This is a granular product? If you wait about 3 mowings after application those grass clippings should be okay to compost. However, far better for the soil your grass is growing in is to mulch mow those clippings right back into the soil, all the time. Those grass clippings will supply about 1/2 the annual nutrient needs of your lawn which means you will need to spend less money on "fertilizers" and you will start to build a healthier soil.

I usually do just mow them into the lawn, but wanted to try composting some for a change.

Hubby is a commercial chemical applicator (I know, it seems wrong but you gotta do what you gotta do). I've asked him this question before.

kimmsr is right, if you can mulch them for a while at least it is better for the lawn. It keeps the fertilizer on the lawn where you intended it in the first place. But if you absolutely want to compost them, you can do it safely.

From my experience, backed up by hubby's training, if you have a hot pile you are good to go. If you have a passive or cold pile then you should probably let the compost cure. What I mean is don't use it right away - maybe this fall or next year.

You can also mulch garden pathways with it or under trees/shrubs. If it is too soon after broadleaf killer and you mulch under something like tomatoes, it could harm them. But next to something like corn it is okay. (Tangent warning: I mulched with treated grass about 17 years ago. I put it under corn and tomatoes. The corn in the row that got mulched was easily 2x taller, healthier, and more productive than the un-mulched corn. However, the tomatoes had a hard time setting fruit.)

You can even compost plant material that has been killed with Round-Up. It gets into the plant through the leaves. Once it has dried and has absorbed, it won't transfer to another plant around it or into the soil. You can actually spray an area one day and plant new plants in it the next day.

Some people are anti-chemical no matter what. They won't even put shredded office paper in compost because of the ink/toner. You can do it but what it comes down to is a personal choice.

Round-Up actually decomposes into some relatively harmless by-products rather easily. However, we don't know what the active ingredient was in kentstar's clippings. Generally speaking I would agree with momstar's advice though.

Actually none of the glyphosate products are as innocuous as the manufacturers would have you believe. While the active ingrediant, glyphosate, apparently does break down in soil fairly quickly when it is mixed with the "inert" ingrediants something changes and you get a long lasting and not very nice poduct that can, and does, float far downstream, as found by the USEPA Ground Water people, which is easily recognizable as the original product.