BLV?
not borrowable on retail level.BUNT
best bet no doubt is ZN, but some people (cnbc viewers) don't know how to open a futures account, or they are afraid if they do they will accidently end up with a carload of corn on their front lawn.
so second best bet is options on a long etf which is usually not shortable.
I think ZN is the most heavily traded contract on the CME. What is that ETF that is long the ten year? TFT or something like that. It is also heavily traded but can't be shorted, but the options are pretty liquid. Depends on your time frame, do you really want to pay a premium to buy the puts? Takes a big bear to do that. Selling the calls? I suppose if you are confident and right on the strike and expiration.Why not ZB?
OP: You might want to try the ETF for zero coupon bonds.