IDENTIFYING the best time of day to apply different pesticides is an imprecise science, but practical experience does give some pointers, says Mike May of IACR Brooms Barn, Suffolk.
"Weather tends to be calm early in the morning, often with high humidity and dew on the plants. For some herbicides and desiccation work this can be an advantage. But avoid this timing if rain is forecast," says Mr May.
Care should also be taken when applying products in the evening if an overnight frost is likely. And remember early season temperature at crop level is usually 2-5C (3.6-9F) lower than the screen temperature. Similarly, even in the early months of the year, the direct sun temperature can easily exceed 30C (86F) the following day.
"The risk of scorch increases in the midday sun, although some try to turn this to their advantage by spraying with a reduced dose," comments Mr May. But temperatures over 20C (68F) can reduce the selectivity of herbicides, especially in crops like sugar beet where crop loss can be significant.
"We are finding in trials that there can be losses even when scorch is not very evident," he adds.
Timing may also be important when considering sequential applications. Product labels say which tank mixes should not be used and what interval should be left between potentially antagonistic products, such as some graminicides and broad-leaved herbicides, or with adverse soil or weather.
Less obvious is the situation where a residual herbicide has been applied to sugar beet in dry conditions and a follow up herbicide spray used. If rain comes both become active and the crop can be hit with a damaging "double whammy".
"On the other hand, with manganese, it has been found that weed control is good if it is applied before or with a herbicide, but applying it after the herbicide can help some weeds grow away," comments Mr May. *