"Slurry" is "A mixture of manure or farmyard waste and water; manure in fluid form," according to the OED, which provides these historical examples:
1965 Punch 22 Dec. 932/2 In a modern fattening house..the manure from several hundred swine falls through slatted floors into tanks beneath the building where a daily dose of water soon turns it into a forbidding quantity of evil-smelling slurry.
1970 R. Jeffries Dead Man's Bluff i. 5 He went through from the dairy into the herringbone parlour and stared..at the two days' accumulation of slurry.
1971 Farmers Weekly 19 Mar. 48/4 It takes one man about six minutes a day to clear away the slurry and a bit longer to put out the hay.