Beer production was also affected by the world wars, with taxes being hiked and breweries being forced to make much weaker beers because of rationing.

Beer historian Ron Pattinson has conducted extensive research on the topic, and he runs an excellent, though quite idiosyncratic, blog. You can look at the work he has done regarding WW I and WW II, but be prepared, it gets really technical with brewing.

After procrastinating writing this for so long, I'm not going to let the perfect be the enemy of the good. I'm going to write what I know, and it won't have all the lovely citations I want it to have or the details, but here is an answer gesturing toward really fascinating history.

The relationship between the World Wars and alcohol in the US is, perhaps surprisingly, complex. In a very real sense, many historians believe that World War I led to prohibition -- the period from 1920 to 1933 when the Eighteenth Amendment prohibited the manufacture or sale of alcohol within the US. Because prohibition went into effect in 1920, many people forget that it was proposed in December 18, 1917. In April 1917, the US formally entered World War I, fighting with Britain and France against Germany and her nefarious allies. Remember that there were Americans on the Lusitania when Germans sunk it in 1915, and that the Zimmerman telegraph in early 1917 made Americans fear that Germans were provoking Mexico into attacking the U.S. Since the Eighteenth Amendment would take too long to ratify, Congress passed a Wartime Prohibition Act on November 18 of 1918, to tide Americans over in the meantime: it prohibited the sale of alcohol beverages, claiming to do so in order to save grain for the war effort. Yet the war was already over as of November 11 of that year, a week before it was passed! Before the "Wartime" Act expired, America ratified the Eighteenth Amendment in later 1919, and Prohibition commenced in 1920.

So what do all these things have to do with each other: grain, beer, World War I, and the U.S. hate of Germany? The answer is that Prohibition, while having long roots into the 19th century allying itself with women's movements, temperance, and moral uplift, was also, in its moment, deeply involved with the social and national politics of its era. In short, Prohibition was as much a display of anti-German and anti-Western European immigrant sentiment, racism, and xenophobia as much as anything else. German-Americans had been one of the most vocal communities speaking out against Prohibition prior to and during the war -- their Biergartens were culturally important social spaces for maintaining community and their German-American identities. In the US in the late nineteenth century and early twentieth, beer drinking at large was connected to immigrant communities and saloon cultures. The marketing team behind Budweiser, briefly renamed "America," might eb surprised to know that at one point, beer was not all-American, but even a symbol of LACK of assimilation into American culture, and clinging to immigrant cultures. While the Women's Christian Temperance Union is more famous for crusading for Prohibition today, equally famed at the time was the Anti-Saloon League. Since saloons were coded language for immigrant drinking establishments, the xenophobia and dislike of immigrants was only just barely simmering beneath the surface in the context of the times. This link between immigrants and brewing also lead to immigrant communities being very active in bootlegging during the prohibition; they had long traditions of making beer at home in America, as in Irish shebeens.

So, without even getting into the economic data proving there was no real shortage of grain driving prohibition (because I don't have time to look it up, not because it doesn't exist), we can see that World War I (and the especially German xenophobia it provoked) led to restrictions on alcohol, not wartime necessity. But how is World War II similar or different? It was a different story, if only because World War II did not lead to another era of Prohibition.

However, the two Wars were perhaps more similar than you might think. Having suffered a brutal defeat with the overturning of their dreams during the Great Depression (since Prohibition was repealed in 1933), Temperance and other anti-alcohol advocates were bruised, but did not consider themselves beaten. The colossal moral failings that led to a Second World War just 30 years after the first one, and after outlawing war in the League of Nations, spurred them to try and seize the opportunity presented by this World War, just as they had seized upon the First World War to push through Prohibition the first time. America needed moral uplift, needed to maintain superiority of body and spirit to most thoroughly fight the evils abroad, they argued. Alcohol was still residually linked to immigrant populations, and it was easy to play off rising tides of xenophobia as immigrants flooded into the United States fleeing the young and expanding Nazi regime. These arguments convinced many Americans -- when war broke out in Europe in 1939, national polls showed that 40% of Americans FAVORED wartime prohibition.

Ultimately beer in particular was declared a necessary material of war -- by the end of the war, 15% of the US's total beer production was being sent directly to the military, and manufacturers of beer were able to gain access to rationed materials like rubber, gasoline, and tin cans, just like armament and munitions plants were able to gain exceptions based on their legal status as a crucial war industry. Scholar L. Jacobson has asked how and why the beer industry in particular was able to shift their public image so radically that instead of World War I producing prohibition, World War II wound up inaugurating beer as an all-American drink of leisure. Sales of beer went from 56.8 mill barrels in 1941 to 80 million barrels in 1945, increasing over the course of the war! Jacobson argues the answer can be found in no small part in the success of lobbying and advertising campaigns from within the beer industry:

"Such wartime images of beer, presented here as a commodified symbol of the 'good life' Americans were fighting to defend, advanced brewers' longtime post-Repeal ambition: to distance beer from its earlier associations with the raucous working-class masculinity of the saloon, fend off renewed prohibitionist attacks, and reposition beer drinking as a wholesome American domestic pastime. [... the beer industry] embraced the softer appeals of masculine domesticity by shifting the class and milieu of drinking to the middle-class suburban home, yet also preserved beer's associations with the virility of soldiers and industrial workers. Finally, it made the pursuit of private pleasure compatible with wartime sacrifice and victory over fascism and want."

TO SEE SOME IMAGES OF ALL-AMERICAN BEER ADS FROM IMMEDIATELY AFTER THE WAR CLICK HERE

However, that is not to say that war did not effect the brewing industry. Average beer ABV shifts from 4-5% to a calculated 3.2%, glass bottles increasingly replace tin cans in a time of tin rationing, and wooden kegs replaced steel or aluminum ones for the same reason. To get at your excellent question, there was indeed a 7% cut in malt and malt syrup allotments for the beer industries, prompting them to introduce corn and rice into their otherwise wheat-y mix. You see the effects of this into the twenty-first century in the use of rice and corn in beers brewed today, including almost every major domestic light brew. Rice and corn can add interesting flavors and qualities to beer, as used within craft brewing (read this article from Serious Eats on identifying these and other flavors in your beer: http://drinks.seriouseats.com/2013/01/how-to-identify-adjunct-grains-in-your-beer-oats-wheat-rye-beer.html). However, the use of rice and corn in major domestics also relates back to World War II history in that it is fueled as much by government subsidies supporting artificially cheap (and often GMO) grains besides wheat. This connects to World War II in that we never actually experienced food shortages as most Americans think of when they imagine World War II. American farm production broke national records for three consecutive years during the war: America was producing more food that it had ever produced before in its entire history. However, the economics and politics of selling food abroad in bulk for better prices through the Lend-Lease Act and other arrangements with allies (plus with fewer middlemen distributors, packagers, and retailers taking a cut of the profits), all this incentivized Americans to ship as much of their surplus abroad as possible, in a strategic geo-political and economic move. In short, yes, beer suffered from wartime rations, but not because it was strictly necessary: because it was profitable and political. And they were still using more total wheat producing 80 million barrels a year by the end of the war than they were producing 56.8 million barrels at the time of US entry into the war.

The main source for my answer and only scholarly work to my knowledge that addresses your question head on is: Jacobson L. “Beer Goes to War: The Politics of Beer Promotion and Production in the Second World War.” Food, Culture and Society 12, no. 3 (2009): 275–312.

For more on immigrant drinking cultures, a still classic source is Roy Rosenzweig, Eight Hours for What We Will: Workers and Leisure in an Industrial City, 1870-1920 (Cambridge; New York: Cambridge University Press, 1983).

For more about the economics of food production in World War II and American farming subsidies see, respectively: Lizzie Collingham, The Taste of War: World War II and the Battle for Food (New York: Penguin Press, 2012). and J. L. Anderson, Industrializing the Corn Belt: Agriculture, Technology, and Environment, 1945-1972, 1 edition (DeKalb, Ill: Northern Illinois University Press, 2008).