Buth has announced to her Tory colleagues that she will leave the Senate this summer and go back to a job tied to her roots in agriculture.
She will head an industry organization, the Canadian International Grains Institute.
Buth had been president of the Canola Council of Canada before being appointed by Harper to the Senate in 2012.
Her departure means the Conservative ranks will be diminished at a time when the Senate continues to grapple with the expenses scandal that began to unfold last spring.
Sen. Claude Carignan, leader of the government in the Senate, lamented Buth's departure Tuesday, saying she will be missed.
"Her stay in the Senate, too short, will be remembered for her valuable contribution to the work of the Senate and to democracy in Canada," said Carignan.
"We will remember the important work she did on the standing committee on agriculture and forestry, as well as on the standing committee on national finance."
Buth is the fifth senator since the end of 2013 to leave the Senate ahead of the mandatory retirement age of 75.
In a statement, Buth said her new role at the grain institute is a chance to return to her roots.
"This opportunity represents coming home to me as I come back to work in an industry that is so critical to Canada and to Canada's reputation internationally," she said.
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