What is the term for a substance that improves the effectiveness of a herbicide?

The term for a substance that improves the effectiveness of a herbicide is "adjuvant." Adjuvants are specifically designed to enhance the performance of herbicides by aiding in various functions, such as increasing the herbicide's ability to adhere to the plant's surface, improving penetration into plant tissues, or enhancing the herbicide's overall efficacy against target weeds. They can also help to minimize the impact of environmental factors, such as rain or sunlight, which might otherwise reduce the herbicide's effectiveness.

The other choices do not relate to the enhancement of herbicide efficacy in the same way. The active ingredient refers to the chemicals in a herbicide that are responsible for its intended effect, but it does not include any substances that modify or improve performance. Adsorption is a physical process where molecules adhere to a surface, which may occur in the context of herbicides but does not constitute an additive substance designed to improve herbicide efficacy. Aeration involves the process of introducing air into the soil or medium, which is relevant in turf management for root health but does not enhance herbicide effectiveness.