Dennis was Director of the Leopold Center for Sustainable Agriculture from 1988 to 2002. He is an Emeritus Professor of Agronomy and of Agriculture and Biosystems Engineering at Iowa State University. He is also a Senior Fellow at the Council for Agricultural Science and Technology, an Ames Senior Fellow at the Department of Soil, Water, and Climate in the College of Agricultural, Food, & Environmental Sciences at University of Minnesota, and a Senior Fellow at the Institute for Agriculture and Trade Policy in Minneapolis. He is also the past president of the Iowa Environmental Council.
He has been involved in numerous projects with IATP, including hypoxia, bioenergy, water quality, water efficiency, and Farm Bill conservation policy. He is also a member of Spira/GRACE Advisory Board, Thomas Jefferson Agriculture Institute Board of Directors, Water for Iowans Board of Directors, and Iowa State Water Resources Center Advisory Board. He grew up on a general dairy farm in central Iowa and is still involved with a grain farm in central Iowa. Agriculture has always been his first interest, love and passion.