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 sativaLeyss) 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−1 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−1, 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.
Alfalfa Bromegrass Economics Energy performance Forage yield Nitrogen fertilizer Protein