The yield plateau experienced on many UK arable farms since the mid-1990s can’t be blamed on either plant breeding or crop protection, NIAB TAG’s director of crops and agronomy Stuart Knight told the HGCA/NFU East Midlands arable conference in Lincolnshire last week.

While genetic improvements have made it possible to increase wheat yields by 0.5% a year and oilseed rape yields by 2%, the on-farm reality has been quite different, he noted.

“Wheat yields have stalled and are rising by less than 0.2t/ha a decade. Oilseed rape yields have fluctuated wildly, although they have gone up in more recent times. But that only puts them at much the same level as they were 30 years ago.”

Mr Knight was speaking on the findings of the Yield Plateau project, which was commissioned by HGCA and DEFRA in 2011, to identify the factors that were constraining yields and to highlight opportunities for improvements.

“Genetics is a success story,” he stressed. “New varieties have got better, especially in the winter wheat hard Group 4 sector, and their uptake has been good.”

Likewise, the number of fungicide and herbicide active ingredients being applied to crops has increased. “Crop protection hasn’t been responsible for the current situation either. It’s another area that has created the potential for yields to increase, despite the threat of resistance and new races.”

However, the effects of climate and agronomic practices have combined to limit advances, he noted. “It’s the same story around the rest of the world, where yields have also stagnated.

“In France, for example, they’ve experienced a climate change effect, with temperatures above 25C during the grain-filling period having a detrimental impact on yield.

“And in Denmark, where there have been reductions in fertiliser and fungicide use, they’ve had more problems with take-all in wheat, and yields have fallen.”

Looking at UK climate and weather patterns, Mr Knight revealed that maximum spring and autumn temperatures have risen by 0.5C since 1980. “The result of these higher temperatures is that the grain-fill period in cereals has been reduced by three days, or 6%, over that time.”

Furthermore, over the last 15 years, the trend has been for April to get drier, he continued. “And that has also had an effect. Crops being grown in the North and the West have shown greater improvements than those in the East, where moisture is more limiting. We’ve also seen almost no progress on light soils or from later sowings.”

So there is strong evidence that the weather has influenced yield trends, he acknowledged.

Agronomic practices have also played a part, he continued. “Rotation is important – yields of first wheats are always better following break crops, with oilseed rape, followed by peas and then beans giving the best results.”

The move away from ploughing – for both financial and environmental reasons – has had an impact too, he said. “Some 40% of the UK wheat crop is established after non-inversion tillage. And we’ve seen that there has been a small yield penalty from this (see panel).”

Once crops are in the ground, the use of nitrogen fertiliser has remained static, despite the requirement of modern varieties being 20kg of N/ha greater, he added. “That’s been responsible for a yield decline of 0.006t/ha. And it’s likely that sulphur use isn’t high enough either.”

Looking at oilseed rape, Mr Knight pointed out that yields were rising, after a fall in the first 10 years of growing the crop.

“But what is clear is that growers are only achieving the higher yields if they are growing the newer varieties, rather than sticking with those that are easier to manage and harvest.”

April weather has again been responsible for variation, he remarked. “You need sunshine for pollination and photosynthesis. A warm, dry spring is very helpful for oilseed rape, especially if it follows a cold winter.”

Rotational changes involving the frequency of oilseed rape crops have had a negative impact, with yield loss estimates of 12% if the crop is grown every other year. “That falls to 6% if it’s grown one year in three and 3% for one year in every four.”

Yield plateau – where next?

Mr Knight had some specific short-term recommendations to help get yields back on track.

• Additional Recommended List information to help choose varieties for specific situations
  
• Better tools for forecasting, monitoring and crop management are required
  
• Bring together all the information on early-sown crops
  
• Update advice on crops at risk from sulphur deficiency
  
• Carry out regular soil testing to target P and K
  
• Highlight implications of pesticide resistance
  
• Consider the implications of timing of operations and inputs