Not that they would have been getting sloshed. The beer supplied for harvesters was usually a low--gravity Mild, weaker than the stuff you'd have found in pubs. Harvest Beer was probably around 4% ABV before WW I, compared to 5% for X Ale.
"DEMAND FOR HARVEST BEER.
The Braintree Rural Food Control Committee met Thursday, Mr.J.Parish presiding.— Mr. Ollett reported that serious trouble was arising with harvest workers over the absence of harvest beer. Farmers from a dozen parishes had applied for beer for their men, and several hundred workmen had stated they could not do harvest work without beer.—Mr. Joseph Ollett, president of the Booking Workers' Union, said members of that Union employed on farms were threatening serious trouble unless they had harvest beer. Men could not be expected do such strenuous work as harvest without beer. — The Chairman said that although he was a teetotaler he supported the demand for beer for harvest work. — The Clerk (Mr. H. B. Fellows) said beer now regarded, as food. — Mr- W. Butler, J.P., Hatfield Peverel, and Mr. W. Roper, White Notley, each supported the demand for beer for harvest workers.—It was decided to telegraph the Ministry of Food stating that grave unrest wad arising among harvest workers through the lack of beer, and asking that authority furnished once for the supply mild ale for harvest workers in order ensure the carrying out of harvest work on farms."
Essex Newsman - Saturday 03 August 1918, page 3.
It shows the power of convention. The workers didn't really need beer to harvest, they were just used to getting beer while they harvested. Eventually the practice did die out, though I'm not sure when. Probably when machines replaced most of the people.