ARS | Publication request: Redesigning alfalfa for use in mixtures with forage grasses

Submitted to: Forage Focus
Publication Type: Trade Journal
Publication Acceptance Date: February 27, 2012
Publication Date: March 1, 2012
Citation: Samac, D.A., Lamb, J.F., Russelle, M.P. 2012. Redesigning alfalfa for use in mixtures with forage grasses. Forage Focus. March 2012. p. 16-17.

Technical Abstract: A lush field consisting of a mixture of grass and legumes is the goal of many producers. Such a production system has many benefits. The most important in times of high fertilizer prices is the reduced need for nitrogen (N) because of the legume's capacity for biological nitrogen fixation. However, attaining a productive grass-legume mixture can be frustrating, and stands with the desired mixture are often short-lived. Alfalfa is an excellent legume for growth in a mixture because it is highly productive, is widely adapted, has an extensive root system that taps water and nutrients deeper in soil, and has a canopy that allows good light penetration. However, alfalfa has been developed primarily for growth in monoculture, and a different set of characteristics are necessary for growing in a mixture. For maximum productivity with grasses, a redesigned alfalfa will need to supply N to the grass component, have enhanced ability to acquire potassium (K) and phosphorus (P) from soil, and resist lodging over longer harvest intervals. N isotope studies and root-imaging studies found that most of the N in soil derived from alfalfa comes from breakdown of fine fibrous roots and nodules. Breeding and selection were done to develop alfalfa lines with high fibrous roots for optimum N transfer as well as optimal K and P uptake. A novel methodology was developed to identify plants with enhanced nitrate uptake. Unique alfalfa lines were developed using this method that preferentially fix N even when nitrate is present. Field trials are in progress to test the competitiveness of the new alfalfa lines when grown as a companion with forage grasses.