In Communist countries such as the Soviet Union, Hungary, North Korea, Cuba and China, farmers were not permitted to grow their own food, but were forced to help cultivate government crops. The harvest was supposed to be distributed to the collective members.
In reality, due to unreasonably high government quotas, farmers got far less for their labor than they did before collectivization. Many refused to work. In one extreme episode in Ukraine, approximately 5-10 million peasants died after Stalin forced the farmers into the collectives. The immediate effect was to lower grain output and almost halve livestock, thus producing major famines in 1932-33.1
In theory, economies of scale plus a hearty community spirit can result in even greater harvests than privately-owned farms. For example, in the first half of the 1980s, Hungary, with a largely collectivised farming, exported more agricultural products than France from an agricultural area little more than a quarter of the French.2 But usually, peasants' incentive to work hard was lost when they took home less food from collective farms.
Some socialist countries permitted farmers to cultivate the land around their house (about one acre) for private consumption, which often spelled the difference between starvation and survival.
In North Korea in the late 1990s, the collective farming system collapsed under the strain of droughts. Estimates of deaths to starvation ranged into the millions, although the government did not allow outside observers to survey the extent of the famine. Aggravating the severity of the famine, the government diverted international relief supplies to its armed forces.
References:
1Eric Hobsbawm: Age of Extremes, 1994
2FAO production, 1986, FAO Trade vol. 40, 1986