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This research work was conducted during the years 1367-68 and 1368-b9 (1988— 80, 1989—90) to evaluate the effect of seasonal climate variation on forage chemical composition of different varieties of Brassica Spp. in North Vararnin region. Considering the data during two years of experiment the level of N.P.K. in the autumn forage was mote than summer forage. On the other hand the level of these three elements in leaf and stem were higher than root of crop. In addition, the level of Ca, Mg, Mn were considerably more in leaf and stem than root, too. The micronutriens contents. In stem., leaf and root of crop especially Cu, was higher in autumn than summer forage during t*co years of ecperiement. Calculated Fe content was higher in autumn forage, however, Fe content was higher in the root of summer than autumn, forage. The zinc content in leaf, stem and root organs of summer corp yield was higher than autumn forage, while the Mn content was reversely effected. The Mn content in stem and leaf was higher than root as compared with Cu and Zn content.
The effect of different climatic factors, heredity and interaction of species x seasons’ effects on level of different chemical content are very important, however, this experiement resulted that the level of micro and macro nutrients content in the different varieties of tested Brassica Spp. are more than the need of livestock animals (especially ruminants).
To compensate the defficiency of these nutrients in the forage yield of different Brassica spp. for feeding livestocks, neccessary fertilizers must be supplied to the cultivated lands.