The spray will work best when the plant is actively growing.
My suggestion - try to keep it sprayed over the summer to kill anything that is growing from the existing seed bed. Labor Day weekend or thereabouts, till the plot and plant your clover along with winter rye and oats. If your plot is 1/2 acre, you would need about 3-4lbs of white clover, 30-40lbs of oats, and 30-40lbs of rye. The amount of oats and rye seed might seem high, but it will work our great.
The oats will provide a quick, lush fall forage for the deer while the clover and rye get going. The rye will be available all winter long....and as soon as it warms up in the spring, they will both jump to life before anything else does. Be prepared to mow the rye in the spring though....as it will get quite tall and generate a lot of bio mass. The residual rye roots will be a great moisture-holder for the clover the following summer.
If you think the rye will be too much to handle in the spring, substitute barley. We have had good luck with the barley seeding out in the early summer (early to mid June), and the turkey hens make good use of the seed with their poults.
Spring plantings are a tough battle with weeds. Planting the fall gives the weeds a limited window to grow before the frost knocks them back....and the following spring, the rye/barley and clover are going so quickly that they suppress most of the weeds that are trying to start.