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Fluid Journal : Fluid Journal 2002-2004
or have not been influenced by P and K addition. Increased mass/shoot can occur as a result of two different mecha- nisms: rapid initiation of new shoots after defoliation ("bud break"), and faster elongation rate of alfalfa shoots after bud break. Both of these factors increase dramatically with the addition of P and K. Increased levels and en- hanced mobilization of stored reserves may be physiological factors contribut- ing to the rapid shoot initiation and growth after harvest. Stems/area. Through the first five years of this study, increased stems/ area has not been positively associated with forage yield (Figure 3). We ob- served nearly a 10-fold range in forage yield at stem densities of 27 to 45 stems/ ft2. Traditionally, 40 stems/ft2 has been used as an indicator of the agronomic viability of an alfalfa stand, with stands possessing fewer than 40 stems/ft2 be- ing considered non-viable. While we observed a reduction in yield with fewer than 25 stems/ft2, high stem counts (>60 shoots/ft2) did not automatically result in high forage yield. Intermediate values of 27 to 55 stems/ft2 were generally asso- ciated with the highest forage yields in this study, but because of low mass/ shoot in unfertilized plots, these shoot densities were also associated with low yield in many plots. Our data indicate that using a cutoff for stand viability of 40 shoots/ft2 is not appropriate for as- sessing the productivity potential of an alfalfa stand. Plants/area declined rapidly after ini- tial establishment of the alfalfa stand (Figure 4). Since establishment of our stand in 1997, plant population has de- clined with time and in December 2002 there were approximately 6 plants/ft2 averaged over all treatments. Contrary to popular belief, extensive plant losses have occurred during summer and not during winter. Competition for light, water, and nutrients in summer, as well as defoliation every 30 days may have increased the frequency of plant death and thinned stands prematurely. In ad- dition, injury during the previous winter may have weakened the plants that sub- sequently die during the summer. Work has begun in an adjacent set of larger plots that will permit us to determine when alfalfa plants die in the summer and to explore the physiological basis for their demise. 2 Fluid Journal Fall 2003 Yield, lbs./A Figure 2. Influence of mass/shoot on forage yield of alfalfa,1998-2002. Figure 3. Influence of stems/area on forage yield of alfalfa, 2000-2002. Yield, lbs/A
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