More European dairy farmers are choosing to put their cows outdoors to graze.
Coachloads of farmers arrived at Westminster for a march organised by campaign group Farmers for Action, which wants the Prime Minister to acknowledge that there is "a major problem" in the agricultural sector.
"Falling prices across the industry are making production unsustainable. People cannot take this any longer," said David Handley, a protest organiser and dairy farmer in Monmouth, South Wales.
Low wholesale prices for goods including milk and cereal have caused income to plummet for many UK farmers.
According to Government figures, the average income of dairy farmers will have dropped by 45% in this tax year to £46,500.