The USDA loan interest rate depends on the program for which you qualify. If you use the Direct program, in which the government actually lends you the money, the government sets the rate. Currently, per the USDA, "Effective January 1, 2018, the current interest rate for Single Family Housing Direct Home Loans is 3.25% for low and very low income borrowers.."
For the Guaranty program, the rates are set by private lenders, similar to the way FHA and VA mortgages work. In that case, you'll want to compare rates and terms between competitive lenders before applying.