The ornamental design for a frontgrillportion of an agricultural tractor, as shown and described.
FIG. 1 is a right front perspective view of a front grill portion of an agricultural tractor showing our new design;
FIG. 2 is a left front perspective view thereof;
FIG. 3 is a front view thereof; and,
FIG. 4 is a perspective view similar to that of FIG. 1, but further includes the front grill illustrated in its intended environment by the area having a stippled textured and outlined in dashed lines. This area is not part of the claimed subject matter.