The buffalo grass must be established before it can withstand any herbicide--even a weed killer that targets only broadleaf weeds. With regular turf grasses, the rule of thumb to determine whether it is established is this: once it has grown enough to be mown twice, it's established enough to survive a herbicide application. However, it's not likely you plan to mow the buffalo grass like you would standard turf grasses. In that case, itd be a good idea to give your newly seeded buffalo grass four or five weeks before applying a broadleaf weed killer. Even then, test the herbicide in one area and wait at least four days to see how the buffalo grass fares. If the weeds get knocked down and the buffalo grass is unfazed, you can safely treat the whole lawn. Avoid fertilizing at this point and stick with a product that is for weed control only. Then when temperatures cool down a little this fall you can apply fertilizer if you wish.