How to manage winter wheat herbicide top ups this November in East Anglia

Article overview

This November, Sam Harvey delves into how to approach grass-weed control in emerged crops through resistance testing and herbicide use.

How to manage winter wheat herbicide top ups this November in East Anglia Content

Crop Progress

Many wheat crops were in the ground by late September into early October well ahead of peak germination of black-grass and Italian ryegrass. In addition, the dry weather pre-drilling created a limited flush of these grassweeds that could be sprayed off before the crop was planted.With useful rain arriving in the latter half of October, black-grass and Italian ryegrass will be racing through the ground as soil temperatures remain relatively warm.

Sam’s agronomy tips for November

1. Do you need a residual herbicide only follow up?

Warm soil temperatures mean not only will grassweeds emerge quickly, but any pre-emergence residual herbicides applied some time ago will be active for less time. We know you get around 30% less efficacy from a residual herbicide applied mid-September compared with one applied mid-October. Dry conditions at application may also have limited activity depending on what rain arrived, and when, and what the applied residual active substances were.On the plus side seedbed quality has been relatively good which will have hopefully aided efficacy. However, higher numbers of weeds to control due to drilling date and recently wetted clods will begin to open-up with seeds germinating.For those crops where grass-weeds have not reached one true leaf it will be possible to rely on another residual herbicide top up, the choice of which will be driven by what has been applied pre-emergence.

2. Or do you need a contact-acting herbicide?

Once a true leaf of black-grass or Italian ryegrass is fully emerged a contact acting herbicide is going to be necessary and that will be a mesosulfuron-containing product. It is still important to partner the contact with an appropriate residual to help control subsequent and protracted germination.With contact herbicides, application is key. Getting the fundamentals, such as water volume, nozzle choice, boom height, forward speed and ensuring application to a dry leaf and allowing sufficient drying time will make all the difference between satisfactory control or not.Although we are into shorter days, make the most of the opportunities of still, dry, sunny days. The key is not to push your luck with too many tank-loads a day otherwise product won’t have a couple of hours to dry on the leaf. Timing is key, it’s important to apply to small actively growing plants that are sat in soils where the previously applied residual herbicide still retains good activity on that weed.Recent mild winters have meant plants can be big in the spring and it is generally better to spray actively growing weeds going into a cooler period as opposed to those coming out of the cold which can be stop-start and which often coincides with the dry too.

3. Understand your enemy with resistance testing

Knowing your enemy when choosing herbicide strategy is key and no less so for contact chemistry. Resistance testing is the only way to know.A recent ryegrass resistance survey reinforced this, highlighting the huge variation of herbicide sensitivities within populations, both within field and between fields which have been treated the same agronomically. It shows that making an assumption about a populations’ resistance status from other resistance test results on the farm, even if the population is thought to be the same can be dangerous. Populations are likely to be more different than you think, and can result in the wrong course of action.The recent ryegrass resistance survey also highlighted how situations with poor or failing control resulted in poor control from reasons other than resistance as often as resistance, highlighting the need to focus attention on the fundamentals such as spray application and timing.Reassuringly the survey identified there was limited cross resistance between contact and residual chemistry which highlighted the need for mode of action diversity in the whole herbicide programme, but again make sure you understand the populations within fields by carrying out representative resistance sampling and interpreting results in the right way to make informed decisions.