A Zimbabwean minister has said that many of those given land since 2000 know little about farming and this has led to food shortages.
Those given land have not received the help they were promised
The authorities have previously blamed hunger on poor rains, while critics have pointed to the seizure of most of the country's white-owned land.
Up to three million people will need food aid this year, the UN says.
At the same time, the UN has criticised Zimbabwe for refusing aid for people made homeless by housing demolitions.
Deputy Agriculture Minister Sylvester Nguni was quoted in the state-owned Herald newspaper as saying that while a few of those given land were committed to agricultural production, many others were doing "nothing" on the farms.
Although he mentioned the poor rains, he also told a meeting of the Zimbabwe Farmers' Union: "The biggest letdown has been that people without the slightest idea of farming got land and the result has been declining agricultural output."
In a secretly filmed report for the BBC, villagers said they had only been eating one meal of porridge a day since May.
A woman said her two children had died after eating poisonous roots because they were so hungry.
Much of the best agricultural land was previously owned by whites, but over the last five years 4,000 white farmers - out of 4,500 - have had their land seized and redistributed to black Zimbabweans.
Critics say that many of the beneficiaries have been government cronies.
On Monday, United Nations Secretary General Kofi Annan criticised the Zimbabwean government for rejecting humanitarian aid to those in need.
Earlier this year, the UN said about 700,000 people had been left without homes or work by an eviction campaign that began in May.
A statement by Mr Annan rejected claims by the Zimbabwean government that it required no international assistance as it had already provided shelter for those in need.
Some 700,000 were left without homes or jobs, the UN said
"A large number of vulnerable groups, including the recent evictees as well as other vulnerable populations, remain in need of immediate humanitarian assistance, including shelter," Mr Annan said.
"Furthermore there is no clear evidence that subsequent Government efforts have significantly benefited these groups," he added.
Annual inflation is running at 360% and about 75% of the population live below the poverty line.
Critics blame the disruption caused by the land seizures to the agriculture-based economy.
President Robert Mugabe has always accused western countries led by former colonial power Britain of sabotaging the economy because of opposition to land reform.