Effectiveness of alfalfa in reducing fertilizer N input for optimum forage yield, protein concentration, returns and energy performance of bromegrass-alfalfa mixtures

Grasses, when grown in association with legumes, may utilize some N fixed by the legumes resulting in improved forage dry matter and protein yield. Field experiments were conducted at Lacombe and Eckville, Alberta, Canada to determine the effectiveness of alfalfa ( Medicago sativa Leyss) in reducing fertilizer N requirements for optimum forage dry matter yield (DMY), protein concentration ( PC), net margins ( returns above N fertilization and forage harvesting costs) and non-renewable energy performance of bromegrass (Bromus inermis Leyss)alfalfa mixtures. Ammonium nitrate was applied in early spring of 1993 to 1995 at 0, 50, 100, 150 and 200 kg N ha to five bromegrass-alfalfa compositions ( pure bromegrass; 2: 1, 1: 1 and 1: 2 ratio of bromegrass: alfalfa; and pure alfalfa) seeded in the summer of 1992. In the zero-N treatment, DMY was lowest in pure bromegrass stands, and increased substantially when alfalfa was grown in association with bromegrass. There was a marked increase in DMY from the application of N fertilizer in pure bromegrass stands, but the increase was much less in the mixed stands. There was a significant increase in PC in forage when bromegrass was grown in a mixture with alfalfa compared to bromegrass alone. Net margins were much greater from mixed stands than from pure bromegrass. In pure bromegrass stands, net margins increased with increasing N rates up to 200 kg N ha, but equivalent net margins were usually attained without fertilizer N in bromegrass-alfalfa mixtures as low as 2: 1. Energy performance of pure bromegrass stands was substantially improved by including alfalfa in the stands, whereas application of N fertilizer caused a strong and steady decline in energy use efficiency. Our findings indicate that seeding alfalfa in mixed stands with bromegrass can generate savings in N fertilizer ( for pure bromegrass stands) equivalent to about 100 kg N ha(-1) or more, without any detrimental effect on forage yield, forage quality or net earnings. However, the short-lived nature of alfalfa in bromegrass-alfalfa mixtures remains a cautionary concern. Thus, producers should also adopt management practices that enhance longevity of alfalfa to maximize long-term benefits of using grass-legume mixtures.