Preheat the oven to 325 degrees F. Line a sheet pan with parchment paper.
In the bowl of an electric mixer fitted with the paddle attachment, combine the two flours, the powdered milk, oatmeal, peanut butter and wheat germ. With the mixer on low speed, add the extra-large egg and 1 cup of water and mix just until it forms a slightly sticky ball.
Dump the dough out on a well-floured board (I use all-purpose flour) and knead it into a ball. Roll the dough out 1/2-inch thick. Dip a cookie cutter in flour and cut out dog bone shapes. Collect the scraps, knead lightly, roll out again and cut more dog biscuits.
Place the biscuits on the prepared sheet pan and brush with the egg wash. Sprinkle with oatmeal and bake for 1 hour, until completely hard. Cool and toss.