Less bittering hops for ag.

[Sozzled Sab]

I have just received a late present from my wife, it is an American book called Clone brews. Many members may be familiar with it.
The recipes are for extract brews with a short note at the bottom for partial mash and ag versions. The partial mash and ag notes say to use 20-30 percent less bittering hopes than the extract recipes.
Why is this?

 

[Jim Brewster]

Longer boil time would be my guess

 

[Bernie]

I have just received a late present from my wife, it is an American book called Clone brews. Many members may be familiar with it.
The recipes are for extract brews with a short note at the bottom for partial mash and ag versions. The partial mash and ag notes say to use 20-30 percent less bittering hopes than the extract recipes.
Why is this?

Tannins extracted during sparging my add to the bitterness.