Established alfalfa is very competitive with most annual weeds

The U.S. Department of Agriculture National Institute of Food and Agriculture awarded the University of Idaho $751,002 on September 15, 2025, under the Alfalfa Seed and Alfalfa Forage Systems Program (CFDA 10.330) to establish weed biomass thresholds in alfalfa forages through designed field studies, on-farm research, and surveys evaluating how individual weed species impact alfalfa biomass yield, forage nutritive value, livestock performance, toxic compound accumulation, milk production, and production economics as a tool for ecological weed management and optimizing economic returns.

The project runs through September 14, 2028, with performance at the University of Idaho's Moscow, Idaho campus. The University of Idaho is executing this as a multi-state, nationally coordinated integrated research and extension project with sub-recipients at the University of Nebraska (awarded May 14, 2026) conducting forage weed research in Nebraska; Kansas State University (awarded May 14, 2026) investigating how weed diversity and biomass impact forage accumulation, nutritive value, milk production, and production economics; and Utah State University (awarded May 11, 2026) performing forage weed research in Utah. The assistance type is a project grant.

However, because alfalfa establishes very slowly, newly planted alfalfa is very vulnerable to weed competition. While weed competition can reduce alfalfa biomass production by up 39%, the reduction in alfalfa biomass is often compensated for by increased weed biomass. In some cases, the weed biomass can be of similar nutritive value as alfalfa. This tendency of some weed species to contribute to the biomass of the whole stand without significantly reducing forage nutritive value has led to arguments that IT may be time to change attitude and view weeds as friends of the forage agroecosystem rather than as foes. However, there are currently no weed biomass thresholds established for alfalfa. Establishing weed biomass thresholds in alfalfa forages requires assessment of how individual weed species impact biomass yield, forage nutritive value, livestock performance, as well as toxic compound accumulations. In this integrated research and extension, national coordination project, we are proposing to utilize designed field studies, on-farm research, and surveys to evaluate weed impacts on alfalfa forage productivity and nutritive value, milk production and production economics as a tool for ecological weed management and optimizing economic returns.