Getting the best from residual oilseed rape herbicides

Date:09-03-2012

As farmers prepare to start drilling oilseed rape, growers are reminded of the importance of removing weed competition early in order to get the crop off to a good start. BASF are advising growers to make sure that they stick to the label change restrictions on maximum dose rates of metazachlor and also offers advice on how to get the best performance from metazachlor-based chemistry this autumn.

Clare Tucker of BASF says that the vast majority of weed control programmes in winter oilseed rape start with a metazachlor-based herbicide, either straight metazachlor as in Butisan S, or as co-forms with dimethenamid-p (Springbok), with quinmerac (Novall) or with all three actives (Shadow). “One of the key planning actions is to make sure that applications are limited to a total dose of no more than 1000g a.i of metazachlor applied in a three year period in the same field. Before applying a metazachlor-based herbicide in rape this autumn, growers need to find out and add up how much metazachlor has preceded the proposed treatment in that particular field, with the total over the three year period not exceeding 1000 g/ha. The three year period is a rolling three year period, not consecutive blocks of three years, but as the restriction was brought in on 22 Jan 2010, there is no need to consider applications before that date as product used to prior to that date was not subject to restriction.”

To help the planning process, the full rate of Shadow (2.5 l/ha) applies 500g of dimethenamid-p, 500 g of metazachlor and 250 g of quinmerac per hectare. The full rate of Springbok delivers 500g of dimethenamid-p and 500 g of metazachlor per hectare and a full rate of Novall delivers 1000 g of metazachlor and 250 g of quinmerac.

Clare Tucker explains that, with the difficulties of grass-weed control last year, many growers will be looking to their rape crop to boost black-grass control across the rotation. “Both dimethenamid-p and metazachlor have good activity on black-grass and by using a co-formulation containing these actives you can remove up to 60 to 80% of black-grass before the weed emerges, providing a lower weed population and an easier target for foliar graminicides. This will be very important given the black-grass burden this autumn. It also means that you are applying just 500 g/ha of metazachlor in that application.”

"Dimethenamid-p, as in Shadow and Springbok, not only gives additional black-grass control, but also wider broad-leaved weed control, including improved activity on all species of crane’s-bill, Shepherd’s purse, poppy and cleavers. It also has a practical advantage under dry conditions as it is taken up by both roots and hypocotyl/coleoptile of the weed and so is less reliant on ‘chasing’ the roots down through the soil, and can wait for shoos to reach it. This means that Shadow and Springbok are less affected by drier conditions which often occur in September when herbicides are applied.”

She reminds growers that when these herbicides are applied pre-emergence, the seed must be covered with at least 15 mm of fine settled soil. “With all treatments, particularly pre-emergence ones, do not apply if heavy rain is forecast within 48 hours. In addition to crop safety, this adheres to the “H²O OK?” VI water protection guidance to reduce metazachlor loss to water.”