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Crop Crack 17.10.09
By Wendy Fearon
As ground conditions improve with each passing day and the good spell set to continue the rush is on, with ploughing and drilling going on apace. Most of the cereal crop is now planted and the potato crop is now ready to harvest which is huge step forward on last autumn.
The late planting this year has delayed harvest but it looks as though the weather will allow digging to get underway, as many of these crops should now have the skins set. The apple crop harvest is now complete in Co Armagh in record time which is due to good weather and unfortunately poor tonnage. Harvesting of the maize crop has not got under way yet so hopefully the good weather will continue.
Cereals
IPU has been a primary component of most autumn herbicides for many years, giving cost-effective control of annual meadow grass (AMG), chickweed and some other broad-leaved weeds (BLW). However because of issues with it appearing in groundwater its approval was withdrawn earlier this year and is now no longer available.
Its loss does complicate herbicide selection going forward. Good control of AMG remains the key issue; when not properly controlled this weed is the most damaging weed of all to NI cereal growers. There are a number of alternative products available that do control AMG, some more effectively even than IPU. However all have limitations when compared to IPU and so there is a greater likelihood of poorer control and increased cost implications if these limitations are not understood. All the alternative actives for AMG control have narrower spray windows than IPU; therefore timing is more critical to obtain a satisfactory result.
Chlorotoluron (CTU) is of the same family of chemistry as IPU, and so has identical mode of action and very similar spectrum of weeds controlled, including AMG, chickweed, mayweed & groundsel. Its’ principal limitation is its potential to scorch; certain varieties of winter barley & winter wheat are not tolerant to CTU, so can only be used on named varieties (check approved variety list). In addition, only use when the seed is well covered and the crop not under any stress.
Flufenacet offers the widest window for good AMG control – it is very residual so will remain active for an extended period after application, controlling from pre-emergent through to beginning of tillering. Ideal timing is very early post-emergent – a new definition for this timing is now being used, peri-emergence, meaning around the time of emergence as distinct from some time after emergence.
Iodosulfuron & mesosulfuron (both sulphonyl-ureas, SUs) will control AMG well into mid-tillering, but have contact activity only, therefore requiring all the AMG to be emerged for control to be satisfactory, so really can only be used early spring onwards. (In addition these actives can only be used on winter wheat, not winter barley or winter oats.)
Pendimethalin (PDM) is also very active on AMG, but pre-emergent only. It is much less soluble than IPU, so persists for longer in the soil. PDM also has a much wider BLW spectrum than IPU.
Diflufenican (DFF) is another widely used active in the autumn; it is residual and has a wide spectrum of BLWs controlled, taking out larger emerged BLWs than many other residuals, but has no AMG activity whatsoever.
Be aware that the length of persistency of any herbicide product that has residual activity is inversely related to soil temperature – the lower the temperature the longer the duration of its persistency; therefore as soils cool over the coming weeks so product performance will improve. Soil acting herbicides also require adequate soil moisture to work, but their performance will fall off in a wet winter when the actives may be leached away before the crop canopy is big enough to prevent a late weed infestation.
Flufenacet will be the mainstay of AMG control in the 2009-10 crop. This active also gives good activity on brome and perennial rye-grass when used at its’ maximum rate. CRYSTAL contains flufenacet and pendimethalin and can be used on all varieties of winter barley & wheat.
BUCKLER contains CTU & DFF. Where the variety list allows this product should be tank-mixed with the CRYSTAL. Its inclusion will extend the spray-window of CRYSTAL, allowing larger weeds to be controlled than if the CRYSTAL were used alone. The CTU will also bring groundsel activity to the mix, whilst the DFF will broaden the spectrum of emerged BLW controlled, including chickweed & mayweed. Only use when the seed is well covered, and not subject to stress as a result of waterlogging or frost.
Where the variety being grown does not allow CTU (ie BUCKLER) be used, then a higher rate of CRYSTAL should be used alone.
Barley Yellow Dwarf Virus (BYDV)
BYDV is a disease is carried by aphids and passed into the crop when they feed on the young plants as they emerge through the ground. Once infected, the disease cannot be controlled. Therefore control is aimed at prevention of crop infection. Early drilled crops are most at risk as they are emerging at a time when the aphids are likely to be more active.
Grassy stubbles and volunteer cereal plants act as host plants for aphids, carrying them from last season’s crop into this seasons’ newly emerging crop. Removal of this ‘green bridge’ is the first prong of control – this should be sprayed off, buried 7days later, and ideally 2 weeks elapse before drilling new crop.
Application of a well timed aphicide is the second prong – this should be applied once the crop has 1 full leaf i.e. along with the herbicide (as long as sufficient crop has emerged). Any delay in application gives huge scope for the feeding aphids to transmit the virus into the crop as the unprotected plants come through the ground.
Like the residual herbicides, product persistency is very dependant on temperature, improving as temperatures fall. Very early emerged and treated crops therefore may require a second treatment to keep maintain persistency until the colder weather takes over.
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