Hi Peter,
The Auto-Complete tool is designed to only work with features in the same feature class. As a workaround you can use the Trace tool. Digitize your new feature (i.e. Crop Field) from one area along the Farm Dam boundary to the other. After clicking on the Farm Dam for the second time (as if you were ready to auto-complete), click on the Trace tool. Click on the last location and begin tracing the boundary of the Farm Dam to the first location.