Of American-English origin, the twofold phrase:
– to get the mushroom treatment, or to feel like a mushroom,
followed by
– to be kept in the dark and fed bullshit, or and fed manure,
means to be kept in a state of ignorance and told nonsense.

The earliest occurrence that I have found is from Indecision Hurts Service Morale—‘Kept in the Dark, Fed Nothing But Manure’, published in The Ottawa Journal (Ottawa, Ontario, Canada) of Saturday 9^th January 1965:

A bitter joke going the rounds of National Defence Headquarters for the past several weeks has officers of all three services describing themselves, to one another and to friends, as “mushrooms.”
They are mushrooms, they explain, because — “we are kept constantly in the dark and fed nothing but manure.”
The origins of that not-very-funny wisecrack lie in the inevitable slowness with which staff cutbacks and changes in responsibility and duty are being decided in connection with Defence Minister Hellyer’s planned integration of the armed forces.

The second-earliest occurrence that I have found is from Agriculture Remains Important To Economy, by Marvin A. Clark, published in The Daily Register (Red Bank, New Jersey, USA) of Tuesday 26^th January 1965:

Monmouth County’s farmers do have troubles, however. In agriculture these are accepted as part of the normal way of life. Part of these might be alleviated with a wider understanding of what is involved. One case is the acquisition of farm land for preservation as open space. When this is done by public agencies under condemnation, farmers are, in their opinion, too frequently ignored. This means that they cannot make plans for seasonal operations, they are reluctant to make needed improvements to land and buildings when unsure of the permanent status in that location, and all these things can be costly as well as aggravating. As one farmer has stated, he has been given the “mushroom treatment — keep them in the dark and feed them horse manure”. This lack of frankness by those who are arranging for purchases of public land is understandably resented.

The phrase is part of an extended metaphor in the following description, published in The Evening Sun (Baltimore, Maryland) of Monday 15^th September 1969—many U.S. newspapers later reprinted various versions of this description, which probably helped popularise the phrase:

Mushroom Treatment

What happens when your company is taken over by a conglomerate?
One executive says: “I don’t know about the others, but we got the mushroom treatment.
“Right after the acquisition, we were kept in the dark. Then they covered us with manure. Then they cultivated us. After that, they let us stew for a while. And, finally, they canned us.”

In The Sun (Vancouver, British Columbia, Canada) of Friday 4^th December 1970, Allan Fotheringham mentioned the term mushroom election—NPA is the abbreviation of Non-Partisan Association, the name of a municipal political party in Vancouver:

One of the NPA aldermanic candidates, who’s been around for years, took another sip of wine and confided to me, “This campaign has been what I call mushroom election.” “A mushroom election?” “Yeah. Feed them lots of horse manure and keep them in the dark.”

An Associated-Press story by James Carrier was published in many U.S. newspapers in March 1974—for example in The Miami News (Miami, Florida) of Friday 8^th, under the title Writing on wall is therapy:

Norwich, Conn.—The writing on the wall of a mental ward is helping psychiatrists decipher the minds of patients.
In an attempt to use graffiti as therapy, the state mental hospital has hung 4-by-6-foot sections of newsprint and felt tip pens on the walls of three wards.
[…]
One angry patient wrote of his Gateway building: “Gateway is like Watergate*. Sometimes I feel like a mushroom. They keep me in the dark and feel me bull.”

(* This seems to be an allusion to the U.S. political scandal in which an attempt in 1972 to bug the national headquarters of the Democratic Party in the Watergate building in Washington, D.C., led to the resignation in August 1974 of Richard Milhous Nixon (1913-1994), 37^th President of the USA.)

The phrase occurred again later that year in another Associated Press story, published for example by the Naples Daily News (Naples, Florida) of Thursday 21^st November 1974:

Orlando, Fla. (AP)—One of the new owners of the Florida Blazers will deliver a partial payment of $2 million during halftime of tonight’s World Football League playoff game against Philadelphia, a team spokesman says.
[…]
Blazer players, however, remained skeptical. One of them posted a cartoon on a dressing room bulletin board Wednesday which said: “I feel like a mushroom. I’m kept in the dark and fed bull—.”

The phrase is also used in Australian English and British English— for example:

– Australian English: On Thursday 13^th November 1969, The Sydney Morning Herald (Sydney, New South Wales) explained that the phrase gave rise to the political term mushroom club—John Grey Gorton (1911-2002) was the Prime Minister of Australia from 1968 to 1971:

Liberal backbencher Mr G. D. Erwin […] spoke bitterly about being dropped from the Federal Ministry.
[…]
He said that, unlike a number of new Ministers, he was not a “mushroom.”
The “mushroom club” was a group of Gorton followers who had dinners when Parliament was sitting.
Mr Gorton attended the dinners as a guest of honour and was known as the “chief spore.”
[…]
“Their motto is ‘keep ’em in the dark and feed them bull . . .’”, Mr Erwin said.

– British English: Nancy Banks-Smith used the phrase in The Guardian (London and Manchester, England) of Friday 31^st July 1970:

All men are born equal, as the American Declaration of Independence says. Women have to work at it. If the house were not at least in joint names, I would not live in it. And if a man said as several did, leaning from choice or necessity against a bar counter, the secretary should be “like good furniture” or “exemplary obedient slaves” or “treated like mushrooms: keep them in the dark and throw dirt on them,” I would strike him about the head with an offensive weapon.