”U.S. Forest Service” wrote:Nutritional value of fireweed varies depending on season and site. Fireweed crude protein averaged 20 percent throughout the second summer following fire; dry matter digestibility was over 80 percent. In another study, crude protein content was 13.7 percent, and protein digestibility (dry matter) was 13 percent. Fireweed collected in July in Alaska had 11.9 percent protein, 62.2 percent dry matter digestibility for moose, and 64.7 percent dry matter digestibility for dairy cow. Fireweed samples taken in July and August in Minnesota had crude protein of 4 to 9 percent and dry matter digestibility of 28 to 69 percent. In Oregon, June fireweed foliage had 17.7 percent protein.
Fireweed flowers contain tannins that have a very high capacity to precipitate proteins, reducing the actual amount of protein available to an herbivore.