NIAB - National Institute of Agricultural Botany

Orson's Oracle

Wheat grain fill complete

Posted on 01/08/2013 by Jim Orson

NIAB TAG Network members have had access to a week by week review of potential yield gain during grain fill of winter wheat over the past few months. A brief summary indicated that it was less than average for the first half of grain fill and greater than average for the second half of grain fill. Overall, potential yield gain during grain fill has been about average but with one of the eight centres monitored (Holbeach) being 7-8% higher than average. 

The estimates are based on the observation that potential yield gain during grain fill is directly related to solar radiation intercepted over a period of around 680 day degrees from anthesis (when the anthers are visible). Day degrees in this context are simply the average daily temperature, so an average temperature of 17 degrees C translates into 17 day degrees.

It would be naïve to take these figures at face value. First, high day and night temperatures can restrict grain fill to less than the potential as can a shortage of soil moisture. In addition, some crops are patchy and will not intercept all the solar radiation; late sown crops in particular may not have sufficient grain sites to absorb all the photosynthate. In addition, these same crops may suffer more from dry soil conditions.

This season, over the eight monitored sites, the average potential yield gains during grain fill were close to their medium-term average of just over 10 t/ha. To get a guide to the potential total yield, stem reserves accumulated in the crop before anthesis have to be added. In modern varieties this is around 2 t/ha for typical crops. This suggests that, on average, timely sown wheat crops at these eight sites have a medium-term potential average yield in excess of 12 t/ha.

These average yields have unfortunately never been achieved in practice; this shows that there are always limitations that prevent such yields from accumulating. The question is whether this year we have had more limitations than usual. I consider that the answer to this is, unfortunately, “yes”. 

The very hot days and nights in July, coupled with very dry soils in many areas, must have taken a higher toll on potential yield gains than in an ‘average’ year. So overall, I’m predicting no better than average yields on timely sown wheat crops. The hot weather and moisture stress would have had an even higher negative impact on late sown crops. I hope that I am wrong and would be delighted to eat humble pie.

I’ve been thinking around the subject of the impact of solar radiation on potential yield gains during grain fill. As I said earlier, it depends on how much radiation can be packed into a grain fill duration of 680 day degrees, which is typically 6 weeks. So the size of potential yield gain is dependent on the average energy from the sun per day degree. As an example, at one site this year the potential yield gain during grain fill was on average 15.3 kg/ha/day degree.wheat harvest

However, there were good days and bad days. The 6th June was a great day with lots of solar radiation but relatively cool temperatures. The potential yield gain per day degree on that day was 32.7 kg/ha. This was unfortunately before grain fill started but if every day was like the 6th June, during grain fill the total potential yield gain would be 22.2 t/ha. In contrast, 23rd July was hot but cloudy and the potential yield gain on that day was a measly 6.5 kg/ha/day degree. Out of interest, the potential yield gains on those hot and sunny days in mid-July were between 15 and 20 kg/ha/day degree and on the cool and cloudy days in mid-June they were between 10-15 kg/ha/day degree. By using this information we can develop a ‘feel’ for good and bad days for grain fill, provided that we still remember it next June!

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