Apex domains cannot have CNAME records.
Question and response
Question
dig usda.gov. NS
Answer
usda.gov. 5 NS ns2.usda.gov.
usda.gov. 5 NS ns1.usda.gov.
Authority
The response has no authority records.
Additional
ns1.usda.gov. 5 A 199.141.126.202
ns2.usda.gov. 5 A 199.141.126.206
ns1.usda.gov. 5 AAAA 2600:12f0:0:ac04:0:0:0:202
ns2.usda.gov. 5 AAAA 2600:12f0:0:ac04:0:0:0:206
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0