Maximising the control achieved by soil-applied herbicides

Abstract

A combination of wind tunnel experiments and field trials examined the use of different spray nozzles to apply pre- and early post-emergent herbicides based on the hypothesis that improving the uniformity of deposition at the soil surface would improve product efficacy. Results of the wind tunnel study showed no major differences in the shadowing effects of spray deposits around clods when sprayed with different nozzle designs and the surface coverage around clods was mainly influenced by the droplet size distribution in the spray. Angling the spray delivery did tend to increase the deposition shadow probably because more of the spray was then deposited in the clods. Results from the field trials gave no significant differences in black-grass control when treatments were applied with a wide range of droplet size distributions in the spray and therefore there is likely to be no benefit from applying pre-emergence sprays as fine or medium quality sprays. The use of sprays with a larger droplet size such as those generated by air-induction nozzles would be expected to give comparable product efficacy when compared with possible alternatives and has the advantage of a reduced drift risk. Results from work conducted in parallel with that reported here indicated that the use of application volumes of 100 L/ha would give no adverse effects on efficacy and again deliver important advantages relating to work rate and timeliness.

Taking the results from this project together with those from associated work enabled the following key messages to be defined:

• Efficacy is likely to be higher when using 100 L/ha rather than 200 L/ha and this also gives timeliness advantages;

• There is flexibility in nozzle selection for pre-emergence applications such that using an air-induction nozzle will represent a good option for delivering high efficacy and drift control - and the potential to improve timeliness;

• There are no differences in the application requirements for chemicals having primarily root or shoot uptake modes of action;

• In coarser seedbeds, there may be some advantages from using application systems that generate mixing of the spray during delivery;

• It is important that factors influencing the spray volume distribution pattern below the boom including nozzle pressure, boom stability and forward speed are managed to keep the larger scale deposit distribution as uniform as practicable.