My post below prompted me to research the history of farmers in the U.S. Senate. The following list includes U.S. Senators since 1900 who were also farmers. The parenthetical comments list their occupation(s) as taken from the Political Graveyard and/or the Congressional Bibliographic Directory site. As you can see, many “farmers” were also bankers, lawyers, sheriffs, etc. It will take significantly more research to determine which of these Senators were primarily farmers, and which were bankers who owned a farm.
Farmer-Senators Since 1900
Ellison DuRant Smith (D-SC) U.S. Senator 1909-44 (engaged in mercantile and agricultural pursuits, organizer of the Southern Cotton Association, field agent and general organizer in the cotton protective movement 1905-1908, known as “Cotton Ed”)
Obadiah Gardner (D-ME) U.S. Senator 1911-13 (engaged in the lumber, lime, and creamery business, and also in agricultural pursuits and in cattle raising)
Henry Wilder Keyes (R-NH) U.S. Senator 1919-37 (farmer, banker, and politician)
Magnus Johnson (DFL-MN) U.S. Senator 1923-25 (lumberjack, farmer, school clerk and assessor)
Lynn Joseph Frazier (R-ND) U.S. Senator 1923-41 (farmer and politician)
Hamilton Fish Kean (R-NJ) U.S. Senator 1929-35 (engaged in banking and agricultural pursuits)
John Gillis Townsend, Jr. (R-DE) U.S. Senator 1929-41 (engaged in banking, also interested in manufacturing and agricultural pursuits)
Robert Davis Carey (R-WY) U.S. Senator 1930-37 (engaged in the raising of livestock and agricultural pursuits, also interested in banking, politician)
Patrick Anthony McCarran (D-NV) U.S. Senator 1933-54 (farmer, lawyer and judge)
Harry Flood Byrd (R-VA) U.S. Senator 1933-65 (newspaper publisher, fruit farmer, politician)
Guy Mark Gillette (D-IA) U.S. Senator 1936-45 (military, engaged in agricultural pursuits, attorney)
George David Aiken (R-VT) U.S. Senator 1941-75 (engaged in fruit farming in 1912, also conducted an extensive nursery business and commercial cultivation of wildflowers)
Zales Nelson Ecton (R-MT) U.S. Senator 1947-53 (grain farmer and livestock rancher)
Earle Chester Clements (D-KY) U.S. Senator 1950-57 (farmer, sheriff and county judge)
Frank Carlson (R-KS) U.S. Senator 1950-69 (farmer and rancher)
Frank Aloysius Barrett (R-WY) U.S. Senator 1953-59 (lawyer, rancher, politician and civil servant)
Henry Louis Bellmon (R-OK) U.S. Senator 1969-81 (farmer, rancher and politician)
Summary thoughts in relation to the newest farmer in the Senate, Jon Tester (D-MT):
There are not many other Senators who were just farmers, the way Jon Tester has been just a farmer for most of his life. Not many farmer-Senators on this list rose as quickly to the U.S. Senate as Jon Tester has. Most were long-term politicians holding a variety of posts and rising though U.S. congressional positions or governor seats to the U.S. Senate.
There is a noticeable decline in the number of elected farmer-Senators about the middle of the last century with only three of the seventeen Senators listed above achieving election after 1950.
My sources profess their own incompleteness. If I missed someone, add them in the comments below.