Thanks for your question, Sharon. We would recommend waiting 7-14 days, as when applying Winter Grass Killer, you need to water it in, and you don't want to leach all the nutrients you've just applied to your lawn. Waiting a short period of time allows the grass to take up the nutrition from the fertiliser, and then you can apply the Winter Grass Killer per instructions. You can learn more about the product here: https://www.yates.com.au/product/lawn/lawn-weed-control/munns-professional-winter-grass-killer/