Oilseed rape growers across the country are finding modern Clearfield varieties establish noticeably better than many non-CL crops grown alongside them as well as yielding at least on a par with them and offering a range of valuable risk reduction benefits beyond cruciferous weed control. So it really isn’t surprising that Clearfield growing has risen sharply over the past two years to account for almost 15% of the crop nationally.
These are the main findings of an Agronomist & Arable Farmer Clearfield growing study involving nearly 100 farms with more than 13,000ha of OSR in the ground undertaken this spring with UK breeding leaders, DEKALB.
Reflecting the increasing popularity of the system, just over two thirds of these are growing imazamox-resistant ‘CL’ varieties this season against less than half in 2017/18. What’s more, fully 85% of the farms growing Clearfield crops last season are doing so again this year, together with just over half of those not growing them previously.
At the same time, the proportion of growers putting more than half their OSR crop into the system has risen from just over 20% to 38%, with over a quarter now growing Clearfield exclusively compared with 16% last season (Figure 1).
A total of seven different Clearfield varieties were identified by the growers in 2017/18, yielding an average yield of 3.53 t/ha against their total oilseed rape harvest average of 3.43 t/ha. Underlining the abilities of modern breeding, the single most widely grown variety, DK Imperial CL delivered 3.68t/ha.
Unsurprisingly perhaps, the variety has maintained top spot in this year’s expanded Clearfield area, although significantly eroded by the growth in DK Impressario CL, taking over from DK Impression CL in second place (Figure 2).
“Growers are clearly voting with their feet as far as the Clearfield system is concerned,” observed study co-ordinator, Mark Shaw of DEKALB. “The lack of any yield gap with non-CL varieties these days has to be a major factor here. But so too, our study suggests, is a noticeable shift in the main reasons for growing them, based on experience.
“Identified by more than 90% of growers, controlling cruciferous weeds remains as much the key driver behind continuing to grow CL varieties as it was in starting to grow them in the first place. Both controlling volunteers for better canopy management and minimising the risk of SU residues interfering with crop establishment also remain the least important reasons for growing them.
“However, the proportion of growers seeing Clearfield varieties as valuable in minimising the risk of erucic acid contamination from weeds and volunteers has increased sharply from 55% to 66%,” he pointed out. “And even more so from the 23% considering this an advantage in the previous study we conducted in the spring of 2017 when far fewer growers were using the them.
“The proportions valuing the system for avoiding either the need for herbicide use until the crops are safely established or any risk of pre-em herbicides interfering with establishment have also increased appreciably within our current study (Figure 3).”
Interestingly too, the study also suggests an establishment advantage from the robust Clearfield varieties being grown over the past autumn – identified as especially challenging by almost half the growers.
While the majority of growers saw their Clearfield crops establish about the same as their other OSR, nearly 30% reckoned they did so better than some or most of their other crops against only 10% rating them as establishing worse than some or most (Figure 4).
“This may be due to the particular priority they are placing on vigorous establishment and rapid autumn development in their variety selection,” Mark Shaw observed. “The former to an even greater extent than high gross output (Figure 5).
“The fact that they are doing so underlines the importance of Clearfield varieties carrying the imazamox resistance trait in the best possible combination with other key characteristics for oilseed rape success; a balance we have always prioritised in our breeding and which is clearly reflected in the popularity of DEKALB varieties among growers.”