ProCam 4Cast data collected from its customers’ farms suggest as oilseed rape seed rates increase, yield decreases, Mr Myers said.

That analysis mirrored trials carried out by Masstock on three varieties established using four different seed rates from 40 seeds/sq m up to 160 seeds/sq m. In each case yields from the lowest seed rate were highest, he said.

Both data sets questioned whether growers needed quite as much seed as they did in the past, he said. “Canopy management is the buzzword at the moment. We want to move away from dense canopies, which have poor light interception. There is a direct correlation between light interception and yield.”

While canopy size could be manipulated by nitrogen management and use of growth regulators in the spring, the key starting point was seed rate, he said. “The aim should be to establish 30-40 conventional variety plants a square metre or 25-35 hybrids a square metre.”

But growers needed to get away from drilling by seed weight, he stressed. “Drill by seed number.” To do that growers had to take account of thousand grain weight (TGW), which could make a big difference. For example, drilling 6kg/ha with a 6g TGW planted 100 seeds/sq m compared with 150 seeds/sq m if the TGW was 4g, he explained.

But in typical conditions 100 seeds/sq m would likely be too high a seed rate, he said. “To achieve that 30-40 plants/sq m growers should typically be looking to sow 60-80 seeds/sq m, assuming plant loss of around 20-40%.”

ProCam analysis of its farms suggested only growers sowing the minimum seed rates recorded were likely to be down to those sort of seed numbers, he said. “It means growers are probably getting rape crops too thick to start with.”