Water, weather and weeds

By Dr. Brian de Villiers

It is well known that cations in hard and brackish water antagonise various post emergence herbicides. However, the question arises at what level this will occur at different cation levels. Growers want to know the threshold values for cations to establish when to use water conditioners. The problem with this approach is that cation threshold values differ depending on different weather conditions and weeds, and it is almost impossible to give an accurate answer. Villa is a firm believer in using our ammonium sulphate-based adjuvants as standard practice when registered with these herbicides. Threshold values then become less important.

Water and weather

Cations complex with certain herbicides as the droplet dries on the leaf. These complexes modify the herbicide molecule, decreasing absorption and efficacy. A given concentration of cations will always complex a certain rate of herbicide, but this doesn’t mean that we can predict the efficacy. Under optimal weather conditions absorption is ideal, and the antagonistic effect of cations will not be that evident. This is because enough herbicide is still absorbed to be effective under those conditions. On the other hand, in poor weather conditions absorption will be compromised, therefore the same concentration of cations will have a bigger impact. The same applies to weeds. Cation antagonism will be more evident on difficult-to-control weeds or weeds that are under stress. Therefore, any factor that compromises absorption will amplify the effect of antagonistic cations.

Adjuvants

The same principle applies to adjuvants. Optimal weather conditions and easy-to-control weeds will be forgiving, and one may get away with using a less effective water conditioner. This is because enough herbicide is absorbed, despite the poor performance of the adjuvant. However, poor weather conditions and difficult-to-control weeds will expose the limitations of ineffective adjuvants, therefore the control will be unacceptable. This is the reason why we at Villa only recommend our ammonium sulphate-based adjuvants for this purpose. They are the most effective adjuvants for our cation sensitive herbicides and will help to deliver the best possible weed control under all conditions. They also have additional advantages that increase efficacy; therefore, they should be an integral part of these applications.

Villa’s stance

Threshold values for cations are almost impossible to determine because antagonism is affected by weather and weeds. We find that our ammonium sulphate adjuvants deliver the best results under all weather conditions. Please don’t take the risk of replacing our ammonium sulphate adjuvants when applying the Villa herbicides. It could have a devastating effect on weed control. Contact your Villa agronomist for more information about adjuvants and water quality.