Real world challenges

Question 1

What is the ideal situation of application to effect?

Answer 1

Application –> transfer processes via movement into soil –> absorption via root uptake or foliage or shoot uptake –> translocation via phloem movement (downwards) or xylem movement (upwards).

Question 2

What is spray drift?

Answer 2

Airborne movement of spray droplets away from treatment site during application

Question 3

Why is spray drift a problem?

Answer 3

Able to damage nearby sensitive crops or contaminate harvest-ready crops

Question 4

How can spray drift be managed?

Answer 4

Specifically designed spray nozzles for application and monitoring of local weather changes

Question 5

What is volatilisation?

Answer 5

Process of solids or liquids converting into a gas, thus moving away from initial site of application. Movement referred to as vapour drift.

Question 6

Why is volatilisation a problem?

Answer 6

Removal of agrochemicals from initial site, release of agrochemicals into atmosphere

Question 7

How can volatilisation be managed?

Answer 7

Weather monitoring - hot, dry and windy weather will increase volatilisation.
Use of active ingredients with high boiling point and molecular weight, as tend to be less volatile.

Question 8

How can volatilisation be measured?

Answer 8

Recording loss of a compound over time in a wind tunnel. Should be able to find a value for T50 to quote with other data on the agrochemical.

Question 9

What is run off and leaching?

Answer 9

Run off is the movement of agrochemicals in water over a sloping surface. Leaching is the movement of agrochemicals in water through the soil, and can occur in all directions.

Question 10

Why is run off and leaching a problem?

Answer 10

Decreases effectiveness of treatment with agrochemical as effective concentration lower. Can cause water contamination and impact livestock and downstream crops.

Question 11

How can run off and leaching be managed?

Answer 11

Properties heavily influenced by solubility of agrochemical. Low solubility (< 1 ppm) will reduce chances of leaching and run off, however reduces changes of reaching the target. Balance is required.
Active ingredients with low melting points, charged or containing high levels of polar functional groups will be more water soluble.

Question 12

What degradation and breakdown processes might impact agrochemical effectivity?

Answer 12

Photochemical breakdown, chemical breakdown, biological metabolism

Question 13

Where does photochemical breakdown happen and can it be measured?

Answer 13

On foliar and soil surfaces, in water bodies. Photochemical breakdown can be measured by recording the loss of the parent compound in an artificial sunlight simulator.

Question 14

What is a major risk factor for photochemical breakdown?

Answer 14

Extended conjugation. Example seen in Case Study 2.

Question 15

Why is chemical breakdown a problem?

Answer 15

Sensitive functionalities can be broken down by simple chemical processes, reducing effective concentration of the drug.

Question 16

What is biological metabolism?

Answer 16

Breakdown of agrochemicals within a living organism, by molecular modification or degradation of an active ingredient, resulting in structural changes.

Question 17

Why is biological metabolism a problem?

Answer 17

Generally results in reduced potency. Metabolism results in change in structure that can mean the chemical no longer interacts at the active site or a change in physical properties to promote excretion or sequestration. Small changes in structure can have dramatic impacts on activity of agrochemical.

Question 18

How can metabolism be managed?

Answer 18

Could use pro-agrochemicals. Some agrochemicals can be activated by metabolism.

Question 19

What are the processes in metabolism inc. the key metabolic pathways?

Answer 19

Made up of two phases, like in humans! Major metabolic pathways are: hydrolysis, oxidation, reduction and conjugation to glutathione.

Question 20

Why are metabolism and selectivity linked?

Answer 20

Metabolism enables organisms to escape toxic effects of an agrochemical. Agrochemical-tolerant plants are able to metabolise the chemical to inactive compounds before it builds up to toxic levels in the site of action, and susceptible pests are unable to detoxify agrochemicals.

Question 21

Where can crop selectivity arise from?

Answer 21

Crop selectivity can arise from selective metabolism of the agrochemical, differences in active site, reduced uptake of the agrochemical and reduced distribution of agrochemical within the crop.

Question 22

What is agrochemical persistence?

Answer 22

Measured in terms of a half-life: time in days required for an agrochemical to degrade to 50% of the original amount.

Question 23

Why is it important to balance agrochemical persistence?

Answer 23

If the agrochemical is too persistent, t > 100 days, encounter problems with leaching and runoff and can cause damage to next season’s crops.
If the agrochemical is non-persistent, t < 30 days, degradation is likely to occur before desired effect achieved.