The farmers were in a pickup truck carrying lettuce when a car bomb exploded in their path, killing them and wounding two passersby, according to a policeman at the scene who did not wish to be identified.
He said a US military convoy had passed through the area shortly before the attack, which happened in the Rashid region on the capital's southern approaches.
Farther south in Mahmudiya, a medic in a local hospital said his facility had received three bodies and a wounded man who later died.
The hospital official, who did not want to be named, said the men were attacked by armed men while travelling on a road west of Mahmudiya.
The roads crisscrossing the area are among the most treacherous in Iraq and corpses of civilians and members of the security forces are often dumped in the open.
In Baghdad, Aljazeera learned that an Iraqi army major was shot dead on Monday by armed fighters in Abu Ghraib neighbourhood in western Baghdad.
In the northern Iraqi city of Mosul, an Iraqi cameraman working for a Kurdish satellite channel was shot dead by armed men in the centre of the city, hospital sources said.
Husam Hilal Sarsam, a Christian who worked for the satellite station of the Kurdistan Democratic Party of Masud Barzani, was kidnapped two weeks ago, a member of his family told AFP on condition of anonymity.
An Iraqi army major was shot
dead in Baghdad
And in the capital, a bomb attack on the car of the director-general of the interim Health Ministry wounded four of his bodyguards, a medical source said.
The attack occurred in Baghdad's western al-Ghazaliya district as Saad al-Amili's car drove by, the source said.
Meanwhile, several Iraqis at a hospital in the Iraqi city of Najaf have staged a sit-in, calling for the interim Iraqi government to close its borders with Jordan in protest of the alleged involvement of a Jordanian in a bomb attack in the town.
Others demonstrated in front of the Jordanian embassy in Baghdad and called for the interim Iraqi government to close the embassy after the attacker's family described their son as "a martyr".