Will you not buy an extra shelf bracket/brace for each new end of the shelf you cut?

Attach that to the drywall using a heavy-duty drywall anchor.

Or, a toggle bolt:
http://www.todayshomeowner.com/testing-wall-anchors-and-picture-hangers/

I would think that would be enough to hold it.

Depending on what you use--some systems make a vertical support that touches floor and ceiling, and supports the "floating" end of the shelf.

http://www.stlouishandymanservices.com/wp-content/uploads/2010/04/wire1.jpg

This pic shows both that vertical support and the brackets at the free end of a shelf.

Thanks for the replies. I'm not worried about the weight of the shelf but the lateral forces resulting from heavy clothes being slid along the rod. Braces don't help for that.

The shelf is not restrained at the open end and the whole assembly could "roll over" if force is applied sideways. Again, brackets, braces and vertical rods are for supporting weight and will not prevent the shelf from rolling over towards the open end.

I think the C Clamps are the solution. I just thought someone would have another idea.

I do not understand how a shelf is going to roll over laterally. Is it that it's not attached to the braces/brackets? In most wire shelving setups, they are.

And I don't get out it's "not restrained" at the open end. Is there not a brace/bracket there? What kind of "restraint" does the top one have that the bottom one doesn't?
Is it just that there's no wall, and the clothes will fall off? (In which case, yes, attaching a C clamp to the pole will serve as a "stopper" at the end of a rod)

If the shelves are attached to their brackets (and every bracket I've ever seen has a way to do that, even if it's just a hole through the bracket that will take a long screw), there is no lateral force from the clothes being slid along the rod that will dislodge it.

And I don't know how I would attach a C clamp to a stud; studs are behind walls, and clamps have to reach halfway around something.

Maybe you're using terminology I don't understand. Pics might help.

I don't know what I can add to my original explanation. A brace supports a vertical load. If a force is applied perpendicularly to the brace, it can roll over and pull out of the wall. It's provides some resistance to sideways motion but is relatively weak in that direction.

I've installed the shelves, and as I expected, after the initial installation it was easy to rock the shelves back and forth sideways just with light pressure. I wouldn't want to repeat it with a large load of clothes and a heavy push,

A C Clamp goes over the back rod of the wire shelf and is screwed into the stud with sheet rock screws. I've already installed several. They did what I hoped: the shelves cannot move sideways now.

This has nothing to do with clothes falling off the rod.

Anyway, I'm pretty much done now and I'm satisfied that it's very strong.

I'd love to see a photo of your C clamp.

This is what I think a C clamp is:

Sorry, I've been away. That's a different kind of C clamp.

Do a Google search for "Rubbermaid C Clamp"

Wayne