In which condition is Ester 2,4-D less effective?

Ester 2,4-D is a systemic herbicide commonly used to control broadleaf weeds, and its effectiveness can be influenced by environmental conditions. In extremely hot weather, the high temperatures can increase the rate of evaporation and volatility of the herbicide, which leads to a loss of its active ingredients before they can be effectively absorbed by the target plants. This can reduce the overall efficacy of the product.

In hotter conditions, plants may also enter a stress state, which can affect their uptake of herbicides, making them less effective. Additionally, scorching temperatures can lead to quicker plant metabolism, meaning the herbicide may break down faster than it can exert its desired effect.

In contrast, under cooler weather, during spring, or during dry conditions, 2,4-D may still be effective, although each scenario has its considerations. For instance, while dry conditions can initially hinder herbicide uptake, it doesn’t necessarily render it ineffective, especially when rain or irrigation follows application. Therefore, it's the extremely hot weather that poses the most significant challenge to the effectiveness of Ester 2,4-D, making it the correct response.