1. Batey, T. and Boyd, D.A. (1967). NAAS Quart. Rev.
78, 47–56.
2. Birch, J.A., Devine, J.R., Holmes, M.R.J. and Whitear, J.D. (1967). J. agric. Sci., Camb.
69, 13–24.
3. Bodlaender, K.B.A. and Reestman, A.J. (1968). Neth. J. agric. Sci.
16, 165–76.
4. Bolton, J. (1971). Rothamsted Report for 1970, 2, 98–110.
5. Boyd, D.A. (1959). J. agric. Sci. Camb.
52, 384–91.