Erm, I'm a bit confused - if the weeds are dead, there isn't anything to compost, is there? Do you mean the dead, brown material that might be left behind?

I wouldn't. The makers of Glyphosate claim it is non-persistent but..........

Where the material is left on the ground the action of sun and rain washes it through into the soil, as it is very water-soluble, where it is very diluted and can break down. In a composter there is only the microbial action, nothing is washed through, so the danger of residue is higher.

Bamboo - that's what I mean ;the dead brown stuff that's left behind

Okay - I'm with Buddleja on this one - in tests, Round up used on ground where carrots were subsequently grown was present in the carrots 6 months later, so its somewhat more persistent than the manufacturers claim. Glyphosate itself, alone, isn't so bad, but Round Up is different, it contains other ingredients too. I'd say put the weeds on your compost heap IF you're not going to be using the compost for a year, and its open to the elements, i.e., open to rain in particular.

Thanks to each of you for your prompt and helpful replies.

Bamboo, can you please point me in the direction of these tests as i use a lot of Roundup and would be interested in the findings. Thank you.

Oh gawd, now you're asking - I did the research at least 18 months ago. When I've got time, I'll try to find the source/s. I do recall the carrot thing, as already mentioned, and the figure 'in some instances, more than 220 days persistence', but it wasn't across the board - in some cases, there was no residue in plants growing in soil treated with Round Up. What I can't recall is why - maybe it was variation depending on what was being grown.