Investigators suspect that the four men, all Indian citizens, were overcome by carbon dioxide fumes from the cow manure. The officials believe that three of them died after jumping in to rescue a worker who had been emptying the tank, the BBC reported.
Two brothers Prem and Tarsem Singh aged 48 and 45 respectively owned the farm, while Arminder Singh, 29, and Majinder Singh, 28, were employees.
Prem and Tarsem Singh registered their farm in 2017. It is about 45 km from Milan. According to Italian media reports, Singhs' farm at Arena Po, producing milk and veal cattle, is one of the biggest in the Pavia region.
During the incident, the manure was being collected for use as fertilizer on the farm's fields. The victims' wives raised the alarm when the men failed to turn up for lunch on Thursday. They rushed to the scene and spotted one body in the sewage.
The wives then called in firefighters, who donned masks and emptied the tank to recover all the bodies.
Italy's public broadcaster Rai said that the Arena Po tragedy brings to 486 the total of deaths in work accidents in the country this year the highest toll since 2016.
Agriculture Minister Teresa Bellanova tweeted condolences and said "safety at work is an inalienable right and we must make every effort to ensure it is respected".
( With inputs from IANS )