There seems to be various qualities of Glyphosate. My farmer pal buys G in quantity from an agricultural merchants. This is industrial strength stuff, with a strong chemical smell, and sprayed plants start to yellow-up in only 7 to 10 days. Your Murphy's, etc. from the local garden centre, is like baby's milk compared to that stuff.... and a hell of a lot more expensive.
I supplied to a plant research centre which did trials on early samples of G. AFAIR their early results were showing that the best time to spray was in late spring, when the plant had grown lots of leaves, but still had plenty of growth hormones (Auxins?) circulating around.