I was listening to a podcast by Dan Carlin about the ancient germanic tribes that Caesar fought. Caesar writes about the Suebi:
They make not much use of corn for food, but chiefly of milk and of cattle
According to wikipedia, corn was domesticated in Mexico and didn't arrive in Europe until the 15th century, so what corn was Caesar writing about?
Corn was the generic term for the predominant local grain. When Claudius spoke of corn, he meant wheat. When the first European explorers saw maize, that's the term that popped into their heads.
Neque multum frumento, sed maximam partem lacte atque pecore vivunt
Is the original Latin. 'Frumentum' is simply harvested grain, related to the english word 'fruit' with a '-mentum' (as in 'monument', 'thing for remembering') suffix. Literally it refers to 'the fruiting', that is, ripe and harvested grain. It does not specify exactly which species of grain, but in the context it probably refers to all kinds of grain, as Caesar here emphasises the disability of the tribe in question of performing agriculture due to the constant threat and skirmishes with a rival tribe. The most common food-grain would have been wheat (just as today), but in the context of the lower Rhine delta (which is the area Caesar describes) barley was also grown.