> But nobody
> said what they do. What is their effect in a *real world* situation and
> how do we know if this effect is due to the screens? Have they made
> possiblen any other changes in management technique? Comments?
>

For the benefits of screened bottoms see
http://www.xs4all.nl/~jtemp/bodem.html
for a study of Helmut Horn on the subject.