From 168:
"When there is a temporary obstacle within the runway strip but outside the area to be cleared and graded, the continued use of the runway may be permitted subject to the pilots being notified of the obstacle. Every effort should be made to remove the obstacle as soon as possible."
Think that last sentence is key, if the obstruction is mobile, I keep it out of the strip.
We generally grass cut from 0600-0800, mainly training flights at our place so they don't normally get affected.
52N