Drilling date dilemma
High nitrogen prices are causing a dilemma for one Oxfordshire estate’s usual weed control practices.
Delay winter wheat drilling for its positive impact on grassweed control or drill earlier to potentially help develop a better root system that will increase nitrogen use efficiency – that’s the conundrum Great Tew Estate farm manager and Association of Independent Crop Consultant agronomist Colin Woodward is mulling over for this autumn.
Usually the policy on the 900 hectares of arable crops on the 1,350ha Oxfordshire estate is to leave wheat planting until ideally mid-October, but with the high fertiliser prices he is considering drilling first wheats earlier this autumn.
“Because we have had good grassweed control in the autumn our seed return is not going to be very high and we want to increase nitrogen utilisation by trying to get a better root system,” he says.
“You generally need more nitrogen when you have a poor root system.
That goes against our usual drilling date policy, but we might take a calculated gamble this autumn.”
Most of his first wheat is following a legume crop after temporarily moving away from oilseed rape because of cabbage stem flea beetle challenges, which will also help reduce his applied nitrogen requirements, along with use of organic materials such as compost, sewage sludge and farmyard manures.
“We are well aware we are going to have to cut our nitrogen back but hopefully achieve the same yields.”
Black-grass is the main weed challenge on the heavier soils across the estate, while there is brome on the lighter land.
Delayed drilling has been one of the central planks to keeping black-grass in check, along with rotational options including spring crop ping, cultivations and judicious chemical use.
Soil structure is an important basic to get right, he says.
“We bought our own drainage trencher and stone cart and do a proportion of the farm every year.
We have most issues with black-grass where it is wet.”
The basics also include pH status, which has implications for phosphate lockup and the competitiveness of the crop against the weeds.
“Obviously you have to be mindful of costs, which is why we do our own drainage, but it is a key to keeping on top of weeds.”
He has taken 5% of the poorest yielding areas out of production, which often coincides with areas with weed issues.
“We now have six-metre margins around every field as part of Countryside Stewardship and are using AB15 [two-year legume fallow] as a rotational option.”
The trick with AB15, he says, is to plant it in May or June if there is some moisture, as that helps avoid it becoming full of black-grass.
Seedbeds are prepared in April following a period of fallow and sprayed off with glyphosate before drilling.
The typical rotation is wheat, winter barley, oilseed rape or legume, wheat, spring barley or beans, but if black-grass is increasing he extends that by a year with spring beans following spring barley for two years of spring crops.
“If it is really difficult, we will continue to grow spring barley until we get it right and in extreme cases plough after wheat before spring barley to bury seed.
But we do not do that very often.”
In fact, the aim is to reduce cultivations, partly driven by rising fuel prices.
A 10m controlled traffic system was introduced last autumn.
“Going shallower should avoid burying seed so it goes dormant and help control more of the shed seed in that year to deplete the seedbank.”
He uses two pieces of cultivation kit depending on whether there is any compaction.
“We use a Vaderstad Carrier where we want to lightly cultivate the surface to two-three inches, but if there is any compaction, we use a Gregoire Besson Helidisc for one-pass 6-8in cultivation.
“We generally leave that for a month to six weeks for the soil to settle out and then spray off with glyphosate before drilling with a Horsch Sprinter.”
In wheat and barley, pre-emergence herbicides are used to reduce the pressure on post-emergence chemistry.
“We use a lot of Avadex [tri-allate] for the brome and wild oats and it helps with the black-grass control.
We have an Avadex spreader on the sprayer so we can do both at the same time.”
Cultural controls are very much key to weed management on the estate, with herbicides acting as the last resort.
On around 60-70% of wheat fields Liberator (flufenacet + diflufenican) plus Defy (prosulfocarb) is applied at pre-emergence and then followed up four to six weeks later with flufenacet plus pendimethalin.
If the grassweed history is more favourable, then the follow up post-emergence is either reduced to just pendimethalin or dropped.
Application is important to getting the best performance from the herbicides, he adds.
“We always apply the pre and post-emergence sprays in 200 litres/ha of water using 06 Defy 3D nozzles and travelling at no more than 12km/hour.”
In the winter barley it is Avadex, plus Crystal (flufenacet + pendimethalin), followed up with flufenacet.
“Sometimes we add some Defy to the Crystal if we have some specific weed issues like cranesbill.
“We always plant hybrid barley now as we find it is so competitive with the black grass – we use that as a tool within our integrated weed management.
“We have had great results with it, which is why we can afford to plant it a little earlier at the end of September.”
It is grown on both heavy and light land.
“We find with the hybrid vigour it produces a good root system, so it survives drought better.
“It gets going in the spring quicker, so it helps with competition in the spring more than the autumn.
“But we are mindful where we grow it.
If we’ve had a black-grass problem in the previous crop we think carefully about growing it and would pull fields out for spring crops or AB15,” he says.
A new Bateman with a 30m boom and pulse width modulation (PWM) might open new opportunities for fine-tuning weed control on the estate in combination with digital tools, Mr Woodward adds.
Each nozzle body on a PWM system is fitted with an electric solenoid that pulses the spray on and off multiple times a second.
Spray rates vary depending on the duration the nozzle is in the ‘on’ position and means operators have individual nozzle control with pressure, spray quality and pattern remaining constant, regardless of forward speed.
Having this ability is increasing the usefulness of Mr Woodward’s digital platform.
“We want to reduce our use of contact herbicides by mapping areas with the platform and then use individual nozzle control to variably apply products, something we found difficult to do without individual nozzle control.”
The estate’s 10m controlled traffic which opens the possibility of using harvest weed seed management – blowing chaff from the combine into permanently tracked tramlines to concentrate more of the weed seed there rather than across the width of the combine.