My sweet corn came in good, but the ends of the ears didn't fill out well. I didn't use any fertilizer on it. Is that the reason? What can I do next year to get fuller cobs? Thanks.

Most likely bugs, like the cucumber beetle.

Could also be the weather, water/rain at the wrong time.
Some times if you get a burst of heat and sun the ear will start to grow again
but there is no silk to pollinate the ear.
When the ear is first starting to form, When it just looks like a few leaves that's when
corn needs water the most. That sets the ear. If you can give it a inch or two
at that time it will give it a good start.
I doubt that is a fertilizer problem if the rest of the stalk did OK.
I would suggest you have a soil analysis dun this fall or buy a kit and do it your self.

Good Luck

If you didn't fertilize the sweet corn, it's entirely possible that was a factor in your poor ear fill. Check one of the above topics about where to get a soil test if you haven't already done so. Sweet corn requires a fair amount of nitrogen, usually in the neigborhood of 100 - 140 lbs./acre of actual N per acre for commercial production depending on what was planted on the area the year before. Nitrogen deficiency will show up as a general pale green color and stunting of the plant. When severe, the leaves will exhibit a yellowing that starts at the tip of the leaf and continues down the midrib, beginning on the lower leaves first. Sweet corn also needs to have relatively decent phosphorus and potassium levels. Rather than blow you away with technical jargon however, here's an excellent site to get you started on understanding your garden soil fertility issues:
http://www.extension.umn.edu/distribution/horticulture/DG1731.html

Another possibility is that your sweet corn simply ran short on or received too much water. Too dry and the ears pollinate but will slough the kernels beginning at the tip of the ear. Too much water can also do the same thing as nitrogen is a mobile nutrient and can either be leached deeper into the soil profile or lost to the atmosphere by a process known as denitrification in waterlogged soils. Compared to regular field corn, sweet corn has a relatively wimpy root system and is more prone to problems recovering nitrogen as a result.

How thick you plant can also make a tremendous difference. Typically, when planting in rows about 38" apart, I like to shoot for a plant every 8 to 11 inches. This equates to a plant population of 16,000 - 20,000 per acre. This is thinner than commercial canning operations but like most gardeners, I'm after big, completely filled ears as opposed to tonnage. The thicker one plants, the smaller the ears are and the more poorly they tend to fill. I like 38" rows because they allow me to perform mechanical weed control and incorporate additional nitrogen fertilizer if needed with the garden tiller until the corn canopies without damaging the root systems.

Another possibility that may have some impact would be silk clipping by insects, primarily rootworm beetles. In MN we contend mostly with northern corn rootworm adults (small lime green beetles ~ 1/4" long) and western corn rootworm adults (normally yellow with black stripes and roughly 1/4" long). These overwinter here in MN whereas the southern corn rootworm or spotted cucumber beetle does not and migrates annually. Particularly in late planted sweet corn, these insects feed on the green silks, keeping them clipped back so they won't accept the pollen shed by the tassel. Have had issues with the rootworm beetles here showing up about the time the last silks are coming out. Those silks are responsible for the ear tips pollinating. Near daily scouting during silking time is key to keeping them from doing serious damage. Several insecticides can be used for their control if this becomes an issue.

Hope there's something here you can use fishermatt. The ears of Supersweet Jubilee I had tonite were awesome; nearly a foot long and very tasty! 2 ears and I was full.

I have a problem with root worm beetle cliping the silks the last 3 years. I think Dotch hit it on the head when he mentioned this. When the corn is about ready to silk or is first starting to silk look for those green 3/8 in. flying bugs. At wally world you can purchase an insicide call 7 which is cleared to use on most garden plants and apple trees it comes in a 18 or 20 oz. plastic bottle that you screw a water hose to. just turn on the water and spray away.

Right on, efgh! Commend you for following some basic IPM principles. I've used Sevin too. The residual is fairly short however and it washes off easily so a repeat application may be necessary if it rains and/or the rootworm beetle pressure is heavy. It's very effective though as are some of the higher powered commercial products I get my hands on.

Thanks for the great information! I have Peaches and Cream and Ambrosia that have been great, my Jubilee got neglected and didn't produce well. I'll try that one again next year.