1. FAO Nutrition Meetings Report Series, No. 52; WHO Technical Report Series, No. 522, (Energy and protein requirements: report of a Joint FAO/WHO Ad Hoc Expert Committee), 1973.
2. Kirchev, Chr. and D. Penkov, Yield of total and digestible amino acids from triticale by experiments with Muscovy ducks, Scientific works Agricultural University Plovdiv, 55, 1, 113-118, 2010.
3. Delibaltova, V.and D. Penkov, Yield of energy and protein from grain maize hybrids using agrotechnical system, optimal for the conditions in the Plovdiv region, J. of Central European Agric., 11, 3, 265-270, 2010.
4. Yanchev, I., D. Penkov and J. Terziev, Yield of crude nutritional substances and energy from maize – second culture in compacted land use, Bulgarian Journal of Crop Science, 36, 475-477, 1999.
5. Tarighaleslami, M., Zarghami, R., Boojar, M. M. A.and M. Oveysi, Effects of drought stress and different nitrogen levels on morphological traits of proline in leaf and protein of corn seed (Zea mays L.). American-Eurasian Journal of Agricultural and Environmental Sciences, 12, pp 49-56, 2012.