by Ethel White and Lisa Black, AFBI Plant Testing Station, Crossnacreevy

Spring weather in 2013 was very different to that in 2012, when warm weather in March boosted spring growth of autumn-sown crops and got sowing of spring crops off to a warm dry start.  Weather over the summer and autumn in 2012 proved challenging for the programme of cereal variety trials across Northern Ireland managed by AFBI Crossnacreevy and funded by the HGCA and DARD.  At harvest 2012, the yield and quality of many crops were disappointing.  This article looks at how weather affects crop growth, yield and quality and attempts to relate performance and weather in 2012 using data from the AFBI Crossnacreevy variety trials conducted from 2008 to 2012.

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Data for yield and other characteristics presented for varieties in the AFBI/DARD Cereal Recommended Lists are averages over the most recent five years.  Mean spring barley yields with fungicide for all varieties in 2012 were reasonably good, not much lower than those in 2010, the highest amongst the past five years. However two trials were excluded because of very variable (and low) yields, so the 2012 average represents the best yields for this year. Spring oat yields with fungicide in 2012 were poor but not as poor as those in 2008, the lowest of the five years. Yields with fungicide of all three winter crops, and of spring wheat (for which there are valid trials for three years only) were all lower in 2012 than in all previous four years.  Yields without fungicide followed roughly the same pattern of variation with year as those with fungicide, as would be expected, with 2012 being the lowest year for the winter crops and spring wheat but not as low as 2008 for spring barley and spring oats.

Grain Weight

Grain weight is the third and final component of yield formed as the crop develops and grows, the earlier-formed components being ear number per m2 and number of grains per ear. Grain weight is expected to be relatively stable because the capacity of the crop to supply material to fill the grains, i.e. its green area and reserves stored in the stem, should be in balance with the capacity of the crop to demand material for filling the grain.  These are in balance because they are being formed at the same time in the weeks before ears emerge in the crop.  Therefore if less radiation is available during June, for example, the ear will be smaller and the reserves stored in the stem will also be smaller.  The balance between the supply of material to the grains and the demand from the grains could be upset during grain filling if either the crop’s capacity to supply material is curtailed because disease destroys green area or radiation is lower than average.  This balance may also be disrupted if the grain-filling period is sunnier than average so that grains are supplied with more material by the crop’s green area.  However the grains may not be heavier, because less of the stem reserves will be used to fill the grain. In the cereal variety trials grain weight is relatively stable from year to year but in 2012 grain weight was lower in all the winter crops than in any of the previous four years, most markedly in the winter wheat trials.

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Specific weight is a complex character reflecting packing within, i.e. density of, the grains and packing between grains, making its interpretation difficult.  In 2012 specific weight was lower in winter wheat and in winter barley (just) than in the previous four years (Figure 4).  Given that specific weights are always lower in winter wheat trials in Northern Ireland than in trials in England and Scotland, the reduction in 2012 to 66 kg per hl on average across all varieties in all trials in Northern Ireland was potentially a more serious problem for those selling grain in Northern Ireland.

Influence of weather

Long-term weather data for temperature, rainfall and sunshine hours are available for Northern Ireland as a whole from the UK Met Office.  These have recently been updated to the period 1981-2010 (http://www.metoffice.gov.uk/climate/uk/averages/).  Monthly summaries for Northern Ireland as a whole for individual years are also available (http://www.metoffice.gov.uk/climate/uk/).  For each month the difference from the long-term mean, i.e. the anomaly, is calculated for the various features of the weather.

March 2012, with a mean temperature of 8.100C was 2.30C warmer than average for March during the period 1981-2010.  March 2012 was also warmer than April 2012, with a mean temperature of 6.40C, which was 1.20C cooler than the long term (1981-2010) average for April.  With the exception of October, which was 1.50C cooler than the long term average, mean temperatures in other months in 2012 were within ±10C of their long term averages.

Total annual rainfall for Northern Ireland in 2012 was 1154 mm, not dissimilar to the 1981-2010 long-term average of 1136 mm.  However this hides enormous anomalies in some months.  Rainfall in March 2012 was 27% of its long-term average.  Most other months fell between 70 and 116% of their long-term averages with the exception of June 2012, when rainfall was 235% of the long-term average, 179 mm compared with 76 mm.  This was the highest June rainfall since 1910 and much greater than that of 126.4 mm in 1990, the highest June rainfall since 1980.  Rainfall was greater than 179 mm in only six out of the total of 156 months (13 years) since January 2000.

However rainfall may not have had the most serious effect on crop growth and yield.  June 2012 had a total of 91.4 sunshine hours, the lowest June total since 1929, and 61% of the long-term average.  Less than 120 sunshine hours were recorded in June in only 9 other years since 1929, most recently 1993, 1994 and 2001.  July 2012 had a total of 116 sunshine hours, 83% of the long term average, which did little to improve the prospects for yield following the very dull June.

Total solar radiation, the light intercepted at the earth’s surface both as direct sunshine and as indirect light when the sky is cloudy, is one of the main factors influencing crop growth.  Photosynthesis conducted in any green tissue in plants converts light to dry matter.  Water and carbon dioxide are the other raw materials used in this particular process.  Growth and yield will be better when the weather is cool and sunny and poorer when it is warm and cloudy.  Radiation and sunshine hours are expected to be high in June every year because of the long days.  In 2012, June had less sunshine than all other months between March and October, inclusive.  On average, since 1929, May has been the sunniest month, with a mean total sunshine receipt of 181 hours, with June having the second highest mean of 161 hours.  Mean sunshine receipts for March, April, July and August range from 101 to 146 hours, against which June 2012, with 91.4 hours, is still seen to be extremely low.

Less sunshine in June 2012 is likely to have been partly responsible for reducing potential crop yield in the autumn-sown crops but this does not seem to be true for spring barley and spring oats.  The reasons for lower yield potential in the winter crops in 2012 may go further back in the season to March. Uncharacteristically warm weather in March 2012 would have encouraged development, i.e. the plants produced leaves (and tillers) more quickly than if temperatures had been lower. Growth (using light) of these leaves and tillers would not have kept up with development despite reasonable, although not unusually high, sunshine receipts for March 2012.  Thus plants would have been smaller than usual for their growth stage, i.e. development stage, curtailing yield potential during subsequent yield formation processes.

Fusarium ear blight

Lighter grains and lower specific weight of winter wheat varieties in 2012 are not so readily explained by lower sunshine hours in June 2012 or warmer temperatures in March 2012.  Yields of winter wheat were highest in 2010 (in this 5-year period) when grain weight was relatively low, 42.5 g per 1000, compared with 44.0 and 48.9 g per 1000 in 2008, 2009 and 2011, but not as low as that of 38.6 g per 1000 in 2012.  High rainfall in June 2012, over the period during which winter wheat was flowering, would have been conducive to the development of Fusarium ear blight. Fusarium had not been recorded in the variety trials in the previous four years but in 2012 infections ranged between 2.0% and 38% in all varieties in the treated trials, and up to 72% in the untreated trials.  In the Crossnacreevy and Hillsborough variety trials, the levels of infection in the treated and untreated did not differ significantly.  Only at Limavady did Fusarium in the untreated trial far exceed those recorded in the fungicide treated plots.  The spring barley trial at Crossnacreevy was infected with Fusarium ear blight although the impact on grain quality in spring barley was much smaller than in winter wheat.

Fusarium infections are likely to have been responsible for the deterioration in grain quality in winter wheat in 2012.  At harvest, Fusarium ear blight can result in shrivelled grains with a chalky white or pink appearance. Infection of ears by Fusarium species can also result in mycotoxin development when the weather is warm and wet at flowering, depending on which Fusarium species are present.  Mycotoxin occurrence may be greater when wet weather delays harvest.  There is some evidence that wheat specific weight is lower when levels of grain mycotoxins are higher.  It should be noted that Fusarium ear blight is difficult to control as fungicides have to be applied at flowering, and available chemicals do not give complete control.

Conclusions

Weather in 2012 on average across Northern Ireland was quite unusual in several respects – a warm March, a cool April, and a wet and cloudy June.  Yield formation processes in all crops were affected to some extent but the winter wheat trials were particularly hard hit by Fusarium ear blight which developed during wet weather over flowering.  As well as reduced yields quality was also affected by these Fusarium infections.

The unusual weather patterns in 2012 weren’t predicted and couldn’t have been predicted.  And so we might ask what can be done to prepare for future unknown weather patterns?  We have gained some information about varietal resistance and susceptibility to Fusarium ear blight but even this may have been influenced by rain events coinciding with particular growth stages in individual varieties.  This can be studied in variety trials in the coming years to consolidate our knowledge of this aspect of variety behaviour.  More generally, we can begin to identify those varieties which have broader resilience to unusual weather patterns.  It may be that such varieties will not have the highest yields when growing conditions are good but they might give growers some security that crop performance can be maintained in the face of climate change.