4 April 1997

Clean linseed has the edge

Weed competition in spring linseed is the theme in this the latest of our series bringing HGCA-funded researchers face to face with farmers. Andrew Blake profiles the research here and farmer reaction on p70

KNOTGRASS is a far greater threat to linseed than chickweed, recently completed HGCA trials indicate.

Linseed is widely regarded as a low input crop. But most producers accept the need for weed control, says researcher Richard Overthrow of the Arable Research Centres.

"Give any linseed grower only one spray and they are almost sure to say it must be a herbicide." For now fungicides, insecticides and growth regulators are much less rewarding on the crop.

Keeping linseed reasonably clean is important, as it is not very competitive, says Mr Overthrow, who spent £60,000 of HGCA oilseeds levy determining which weeds hit the crop hardest.

The three-year project involved sowing seeds of four key weeds at several different densities in standard crops of Antares (700/sq m). Trials were at three ARC sites and at Morley Research Centre and IACR-Rothamsted. Plots were hand-weeded to remove other species and yields measured to determine precisely how much damage each sown weed caused.

"From there we were able to look at the financial implications, balancing herbicide costs and the percentage yield loss from each weed to see if a spray would be cost-effective."

Main aim was to help growers assess whether treatments would be worthwhile for specific infestations.

But weeds rarely occur uniformly across fields, he concedes. "That is why we stopped short of producing a ready reckoner. What we have created is more of a decision support system for growers to keep at the backs of their minds."

Weeds chosen for sowing were chickweed, fat-hen, knotgrass and spring oats, the latter just as competitive as wild oats, says Mr Overthrow.

Dry weather in all three seasons meant weed germination was often poor. But useful trends and the odd surprise emerged, he notes.

Overall, knotgrass and especially oats were the most competitive of the four sown species. There was an unexpected result at Wimborne, Dorset in 1994 when germination was good. There the densest chickweed left a better yield than a lighter infestation.

Although chickweed often seems aggressive, forming dense mats in the crop base, the discovery reinforces those from earlier Rotham-sted work, explains Mr Overthrow.

It suggests that in very weedy stands the prostrate chickweed plants soon start competing with each other rather than the linseed, which quickly grows away from the mat. "Our trials show that even at very high populations of chickweed and fat-hen you may not lose yield," he adds.

Yield potential clearly has an effect on the economics of treatment. "As the yield comes down you can tolerate higher levels of weeds before you need to control them," says Mr Overthrow.

Just to cover the cost of a popular herbicide, say Ally (metsulfuron-methyl) at £18/ha (£7.30/ acre), growers need an extra 0.15t/ ha (1.2cwt/acre). (That assumes a linseed price of £120/t.) But linseed yields vary a lot, so 0.15t/ha must be redefined as a percentage yield response in order to gauge treatment thresholds, he explains (table 1).

A 3t/ha (24cwt/acre) crop requires a 5% yield response merely to cover the cost of the broad-leaved herbicide. Oat weed-killers are more expensive, so the yield response needs to be 0.25t/ha (2cwt/acre). The research highlights big differences in the numbers of weeds justifying spraying such a crop (table 2).

"We are not saying you should not control weeds below those levels," says Mr Overthrow. Avoiding seed return and harvesting hassle may dictate sterner tactics. But the ARC analysis shows that of the tested weeds, knotgrass is by far the most damaging, oats and fat hen about equal, and chickweed, perhaps surprisingly, the least worrying. &#42

Richard Overthrow (centre) gives the Moore brothers, Geoffrey (left) and Roger, the lowdown on weed treatment thresholds for spring linseed.

Table 1: Yield response represented by 0.15t/ ha in linseed crops of different potential

Potential % yield response

yield t/ha







Table 2: Weed infestation spray thresholds in 3t/ha linseed crop*

SpeciesPlants a sq m





*Linseed @ £120/t. Broad-leaved herbicide @ £18/ha. Graminicide @ £20-37/ha.