Q:

usage of 'than' here

Russia will have less grain than if Stalin had not insisted upon the
  adoption of Lysenko's theories.

I can understand the meaning of the sentence but am not familiar with 'than' here. Can you explain the usage of 'than' here? 

A:

In this case the sentence is like so:

X is comparably different than [X would be] if Y had happened.

This sentence has the omission of X would be after the word than.

This sentence is shorter than if I had not omitted part of it.

This sort of sentence compares a current situation to a hypothetical one. In your sentence:

Russia will have less grain than [it would have] if Stalin had not insisted upon the
  adoption of Lysenko's theories.

You can also reverse this comparison like so:

Russia would have had more grain if Stalin had not insisted upon the
  adoption of Lysenko's theories.

This sentence implies that Russia will have less grain compared to the hypothetical situation.

