What is a GOOD product form that will kill grass and weeds in a gravel driveway for a long period of time?

Im looking for a GOOD product in concentrate form (OTHER THAT ROUND UP) that will kill grass and weeds in a gravel driveway for at least a month or so. We have several feet of gravel driveway that i have been spraying with round up for 2 years to keep the grass and weeds out of it, but it keeps coming back. I have to re-spray it every 2 weeks or so to keep all the grass and weeds out. I was wondering if there was a product in concentrate form out there that is better that round up and will keep grass and weeds away longer, if so what do you recommend, how much does it cost and where can i get it? I have tried several other stuff from walmart Home Deopt and Lowes too, and its no better that round up. We live on a small farm so im familiar with the ole diesel fuel in a pump sprayer trick, i was just wondering if there was anything from CoOp or Tractor Supply that might not smell so bad and more user friendly.