If you don’t take blackgrass levels into account when choosing which wheat variety to sow in which fields, perhaps you should, Masstock Arable trials suggest.

Final blackgrass ear numbers in plots untreated with any herbicide differed hugely depending on variety and seed rate, notes the firm’s technical manager Clare Bend.

“We believe variety choice must be considered alongside other integrated control methods, such as stale seed-beds and pre-emergence herbicides, to reduce the selection pressure on blackgrass that may be resistant to Atlantis*,” says Mrs Bend.

“Reducing blackgrass populations before spraying will help.”

The trials, at Stow Longa in Cambridgeshire, examined the competitiveness of three winter wheat varieties drilled on 19 September at two seed rates (175 and 350/sq m).

Treatments consisted of no herbicide, a weak programme or a strong Atlantis-based programme. Blackgrass ear numbers were assessed in June.

The varieties were chosen for their different growth habits, says Mrs Bend. “Robigus tends to tiller well and Gladiator less so, but achieves good ground cover pre-Christmas. Hereward is known to be a shy tillering variety.”

The untreated plots of Robigus, predicted to be the most competitive variety at the higher seed rate, had 62% fewer blackgrass heads/sq m in June, compared with the least competitive variety, Hereward, at the low seed rate.

“Doubling seed rates in Hereward and Robigus reduced blackgrass levels in untreated plots by 35-45%, and the yield difference between the best and the worst was a staggering 3.9t/ha.

Choice of variety and seed rate was also particularly relevant where a weak rather than a strong herbicide programme was employed.

“Our work suggests that the most competitive varieties should be sown in the worst grassweed fields to cut populations before applying Atlantis or other herbicides where resistance could be an issue.

“Masstock has profiled many other wheat varieties. Careful increases in seed rate may also be appropriate.

“But this should really be done with an agronomist, as it could have other agronomic impacts such as increased lodging risk.”

*Active ingredients: Iodosulfuron + mesosulfuron