Hay Reseeding Help

I’m looking for some help . We are fairly new to the hay business and are at the point of replanting. Located in southwest Ontario Canada on a light sand soil . The alfalfa in the field is 6 yrs old and 50 to 70% died off and about the same for Timothy .
Can I get away with taking the first cut off in june Then spray down with roundup. Replant with 50/50 orchard grass / alfalfa . Will the toxic effects of the old alfalfa cause problems? I could seed the orchard grass in August and then drill in alfalfa the following spring? What are your thoughts? TIA

Hay Reseeding Help