Both poplar and willow have been used as drought forage for sheep, cattle and deer. There has been a little New Zealand research on these forages for stock focusing mainly on sheep and cattle, including nutritive value, dry matter yield, palatability and as well as management of the tree blocks.
International research shows that foraging deer add a considerable amount of tree and shrub species to their diet. Their intake appears to be determined by forage architecture (height, leafiness, branch length, and shade) and bite size and frequency (ease of prehension). This correlates to deer being an intermediate forager and as such highlights an opportunity open to explore further.
The feed value of poplar and willow is between 65-70% dry matter digestibility and the crude protein level around 15%. The leaves also contain condensed tannins (higher in willow) and phenolic glycosides. Willow leaves are also high in zinc and magnesium however sodium levels can be low therefore salt blocks should be provided.
Show me the science!
Charlton, J.F.L. (Ed.), 2003. Using Trees on Farms. New Zealand Grassland Association, Wellington