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We're back on the topic of harvest beer during WW I. With shortages of food, a good harvest was a high priority for the government. As far as farm labourers were concerned, there could be no harvest without beer. But there already wasn't enough beer to go around, even without providing extra supplies for harvesters.

"Both the Board of Agriculture and the Ministry of Food are up against a problem on the question of harvest beer. This is not such a simple matter as a good many people imagine. Harvesting, as those who have tried it know, creates a thirst not easily assuaged by water or barley decoction, and from time immemorial a free supply of harvest beer has been regarded as one of the privileges of the workers engaged. The restricted supplies of beer have led to the appalling prospect of a good many farm labourers having to be content with a less satisfying liquid, and a spirit of unrest has set in. Reports from a good many agricultural districts indicate that the men are taking up the position of "no beer, no harvest" and the food authorities are becoming anxious. Replies given by Mr Clynes to the questions put in the House on the subject fully recognised the gravity of the situation. The way out seems to be the provision of larger supplies for the rural districts at the expense of the urban population, as it is not proposed to increase the barrellage, or of enlarging the available supplies by decreasing the brewing of the heavy beers in favour of mild ale. In any case, it is hardly conceivable that the authorities concerned will permit the harvest to be endangered for lack of harvest beer."
Dundee Courier - Saturday 03 August 1918, page 3.
There's a terrible irony that young men were sent to France and Belgium to be slaughtered without a murmour of protest, yet a harvest without free beer was unthinkable. I'd have been happy to harvest beer-free where no-one was lobbing shells, gas and grenades in my direction all day.

As for cutting down production of heavier beers, I don't see how that was possible. From what I've seen in wartime brewing records, these had been discontinued long before August 1918. This is what Whitbread were brewed in August 1918:

Whitbread beers in August 1918
Beer OG Barrels brewed
MA (strong) 1037.6 23,577
MA (straight) 1022.7
MA (weak) 1011.5
IPA 1032.4 6,966
PA 1036.9 2,910
London Stout 1042.8 3,331
Porter 1037.6 2,964
Total 39,748
Whitbread brewing records held at the London Metropolitan Archives document numbers LMA/4453/D/01/083, LMA/4453/D/01/084 and LMA/4453/D/09/112.

Anything in there you would describe as "heavy"? Unfortunately the output of each of the three types of MA aren't given separately. But I can tell you that not much of it was the strongest version. None of the beers are particularly strong and far more was produced of the weaker ones.