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- [3] GRASSLAND RESEARCH AND PRACTICE JOURNAL OF THE ROYAL AGRICULTURAL SOCIETY OF ENGLAND, 1968, 129 : 85 - +
- [4] THE GERMAN INSTITUTE FOR ECONOMIC RESEARCH, INSTITUTE-FOR-BUSINESS-RESEARCH JAHRBUCHER FUR NATIONALOKONOMIE UND STATISTIK, 1988, 204 (05): : 478 - 483