduced mean yield (kg/ha), black line represents the mean yield (kg/ha). Grey columns are the estimated optimal flowering period defined as 95% of the mean yield for the 51 year simulation (1963-2013)
In order for growers to maximise yield, they must target cultivar selection and a germination date that results in their crops flowering during the optimal period. Historically, much is made of the need to manage the risk of frost damage by spreading flowering dates across a sowing program as much as possible. We argue that this a very poor method of managing frost risk. Firstly, it is difficult to spread flowering dates substantially, as the photoperiod and vernalisation requirements inherent in all wheat cultivars will result in their flowering dates converging even if they are sown over a broad range of dates (e.g. even spring wheats sown ~30 days apart will end up flowering only 15 days apart). Secondly and most importantly, reproductive frost affects wheat at a much broader range of development stages than just flowering alone. Plants become susceptible during booting, and stay susceptible well into grain fill as late frosts in recent years have demonstrated. It is thus impossible to spread sowing dates sufficiently in order to ensure that some areas of crop are not at a susceptible stage when frosts are prevalent, without sowing so late that substantial yield losses from drought and heat are certain.
The key is to MINIMISE the risk of frost (and drought and heat) by targeting the optimal flowering period, but MANAGE the risk via a combination of more effective means e.g. spatially (avoiding high-risk crops on frost prone paddocks, sowing these at the end of the optimal window), incorporating enterprises that are not as affected by frost (e.g. hay & livestock) or via financial means (multi-peril crop insurance, diversification of off-farm income streams).
Experiments across Victoria and South Australia conducted as part of the GRDC Early Sowing project have shown that in most mid-low rainfall districts that experience frost, optimal flowering period is achieved with fast developing spring wheats (e.g. Mace, Scepter) when they are sown in early May and that yield declines at a rate of 35 kg/ha per day that sowing is delayed past this time. However, yield of fast developing types also declines once they germinate earlier into April, and growers who wish to have crops germinating at this time need to minimise their frost risk by using cultivars in which development is slowed by either photoperiod (day length) or vernalisation (cold) requirement. Photoperiod sensitive spring wheats such as Cutlass or Phantom can germinate from 20 April to very early May and will flower during the optimal period.
For germination dates prior to 20 April, winter wheats are required. Winter wheats must experience a period of cold before they will transition from vegetative to reproductive phases, and this makes them the most suitable option for minimising frost risk at early sowing times. Growers in the medium-low rainfall districts of SA & VIC have not traditionally had access to adapted winter wheat cultivars, but this has changed with the first of a new wave of winter cultivars becoming available in 2017 (link to Russell Eastwood GRDC update paper). Trials conducted in 2017 as part of the GRDC Management of Early Sown wheat project have shown that in warmer low rainfall regions ‘fast’ winter wheats (e.g. Longsword) are needed to ensure flowering during the optimal period and maximised yields (Fig. 2). In cooler medium rainfall areas ‘mid’ winter wheats (e.g. Kittyhawk, VO9150-01) are required. The trials have also indicated that yields of winter wheats are maximised when they are sown between late March and late April, with yields declining once sowing dates are pushed earlier into March. This first year of trials has confirmed earlier research that demonstrated that winter wheats are an effective new technology available to growers in the southern region that can help them achieve the yield benefits of early sowing whilst minimising risk of frost damage.
Figure 2. Mean grain yield for a selection of new winter cultivars across different times of sowing in 2018 from either low or medium rainfall sites.
2018 low rainfall sites (Minnipa, Booleroo, Mildura, Loxton) 2018 mid rainfall sites (Hart, Boirchip, Horsahm, Yarrawonga)
Dry sowing (i.e. planting seeds into a dry seed bed such that they will germinate following future rainfall) is still an effective tool to assist in achieving the optimal flowering period in seasons with a dry start. However, this is best done with fast developing spring types which can be sown during or just prior to their optimal germination period. Dry sowing slower spring types or winter wheats can potentially result in them flowering outside of the optimal period if rain does not germinate them in time.
GRDC Project Title: Management of Early Sown Wheat (2017-2020)
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