Which of the following is an example of mechanical control of weeds?

Mowing in alfalfa fields exemplifies mechanical control of weeds because it directly involves the physical removal or disruption of weeds through cutting. This method can significantly reduce weed biomass, limit their ability to photosynthesize, and prevent them from setting seed, thereby effectively managing weed populations in a given area.

Mechanical controls encompass techniques that rely on physical actions to combat pests and weeds rather than chemical interventions. Mowing is particularly beneficial in certain agricultural practices, as it can allow for the control of early-stage weeds without adversely affecting the crop itself, particularly a perennial crop like alfalfa.

In contrast, applying contact herbicides involves the use of chemicals to kill weeds, which does not fall under the category of mechanical control. Using cover crops is a cultural practice that suppresses weed growth by outcompeting them for resources. Adjusting irrigation practices can influence weed growth indirectly but does not involve a direct physical method of control.