Areas of fur that receive bright or direct sunlight do not show individual hairs. Nor do areas that are either dark because of color or dark because they are in shadow.
The cow’s neck is a deeper color than the white fur of her face. Her shoulder area is almost black in color. Some hair lines will show in the neck coloring area but the shoulder area will appear solid in texturing.
In the shadow areas and medium color area I have begun to add individual hairs by burning thin fine lines that flow in the direction of that hair clump. Each hair will be slightly darker at the shadow area and become pale at the end or outer edge of the clump. A smooth pulling stroke with your wood burning pen will create this changing color in your lines.
You can use a very fine v-gouge carving tool to cut fine white lines into the darkest areas of a burning. I have used my v-gouge in the ear area where the long hair along the ear’s upper edge flows over the very black shadow inside the ear. Cut these hairs in several light strokes until you are back to the original color of the wood.