Wigley, K., J. Owens, J. Trethewey, D. Ekanayake, R. Roten, and A. Werner. “Optical Sensors for Variable Rate Nitrogen Application in Dairy Pastures”. Journal of New Zealand Grasslands, vol. 79, Jan. 2017, pp. 223-7, doi:10.33584/jnzg.2017.79.533.