W. C. Young, Silberstein, T. B., Chastain, T. G., and Rowarth, J. S.,
“The relationship between applied nitrogen, nitrogen concentration in herbage, and seed yield in perennial ryegrass (Lolium perenne L.). II. Cultivars in Oregon”,
J. Appl. Seed Prod, vol. 16, pp. 115–124, 1998.