Don, I think this is your problem: You are looking for "direct burial" line but your problem is really finding a line with the ruggedness to withstand animal contact. "Direct burial" only means the jacket is made to resist breakdown from contact with soil and there is some attempt at waterproofing to keep the dielectric from wicking up water. None of this guarantees the feed can stand up to a mole chewing on it. In fact, some of the direct burial line has jackets that are made of soft chewy plastic.
This place has some Andrew RG11 direct burial https://www.mjsales.net/items.asp?FamilyID=861&this_Cat1ID=266&Cat2ID=34&Cat3ID=27
but again, it is probably not what you really need.
What it boils down to is if you want professional grade reliability, you are going to have to bite the bullet and spend the money on real professional/commercial grade feedline. If it were me, I'd start getting prices on 1000 feet of 3/8 inch Andrew heliax.
There are three things it has going for it: 1. Heliax has the toughest jacket I have ever seen. It is almost like a PVC pipe for a jacket. 2. If the jacket does get breached, the shield is solid corrugated copper. The dielectric is still protected by the solid shield -- in fact the line would work fine electrically with no jacket at all. 3. If any of this fails, and you do have to make a repair, the splice will be made with Andrew connectors. These are waterproof and the only soldering is with the center pin on each male. A couple of N males and a N bullet between them and you are back in business.
I really can't think of any other coax cable solution for animals, other than buying less robust cable and putting it in PVC pipe but that would probably be a lot more work and expense than just getting heliax.
Another option is to bury so deep (a few feet) that you get below the below grade region where most animals burrow. But now we're talking about trenching 600 feet and if you have anything go wrong down there you have another problem (usually when radio stations run deep feedline, it is in a big pipe with a few pull lines so they don't have to excavate and have plenty of room to pull another line).