Tim Tyler writes
K wrote or quoted:

Incidentally it seems possible that pinching out
the young male catkins from Morus Nigra specimins
could result in more resources being available
for the fruit. I wonder if this hypothesis is
reasonable, and whether it has been tested.
Do you still get fruit if you pinch out the male catkins?

Maybe:

``In monoecious species with male and female flowers
on the same tree, pollination typically requires
pollen from a different tree because the maturation
sequence of male and female flowers on the same tree
is incompatible.

Some species and varieties produce fleshy, seedless fruits
without pollination. These fruits are called parthenocarpic
because they mature without pollination and seed formation.''

- http://waynesword.palomar.edu/fruitid6.htm

Why a tree would bother to produce fruit without
seeds seems puzzling - perhaps it doesn't happen
naturally very frequently.

What do the male catkins look like, anyway? I've had a good look on my
tree this morning and all I can see is what I presume are female
catkins, basically looking like embryonic fruit.
--
Kay