Half term report

9 Jul 2013

Around this time last year I wrote a blog suggesting that winter wheat yields could be ‘disappointing’.  This was based on the lack of solar radiation in the first half of grain fill. What I didn’t foresee was the devastating impact of waterlogging on yields in some parts of the country.

This year it’s likely that the end of the first week of July was about halfway through grain fill for the majority of winter wheat crops. Again solar radiation has generally been well below the average – the map shows the total accumulated surface solar radiation for the period 15th-30th June 2013, approximately the first two weeks of grain fill, expressed as a percentage of the 1983-2005 average for the same period. Once more this suggests that yields will possibly not be that great unless we have a strong second half.

Surface solar radiation 15-30 June 2013

The big difference this year is that we don’t have a surplus of water. In fact it’s very dry in many areas and that cannot be good for yields. Ideally we need more than average of both solar radiation and rainfall over the next two or three weeks.  Clearly, if we get one of these we are unlikely to get the other unless we have a series of thunderstorms at night.

There’s also a huge area of late sown winter wheat this year.  In general, these crops have come together extraordinary well through the prolonged cool spring and are not far off providing sufficient plant cover to maximise the interception of solar radiation. I’ve been thinking about the possible implication of lower than average solar radiation levels on these crops in particular.

The central issue of my debate with myself is whether less than average solar radiation levels are as important to these late sown crops as to those crops with a higher yield potential. Typically, later sown crops are more likely to have a lower ‘sink’ for the products of photosynthesis i.e. the amount of grain fill is limited by the number of grain sites. Therefore, it may be that the lack of solar radiation so far this year may not be as important because of this huge area of late sown crops and their associated lower yield potential.

However, I’m not going to fall into the trap of thinking that just one factor alone can determine yields. Late sown crops tend to be more vulnerable to drought and, as I mentioned earlier, some areas of the country are now particularly dry.      

The current unbroken sunshine can only be good for yields provided that the wheat has a reasonable supply of water. Excessive heat can lead to the ‘cooling system’ of the plant breaking down which can limit eventual grain size. This is particularly damaging at flowering but it’s never welcome. Very warm nights are also not welcome because the crop will have a higher respiration rate, using up more of the photosynthate that was accumulated during the day.

It’s no wonder that farmers are fixated by the weather. Every twist and turn can have an impact on the business but in the end only the harvested yields will determine how good a growing season we have had. At the moment I’m not bullish about yield prospects.

NIAB TAG Network members can log-in to their area of the Network website to see the daily solar radiation received at eight sites in England in June and July (kindly provided by UEA/Weatherquest). The data is updated on a weekly basis.