Until they are about 2 inches long, grass carp feed almost exclusively on microscopic animals called zooplankton. They become dedicated herbivores, however, after they reach a length of about 4 inches. The amount of vegetation they will consume depends upon several environmental conditions, such as water temperature, water chemistry, and the kinds of plants available. Consumption rates also vary with fish size. For example, until they reach weights of about 6 pounds, grass carp may eat 100 percent of their body weight in vegetation per day. (This is equivalent to a 150-pound human eating 150 pounds of food per day.) As they grow larger, consumption decreases; up to about 13 pounds, they will eat 75 percent of their body weight per day, and above 13 pounds, they slow down to about 25 percent of body weight per day.

Grass carp prefer soft, low fiber aquatic weeds such as duckweed and various underwater plants. If the more desired species of plants are not available, they will feed on plants above the water surface; and in cases where no aquatic food is available, they have been observed feeding on overhanging brush and tree branches. It is this voracious appetite for plants that makes grass carp useful in controlling aquatic weeds.