I'm not an expert but I'll tell you what my experience was w/ my domestic sows. I built a pen measuring 24 x 54 and they were about 150 lbs. when they went in there. It took them two months to clear it and I was giving each 2.5 pounds of hog feed everyday. The only cluster of myrtles, is still standing. They rooted around it and thinned it out considerably due to the root damage but they didn't kill it in the five months they were in there. My wild hogs are twice as fast to clear nutgrass but they are just as slow to kill trees. Long after the grass is gone the trees are still standing. A neighbor down the road put in about 6-8 wild pigs in a treed pasture that looks about an acre (could be less). Within four months the grass was gone. I don't see the pigs in there anymore but the trees look great. The pigs thinned out some palmetto scrubs but didn't kill any that I can tell. What may happen is that you will have to supplement them with feed after the grass is gone. They will kill trees but it has been my experience that the trees die due to root damage not from being eaten. Best wishes with your pasture. I use my wild pigs to clear my gardens also and never have to buy fertilizer 
