Article taken from Crop Production Magazine, March 2018.
Treating fields early to scupper the chances of competitive broadleaf weeds from eroding yield is a straight-forward concept when taken at face value. Factor in the vagaries of British springtime weather and the picture becomes somewhat cloudier.
Agrovista agronomist Rob Sheets says the strategy won't be possible on every farm in every year, but where sprayers can travel and a weed problem exists, spraying early is a 'no brainer'.
To illustrate his point, he uses a problem he found last spring in 14ha of winter barley in Northants as an example. ""No pre-em was applied to the crop, so during the first week of March it was like a carpet of weeds and you couldn't see the soil in parts," he explains.
Cleavers, poppy, shepherd's purse, cranesbill and other weeds such as fumitory posed the main challenges. Using an SU at full-rate - such as Finish SX (metsulfuron-methyl+ thifensulfuron-methyl) - followed by a mix of Starane (fluroxypyr) plus CMPP would have been the tried and tested route to tackle the problem.
That would have required a pass at the T0 and T1 fungicide timings and would have ramped up the pressure on the SUs as well as further complicating the fungicide tank mix.
""Having looked at Whorl (halauxifen-methyl+ fluroxypyr) and the spectrum of weeds it controls, it matched up well so we decided to go with that in the tank at 0.375 I/ha with an SU at a lower rate. Within a day or two, it was already showing activity. That would have been the Arylex kicking in because the SU takes longer. Four weeks later everything was dead.""
The farm is likely to get a similar treatment this year as no pre-em was applied in the autumn to its wheat crop. ""A lot of the farms I deal with are stacking pre-em chemistry in the autumn, so they haven't got a huge broadleaf weed spectrums come March.
""Those who don't get a pre-em on because the weather closes in, or there isn't a grassweed problem to contend with, have significant challenges in the spring. Travelling is often the limiting factor when it comes to early application, and that will come down to the soil type and conditions," he says.
Late-emerging weeds will also be a concern for growers who don't want to spend early, only to get problems in April and May, he points out. ""It's a case of knowing your weeds and knowing your fields. If you're doing weed control early and you have a competitive crop, then it should be able to pull away from any late-emerging weeds.""