During both World Wars central government exercised considerable control over the brewing industry,. Dictating what materials could be used, beer gravities and even, through fuel economy measures, brewing techniques.
Soon after the beginning of the war, brewers had been obliged to start using flaked barley. When supplies of this ran short in 1943, oats were a compulsory substitute.
In September 1941, the use of flaked barley was introduced by the Ministry of Food in order to conserve malt supplies, and a large proportion of the brewing trade has achieved a substantial reduction in the use of barley, whether flaked or malted, while maintaining the present level of output of beer. Supplies of flaked oats will be available from the present suppliers of flaked barley, and in most districts these supplies are already sufficient. A proportion of the barley crop is reserved for the loaf, and it has become necessary in the national interest to ask every brewer to take steps without delay to obtain delivery of flaked oats and to carry out experiments in order to ascertain the maximum proportion of this material that he can use. It is hoped that brewers will lose no time in putting the results of their experiments into effect. In some cases breweries are working to the full capacity of their mash tuns, and the greater bulk of oats, necessary to replace a given proportion of barley malt, presents a difficulty. It may be possible to overcome this difficulty by using ground oats in place of flakes, and there is no reason why a brewer who can more conveniently use oats in this form and has the facilities for grinding should not take his supplies in grains instead of flakes. It should be said, however, that the malting of oats will not be permitted, as this would be uneconomic in malting labour having regard to the lower output of beer obtainable as compared with that from a corresponding quantity of barley malt."
"The Brewing Trade Review 1943" page 63.
Because I know there are a few of you who love technical brewing details, I'll be boring all of you with further posts on how to brew with oats.