In Ukraine it turned out that the Romanian occupied area was comparitvely prosperous and well fed to the German occupied areas. The main reason? Not murdering everyone.
Oh, that was the main reason? The Romanians weren't wholesale murderers? Have you heard about what happened in Odessa in October 1941?
Now, start by looking up the food balance of Germany and of Romania pre-war. Did Germany have a food surplus, or a food deficit? Did Romania have a food deficit, or a food surplus?
If the German troops in the field did not "live off the land", i.e. by stealing food from the locals, where would their food come from, do you think?
Now look at the food balance of Southern Ukraine and, say, the region occupied by HGN beyond Lithuania. Which was called "the breadbasket of Europe"? Which had more than its fair share of forests and lakes and swamps?
They don't need to feed the people, just let them grow their own food.
Sure the Germans can let the farmers in the occupied areas of the Soviet Union grow food. That's exactly what they did. Only, they also need to requisition that food to feed the army. Oh, and they will also need the horses. And the carts, to bring the food to the army units.
How do you think the farmers will fare?
And they are the lucky ones. They're close to the source, firstly, and secondly the Germans need them. So they can, if barely, survive with what little food is left to them.
Now let's look at the urban population. There's the factory worker, who might be useful to the Germans - if they wanted a factory in the Soviet city and if the factory's machines had not been removed to the Urals. Then there are the university professor, the violin player, and the party official - all members of the intellighentsia, which the Germans don't want to exist. Then there are the insurance claims clerk and the specialized shopkeeper, the streetsweeper and the dentist's assistant - what use are they to the Germans? Aren't they, you know, "superfluous eaters"? (I'm not using this term at random, look it up).
What's more, producing the food is only part of the problem when it comes to occupied Soviet cities. Assume the Germans have enough food in the countryside; assume they're willing to feed the city dwellers. What will bring the food to the city? Carts and horses that could gather the produce to the nearest railway stations have been requisitioned. The locomotives, rolling stock, and rail lines themselves are working non-stop for the Heer. So?
Hell, the Germans had a labor shortage through a great deal of the war, for which they had the French round up a hundred thousand plus Frenchmen to work in their factories. Having Soviet POWs or locals getting away from fighting and simply getting to work in German factories, in farms, in mines, whatever, it could be managed. Not as if they would start delivering rations to Eastern Europeans.
What makes you think the Germans did not import Slavic slave labor (the allitteration is not casual, it tells you how central Europeans historically looked at these populations) to Germany? Who do you think was driving the plough in Fritz's farm while he was at the front in 1943, his wife Gretchen? No, Gretchen ran the farm, but wasn't strong enough for the plough. It was Ivan. And in the city, who do you think took care of Frida's house and children while she was working as a clerk in the procurement office of the nearest factory, thus replacing Hans who had died in Stalingrad? It was Masha. Down in the coal mine, who loaded the trolleys, given that Horst, who used to do that, was a POW in India? It was Piotr.
IOW, the above is a proposal for the Germans to do what they did do in OTL.
And, of course, these imported slaves were eating worse and less than the Germans. Ivan ate relatively well because he was close to the source and could steal a potato or two. Masha might have fared not so bad if Frida wasn't a harsh mistress. As to Piotr, well, he lasted six months, then fell ill because of exhaustion and malnourishment, was not treated, and died.
Read about Generalplan Ost before posting again.