NITROGEN FERTILIZATION OF ALFALFA (MEDICAGO SATIVA L.) IN LATE WINTER IN MEDITERRANEAN ENVIRONMENTS

Fertilizing established alfalfa with nitrogen is a practice that is generally questioned. However, in cold springs, some alfalfa growers believe that N fertilization might increase the rate of spring regrowth. To evalúate the effects of N fertilization in spring, field experiments were conducted during four years under irrigation in a Mediterranean environment. The treatments were 0 and 30 kg ha' of N applied annually in late February on different alfalfa varieties. N fertilization increased the forage dry matter (DM) yields in the fírst spring harvest in one experiment, from 3.74 to 4.21 Mg ha1, whereas the proportion of weeds in the DM, in this harvest, rose from 22.8% to 32.8%. No DM yield differences between treatments were found in any of the other five harvest of the year or in the total annual DM yield. When weeds were controlled by herbicide, the N fertilization in late winter did not affect the herbage DM yields, weed proportion of the herbage, plant height or crude protein concentration of the forage. The results suggest that in the conditions of the experiments, the yield improvement with N fertilization was due to an increase in the weed contení (from 22.8% with 0 N to 32.8% with 30 kg/ha N)