Most farmer spray their grains post emergent, because for pre-emergent you can just use roundup for a burndown if planting no-till. Then wait until the grain is several inches tall and hit it with 2,4-D for broadleaf control. These two products are easy to use and cheap to buy.
If doing conventional tillage I'd depend on the tillage for pre-emergent weed control rather than bothering with roundup, and then wait until the grain is several inches tall and hit it with 2,4-D for broadleaf control. A lot of grains suppress grass on their own through allelopathic action.
If you do want a pre-emergent for broadleaf control in grain you could use Sharpen (Saflufenacil) herbicide, which is one of BSAF's new Kixor herbicides. I haven't used it yet, although I have my eye on it, btu it's $769.95 per gallon, one gallon does 40 to 60 acres for a cost of about $16 an acre.
Below is their website spiel about this product;
Sharpen herbicide, powered by Kixor herbicide technology, is designed to provide fast, flexible burndown of broadleaf weeds applied preplant and/or pre-emergence in a wide range of crops. Research has also shown that Sharpen herbicide provides excellent residual control when applied at appropriate rates. Today, award-winning Sharpen® herbicide, powered by Kixor® technology, leads the way in no-till fields featuring a unique chemistry that provides foliar and soil activity on today’s toughest weeds. Kixor® is the latest in a long line of modern herbicides developed by BASF. Used on its own or in a mixture with glyphosate, Kixor® is applied prior to crop planting for fast and complete burndown of over 80 different weeds, including those resistant to glyphosate. This provides improved planting efficiency and greater timing flexibility. Kixor® can also be used as a pre-emergence herbicide in corn and sorghum to control many major weeds without triazine.