Growers across the country must address the serious challenge of oilseed rape volunteers by placing more priority on reducing seed losses in the run up to, and at, harvest
Growers across the country must address the serious challenge of oilseed rape volunteers by placing more priority on reducing seed losses in the run up to, and at, harvest.Two years of trials undertaken on a Nottinghamshire farm in 2011 and 2012 designed to improve rotational hygiene have underlined the potential for volunteer oilseed rape problems even with modern harvesting equipment and pod-sticking sprays. At the same time they have revealed the extent to which they can be reduced by growing varieties with genetic resistance to pod shatter, advises Openfield arable technical manager, David Leaper (left).“The impact of OSR volunteers is becoming increasingly important with closer rotations and a declining selective herbicide armoury,” stresses David. “Especially so, as priority cultivations for black-grass control ahead of wheat planting increase the dormancy of shed rapeseed. This means far greater survival and emergence in subsequent OSR crops which, in turn, compromises plant populations, crop uniformity, productivity and disease management.”For the best oilseed rape performance growers need uniform crops with superior agronomic traits grown at populations that produce the most efficient canopies. So they’re increasingly prioritising careful variety selection, lower seed rates and variety-specific agronomy. All of which is undermined where large volunteer legacies leave the sort of mixed populations and 120 per cent-plus establishment rates many are seeing.”Add to this extra slug problems and green bridge and seed disease carry-over risks, and minimising OSR volunteers must be taken seriously.”Against this background, detailed recording in the initial Openfield trial in 2010/11 showed volunteer plant levels of over 100/m2 three weeks after harvest in four of the 13 strips, the worst being more than 700 volunteers/m2. What’s more, no less than 14 of the 22 strips harvested in an extended trial in 2011/12 had more than 70 per cent volunteer cover after two months.
This was despite harvesting with a top of the range Claas Lexion combine fitted with Vario header and the inclusion of a proprietary pod sticking agent with the pre-harvest Roundup spray. Interestingly too, while being acutely aware of problems during his combining in 2011, last harvest the grower involved reported little or no visible pod shattering in the crop.”Seed losses at harvest may not noticeably comprise yield in most cases,” David points out. “Yet their impact on volunteers is invariably considerable. For instance, at a typical thousand seed weight of 6g a loss of just 30kg/ha is over 500 seeds/m2. Which, even after accounting for viability losses of as much as 90 per cent over a couple of seasons, is the equivalent of a typical hybrid seed rate added to the next oilseed rape crop.”As well as highlighting the scale of what are often unrecognised losses, our trials also show they can be dramatically reduced by growing varieties carrying genetic resistance to pod shatter,” he explains.”In 2011, we recorded less than 10 volunteers/m2 three weeks after harvest in the strips of all three varieties known to have pod shatter resistance against an average of just under 170/m2 in the other ten variety strips.”Equally, in 2012 we recorded less than 10 per cent volunteer ground cover two months after harvest with five varieties and just three per cent with the best performer, DK Extrovert. This compared with an average of just under 80 per cent volunteer cover for the other 17 leading varieties in the trial and over 95% from the six worst performers (Figure 1).Figure 1: Volunteer OSR growth two months after harvest (Openfield trial 2011/12).“Altogether, our evidence suggests pod shatter resistance in a variety can typically give a ten-fold reduction in volunteer problems arising from seed losses around and at harvest,” David reports. “And the advantage can be considerably greater where the weather is unsettled or harvesting conditions are difficult.”There are, of course, a number of other cultural opportunities for improving oilseed rape rotational hygiene. As well as employing extension headers and ensuring the correct combine settings to minimise threshing losses, pod stickers may help in some circumstances. As will subsequent stubble and seedbed management to encourage maximum volunteer germination ahead of the next crop for pre-planting control with Roundup.David urges growers to appreciate that as ‘contact’ products, the effectiveness of pod stickers depends on achieving the best possible distribution throughout the canopy which, in his experience, certainly isn’t easy pre-harvest in many crops. He also accepts that leaving stubbles uncultivated for the four weeks needed to ensure the best chit and pre-planting control of volunteers may not be practicable with many early wheat establishment regimes; particularly where the most effective pre-planting black-grass control is required.
“Given these limitations, the pod shatter resistance available alongside a growing range of yield-protecting traits in Dekalb hybrid varieties is a real boon for rotational hygiene,” he concludes. “Our trials and anecdotal evidence from a growing number of customers confirm its value in minimising seed shedding even under conditions where it is not considered a significant problem.”