14 August 1999

Cure-alls for rape

The saving grace for rape this year seems to be its agronomic benefit as a good entry for wheat – but it has to be profitable. Herbicides are the obvious target for cost savings, writes Lucy Stephenson.

TIPPING the balance back to profit in oilseed rape means cost-effective weed control – and that means knowing when products work best, and where their strengths lie, says ADAS agronomist Brendan OConnor.

Herbicides can take the lions share of spend on oilseed rape. In difficult weed infestations, costs can escalate to over £100/ha. The primary reason for controlling weeds in oilseed rape is to prevent populations building up, says Mr OConnor. Weeds can also act as a green bridge for disease carry-over to the following wheat crop.

Its tempting to blitz weeds simply for tidy looking fields. Poppies dont turn just the field red – theyre so noticeable that growers blush too, he says. There are practical problems too: cleavers are still green at harvest and tangle in the combine.

As a general rule broadleaved weeds cause little yield loss, says Mr OConnor. However, research has shown that weeds left untreated can cause yield losses of 0.64t/ha from broadleaved weeds and up to 1t/ha from volunteer cereals.

But with area aid and prices at new lows it doesnt pay to get rid of the last weed. "Anyone who spent lots of money on weed control lost money last season. A gross margin of £430/ha doesnt cover fixed costs which on most arable farms are about £500/ha," says Mr OConnor. Broadleaved weed control in rape is often unreliable, because oilseed rape is a sensitive broadleaf itself. Costs can be kept lower if broad-leaved weeds are tackled in cereals so that they are less of a problem in rape, he says. Conversely, grass weeds are much easier to eradicate in rape than cereals.

Mr OConnors standard rape herbicide programme costs £45/ha. It begins with a pre-emergence spray of Butisan (metazachlor) at 1.5 litres/ha costing £35/ha, followed in late winter with a single post-emergence spray of Falcon (propaquizafop) plus oil at 0.3 litres/ha, costing £10/ha. This basic recipe is modified according to the weeds present on each farm, he stresses.

Cheap option

The first, least expensive herbicide option is Treflan (trifluralin), at £5-6/ha. It suppresses a broad range of grassweeds and broadleaves, particularly chickweed, but is ineffective in dry conditions.

Pre and early post-emergence herbicide Butisans strengths are chickweed, mayweed, poppies and shepherds purse. But Butisan has a particular weakness against cleavers. "If cleavers are a problem then use Katamaran (metazachlor plus quinmerac), which is also strong against poppies," says Mr OConnor.

No pre-emergence herbicides can be relied upon without moisture, which the weeds need to take up the chemical.

Surprisingly, seedbed conditions are more likely to be moist on light soils than on heavy land. Heavy soils might be wetter most of the year but cloddy seedbeds are often dry, reducing germination, explains Mr OConnor.

If conditions are ideal the pre-em rate can be as low as 0.75 litres/ha for Butisan and 1.25 litres/ha for Katamaran. However, in dry conditions full rates are more reliable, he advises.

If cranesbill or cleavers are a problem, Galtak (benazolin) is particularly effective. Fortrol (cyanazine) can do a good job on charlock and runch, and on bugloss if it is frosty, says Mr OConnor.

Small sow thistles and stubborn mayweed are best cleared with Dow Shield (clopyralid). But dont just look in the gateways before deciding to spray – mayweed is always worse there and on the headlands, he warns.

Grass weed control is less complicated. Mr OConnors other staple, Falcon, is a cheap and cheerful graminicide with general activity. Early autumn treatment is required to remove competitive levels of volunteer cereals. Small volunteer cereals and blackgrass can be knocked on the head with just 0.2 litres/ha plus oil, he says.

Substantial savings can be made by reducing rates of Falcon, Laser (cycloxydim) and Fusilade (fluazifop-P-butyl). "I cut the rate of Falcon down to 0.2 litres/ha. If brome is a particular problem though I use a late winter treatment of Fusilade or Laser," says Mr OConnor. However, these graminicides are ineffective on resistant blackgrass, which should be treated with full-rate Kerb.

Ben Freers prescription for heavy soils:

1) 2 litres/ha Triflur (trifluralin) tank-mixed with 0.75 litre/ha Butisan S (metazachlor) applied pre-emergence, or 1 litre/ha Katamaran (metazachlor plus quinmerac) where there is a known cleavers problem.

2) Trifluralin is more effective if incorporated into the soil pre-drilling, but this can fluff up and dry heavy soils. Spray it on prior to drilling, then apply Butisan S as a post-em at 1 litre/ha.

3) Wait until October and put on Kerb (propyzamide). Metazachlor, propyzamide and trifluralin are good as part of an anti-resistance strategy for blackgrass because they reduce your reliance on fop and dim graminicides.