Invited Perspective and Commentary: History of the discovery and 50-year development of monensin for beef cattle

Purpose The purpose of this review is to document the events that led to the discovery and development of the utility of monensin for improving feed efficiency in beef cattle. Monensin development is an example of an internally funded commercial research program whose objective was to discover and develop a product profit-able for the company as well as its customers. The pro-gram was justified on the market potential of the hoped-for product and, during the process, was subjected to numerous reviews. This review also illustrates all the research work necessary to obtain approval for product use, by agencies such as the US Food and Drug Admin-istration. Sources The sources for this article include the per-sonal records and observations of the authors, who were principally responsible for this discovery and development, peer-reviewed scientific literature, abstracts from scientific meetings, academic technical reports and company re-ports, and symposia. Synthesis This discovery of the utility of monensin in ruminants by Eli Lilly & Co. (Lilly) scientists was not the result of serendipity, but the result of a designed pro-gram. The goal of this program was 2-fold—first, to design a test system to find compounds that would modify ru-men VFA production, and second, to determine whether VFA production changes would improve feed efficiency in ruminants. Neither of these effects had been previously observed. Before monensin the only practical way to mod-ify VFA production was through ration change. This was not the result of a large corporate research program, but one that resulted from use of redirected limited resources. Once the activity of monensin in in vitro and in vivo tests was recognized, the development of monensin for cattle proceeded rapidly under the direction of a multidiscipline research team with unlimited resources. The final approval of the use of monensin in cattle was the result of the timely efforts of a large number of Lilly and academic scientists. Conclusions and Applications Monensin was ap-proved for inclusion in beef cattle rations 50 yr ago. It is currently (2026) used in an estimated 90% of US feedlot cattle and 70% of the US dairy cows. It is now used in 26 different countries, with 14 different applications. The effects of monensin on all aspects of ruminant physiology are estimated to be the subject of about 22,600 (Google Scholar, search for monensin and ruminants) scientific pa-pers and technical reports.