Why is calm and dry weather preferred for herbicide application?

Calm and dry weather is preferred for herbicide application primarily because it prevents spray drift and improves the efficacy of the herbicide. When the wind is calm, there is less chance that the spray will be carried away from the target area onto non-target plants or into water sources. This direct application ensures that the herbicide is effective in controlling the intended weeds because it reaches them without being dispersed elsewhere. Additionally, dry conditions help avoid the dilution or runoff of the herbicide, which can occur in wet conditions and can reduce its effectiveness.

The other options do not align with the reasons for choosing calm and dry weather for application. Calm winds contribute to precision in placement, while dry conditions maintain the integrity of the herbicide on the foliage for better absorption.