Tuesday, August 20, 2019

Dry summer continues to dictate crop management

July 31, 2018 by  
Filed under Crops

Exceptionally dry weather in the run-up to harvest has implications for autumn establishment, says Richard Overthrow.

Lucky growers may have some moisture in post-harvest seedbeds. But oilseed rape drilling should be delayed where lack of rain means soils remain dry because sown seed can become dormant if drilled into parched ground.

While this dormancy is mostly shorter than for volunteers or shed rape seed, it can still lead to a staggered emergence making early agronomy timings very difficult. But any such forced delay in rape sowing could have hidden benefits too.

Given few, if any, chemical control options, sowing date is the most effective cultural method for flea beetle control, and hence the best control method full stop. Early or late sowing, outside the normal window of activity of the beetle, can lead to the crop avoiding serious damage.

Late sowing has been more effective than earlier, and many growers have reported little or no damage with September-sown crops. So patience is often its own reward – although waiting can require a lot of nerve.

The risk of running into colder, wetter conditions of autumn would mean insufficient plant growth going into winter. But as with winter cereals, delayed sowing can be a valuable agronomy tool even if, as mentioned above, it is due to excessively dry seedbeds at normal sowing time.

Safe sowing window

Even if adequate rain has fallen and good seedbeds created, it is still worth considering delayed sowing where the history of flea beetle damage suggests a large threat. Experience suggests that in this region the ‘safe sowing window’ for the crop extends to mid September anyway.

Other benefits will show such as a longer window for stale seedbeds for grass weed control and avoidance of early aphid activity that might introduce Turnip Yellows Virus (TuYV).

For many, flea beetle has forced a change in approach to weed control. Where it has been a serious problem, pre-emergence herbicides are less commonly used – not just because growers want to see an emerged crop before spending money on it but also because such herbicide timings can slow down emergence and subsequent ‘escape’. Early post-emergence treatment is becoming more popular as a result but will be less effective against early weed flushes.

Pre-emergence herbicides cannot be used on broadcast or autocast crops. Here, weed control will have to wait until post-emergence but the low soil disturbance associated with these cultivation methods usually means early weed pressure is low.

Competition for moisture

With more intense cultivation early flushes of cereal volunteers will be a problem and it is essential these are taken out with a graminicide as soon as possible, and certainly before they start to tiller. Their main threat is competition for moisture and if this is already limited than effects on crop plants could be severe.

Other grass weed control issues will mainly involve the later post-emergence options, though some contribution can come from earlier residual treatments.

Starter fertiliser is also essential for oilseed rape, again to help grow away from flea beetle attention and early weeds. Phosphates can be as effective as nitrogen and combinations of these are now commonly used. They should be applied at or very soon after sowing to ensure they are available from the start of crop emergence.

Careful stubble management, to create good stale seedbeds, is important ahead of cereal crops. Good seedbeds will also help residual pre-emergence herbicides, and these are now an important part of grass weed programmes.

Herbicide timings

Recent autumns have been sufficiently wet to allow good seedbeds and pre-emergence performance has been good whilst still allowing timely drilling but soil conditions should always dictate herbicide timing and hence crop sowing so, if seedbeds are dry, do try to wait a while.

Where grass weeds are particularly numerous sowing should be delayed anyway, this will also allow more time for seedbed preparation and pre-sowing control.

If there are no grass weed problems it is feasible to sow wheat in early September, though good seedbeds with adequate moisture are still important.  If planning to sow early you need to spend much of this month planning for this. Variety choice, seed rate, herbicide and aphicide strategies can still be complex.

Richard Overthrow is a regional agronomist with NIAB TAG, the UK’s largest independent agronomy organisation with several research centres in East Anglia. For more details, call 01223 342495.