Does "three leaves and one heart" include the cotyledon?

"Three leaves and one heart" includes the cotyledon. It refers to the very top of the stem having three leaves that have already grown out, have the basic shape of a leaf and are relatively complete, as well as one cotyledon that is still developing.

Corn herbicide must absolutely wait until the corn has reached the three-leaf-one-heart stage before the herbicide can be applied.

The corn three-leaf-one-heart stage, that is, the corn plant already has three unfolded leaves and one new leaf that has not yet unfolded; this is the corn three-leaf stage, commonly known as three leaves and one heart. Farmers must learn to clearly identify what the three-leaf stage is. If you mistakenly take two leaves and one heart for the three-leaf stage, then you are applying the herbicide too early, and the corn is too small to withstand it.