Ash would imply combustion, which would remove most of the weight of a living object. C turns into CO2, and plenty of other combustible and/or volatile substances also are removed in combustion.
I’m pretty sure biomass is just the dry weight as the OP asks for. Looking at definitions online, I find many that specify dry weight.
I don’t know if an oven is necessarily the best technique, but I’d recommend a temperature of 150-200 F. (This is about what’s used for making beef jerky). The length of time depends on how thin your specimen is. For 1/4" beef jerky on a rack, 8 hours is sufficient, but I suspect a whole roast would take several days.
Of course, you could use a dehydrator that relies more on moving air than on heat. My dehydrator (which we use for fruits) puts out only a little extra heat; I doubt anything in there gets over 90 F. Again, time depends a lot on thickness. 8-12 hours is good for most slices.
Neither of these methods can get you to exactly 0% moisture content, but they’ll get close.