Roundup will kill the poison ivy on your farm. I never heard of cold weather having much impact.
The birds will replant it for you though.
Round up is fine for the back yard and we do use it but 8 acres is pretty tough. I spent the last two weekends detangling bind weed, brambles (aka wild rose) and poison ivy which had grown up with a few trash trees under the biggest tree on our property. I scrub down with poison ivy soap and creams and shower and wash my clothes but still get it. I think I need a gas mask and a flame thrower. This area had been sprayed with round up over the past three years.
Roundup will kill the poison ivy on your farm.
One chem. better than Round-up, ask for 2-4-D, kills broad leaf but not grass..... Tractor Supply.