At this size, you don't have much in the way of herbicide options and mechanically cultivating, pulling the weeds, or covering with newspaper and mulch are your best options. 2,4-D is used on very small corn, but tends to cause the stalks to bend and makes them brittle once the corn is over a few inches high. Last thing you want is to have the stalks break off just when the ears are almost ready to eat! You can apply it if you keep your spray nozzle very low and try to minimize how much gets onto the base of the stalks, but you may still end up with some brittle ones that will break in the wind. Tomatoes are very sensitive to the vapors of 2,4-D as are some other broadleaf garden vegetables, so don't use if they are near. Other broadleaf herbicides available to homeowners will also likely damage the corn at this stage. Grass killers will also kill your corn, they are out. The herbicide version of Preen contains trifluralin, which kills weeds when they germinate. If you cleaned the weeds out, you could put it down to prevent future flushes of weeds. Just mix it into the top inch or so of soil or water it in with about 1/2 inch of water. Trifluralin is sensitive to sunlight and degrades quickly if not worked into the soil. Corn is sensitive to trifluralin and it will stunt the roots if the corn is planted in treated soil. Once it is emerged and growing, it shouldn't affect the plants but I've never tried that and you might find out otherwise. In a raised bed, I'd just pull the weeds when the soil is moist, then put down a good layer of mulch to prevent future ones. Good luck!

Wow. Thanks for the info. I didn't know this, but am glad I do now. I live near a pond and I would hate to think I've been contributing somehow to its demise. Thanks again.

I do 15lbs/1000sqft of corn gluten meal and 15lbs/1000sqft dry molasses. Corn gluten meal is much cheaper than and the same product as Preen. I do this every spring and one more time before fall.

Straw, compost, hay, grass clippings, shredded newspaper, cardboard, wood chips, weed preventing fabric, etc. Lots of discussions on mulches over on the Soil, Compost & Mulch Forum. I use hay, straw when I can get it, compost, and shredded leaves and have no problems with slugs. They don't like the sharp edges I guess. ;) See the article below for several more options. Here is a link that might be useful: Vegetable Garden Mulches