Stalin and his cronies (we cannot exempt the cronies from complicity in his crimes) destroyed such a viable agricultural region to the extent that after his Reign of Terror, the Soviet Union needed 10% of the world's total grain to import to feed a starving populace.
Actually, as late as the early 1970s, the Soviet Union was a grain exporter. Russia endured agricultural problems during and after the 1970s due to a large extent to frequent drought. While Russia endured three famines between 1921-1991, Russia had endured dozens of such famines in the 19th century. The Finnish famine of 1866-68 killed a staggering 20 percent of the population.
If it's Stalin's era you want to talk about, the fact that his policies helped to greatly modernize Russian agriculture. Above all, the introduction of the tractor to Russia helped to greatly increase agricultural productivity.
Here is something for you to read:
Around 20 million (citing The Black Book of Communism: Crimes, Terror, Repression by Stephane Courtois et al) to 35 million (citing A Century of Violence in Soviet Russia by Alexander Yakovlev) killed in all, from 1917 to 1991
250,000 executed by the Cheka during the "Red Terror" and Russian civil war. (citing The Cheka: Lenin's Political Police by George Leggett) But it could be much higher (see my sig)
Between 300,000 and 500,000 Cossacks killed or deported in 1919 and 1920 (known as "de-Cossackization"; not sure how many of these deaths overlap with the aforementioned Cheka executions - if at all). (citing The Black Book of Communism: Crimes, Terror, Repression by Stephane Courtois et al)
Between 7.2 to 10.8 million deaths during dekulakization and collectivization - which caused a famine the regime used as a weapon against supposed "class enemies" (citing Stalin and His Hangmen: the Tyrant and Those Who Killed For Him by Donald Rayfield)
Around 700,000 executed during the Great Terror of 1937-38 (citing Stalin: the Court of the Red Tsar by Simon Sebag Montefiore); this does not include those who were beaten/tortured to death during "interrogation" or deaths in the gulag during this time, which would put it over a million.
Over 1 million Polish citizens deported by November 1940; 30% of whom were dead by 1941 (citing Stalin: The Court of the Red Tsar by Simon Sebag Montefiore) and 21,857 executed outright (i.e. Katyn) by the NKVD during the Nazi-Soviet pact (citing Autopsy for an Empire by Dimitri Volkogonov)
A total of 34,250 Latvians and around 60,000 Estonians and 75,000 Lithuanians murdered or deported during Nazi-Soviet pact (citing Stalin: The Court of the Red Tsar by Simon Sebag Montefiore)
An estimated 4.5 million (citing Gulag: A History by Anne Applebaum) to 12 million (citing How to Prevent Genocide: A Guide for Policymakers, Scholars, and the Concerned Citizen by John G. Heidenrich) deaths in the Gulag from 1918 to 1956.
(I'm leaving out Stalin's ethnic cleansing of minorities in the USSR during WWII - Chechens, Crimean Taters, Kalmyks, Volga Germans, etc. - accused of "collaboration" with the Germans. I can't think of a source for that one off the top of my head. I'm sure hundreds of thousands perished though)
Haven't read as much on Mao Tse-tung, but the new biography of him by Jung Chang and Jon Halliday (Mao: The Unknown Story) estimates "well over 70 million" perished as a result of Mao's policies which, if true, makes him the biggest mass killer in history.
Broken down looks like this:
3 million deaths during land reform and the "campaign to suppress counter-revolutionaries"
38 million deaths during "Great Leap Forward"
3 million deaths during the Cultural Revolution
What do you think about that?