Zombies love brains, so any zombie with a herd of humans will want to pay attention to folate since without enough folate, the humans are born without brains. The relationship between folate intake is strong and linear, and it appears reduce the incidence of neural tube defects all the way up to 700mcg, with most articles recommending at least 400mcg. This article is written from the vegetarian standpoint, since any Zombie farmer is likely to reserve livers for themselves.
So an obvious question is– Is there enough folate in the naive diet? After all, there is some folate in lots of common foods, and most bread and grain products have extra folic acid added to them. Probably not– the average from dietary sources alone is about 470mcg, and 20% of women don’t hit the minimal level from diet alone, probably fewer hit the higher number.
Like my previous article on vegetarian protein, nutrition labels can be full of mathematical traps if you are trying to get to a certain dietary intake of something, in this case 400mcg to 700mcg of folate. The trap is that nutrional facts are listed based either on 100grams, or some other unit of weight disconnected from typical portions. For example, Romaine lettuce has a lot of folate… but no one is going to eat a bucket of Romaine.
Brewers yeast, is packed with B vitamins, including folate. But typically it is consumed in small quantities, e.g. a tablespoonful here and there. Depending on who you read (or maybe brand) it’s a great or lousy strategy for getting folate into the diet, requiring 2 to 33 tbsps to get a days worth of folate.
I think the conclusion is that one would want to follow a mixed strategy–no single source can provide a days worth of folate in a typical portion. Or take a pill, but there’s no challenge in that.