Article taken from Agronomist & Arable Farmer, May 2017
Spring wheat directly drilled into cover crops can be given appropriate doses of nitrogen without encouraging excessive blackgrass growth, producing a high-yielding, commercially viable crop. However, spring wheat drilled after autumn fallow struggles to compete with blackgrass, and adding N makes the problem worse.
The findings come from the first year of an Agrovista trial at Draughton, Northamptonshire, supported by Yara.
""The site had a very high population of blackgrass, so provided a real test for the various techniques we employed," says Agrovista technical manager Mark Hemmant.
Three types of cover crops - a black oat/berseem clover mix (N-Structure), a deep-till radish and a mixture based on tame oats - were drilled into large farm-scale plots alongside a fallow plot.
The cover crops were drilled on September 9, 2015 following a light cultivation with a Simba X-Press. ""Two months later the fallow was like a lawn with blackgrass, and there was plenty to be found in the cover crops as well," says Hemmant.
The cover crops and fallow plots were destroyed with an application of glyphosate in November, and a repeat dose was applied in February to clean up any new growth.
All plots were drilled on April 28, 2016 with KWS Willow spring wheat using a Great Plains Saxon drill. This has performed well in blackgrass control trials at Agrovista's nearby Project Lamport site, thanks to its minimum disturbance action, which reduces the germination of blackgrass seed.
All plots received the same pre-em herbicide treatment of 0.15 litres/ha of Herold (diflufenican + flufenacet) on an EAMU and l,000g/ha of pendimethalin + Remix.
Five different seedbed N regimes were used across the plots, ranging from 0-280kg/ ha. This produced some stark differences in blackgrass head counts and crop yield.
""The key difference was that on fallow plots, N increased blackgrass head numbers," says Hemmant.
Where no N was applied after autumn fallow, just over 100 heads/sq m were recorded at the end of June. However, at 160kg/ha N this rose to about 225 heads/sq m, tailing off to a still substantial 150 heads at higher doses (graph 1).
By contrast, N had almost no impact on blackgrass numbers in the N-structure plot. Interestingly, where no N was applied the blackgrass head count was approximately half that seen in the fallow plot and in other cover crop plots. This is probably because the N-Structure trapped, rather than smothered, blackgrass in the autumn, allowing more to emerge and be destroyed with the glyphosate pre-drillling.
The blackgrass head count figure remained almost unchanged at 160kg/ha upwards - some reduction was seen where just IO0kg/ha was applied.
An almost identical pattern was seen with the deep-till radish cover, although the head count was about twice that seen in the N-Structure plot. Tame oats produced a similar result.
""We think the reason that N had little impact on blackgrass count after a cover crop is that cover crop residues helped minimise soil disturbance at drilling," says Hemmant.
""This reduces blackgras s germination, as observed with black oat mixtures at Project Lamport.
""Cover crops also capture nitrogen during the late autumn, and bacterial breakdown of crop residue also has a nitrogen cost. This could be influencing blackgrass emergence and vigour post drilling.""
The plots were harvested on September 9, revealing the impact of cover crop and N on yield. Graph 2 shows wheat after the N-Structure cover produced around 8.5t/ha from 100kg/ha upwards, outperforming other plots following cover crops and leaving the crop after fallow trailing.
""We have to be careful about results after just one year," says Hemmant. ""However,there is a very strong trend that suggests the more N you put on without a preceding cover crop, the more blackgrass you get."" Graph 3 shows the overall yield results and blackgrass head counts at 160kg/ha N.
""With wheat after autumn fallow we ended up with just 3.3t/ha at 160kg/ha N and a very high seed return into the bargain, "" says Hemmant.
""It turns out that the best return was from no nitrogen at all, but that wouldn't be economical. That's a big contrast from the 8.4t/ha achieved after N-Structure, and a blackgrass head count of just 56 heads/sq m.""