Remobilization of leaf nitrogen in stockpiled tall fescue
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Missouri's beef industry is supported by more than five million hectares of pasture containing the grass tall fescue (Lolium arundinaceum). Stockpiled (accumulated autumn growth used for winter grazing) tall fescue leaf macronutrients decrease in concentration from late autumn through winter. The objective of this study was to elucidate the fate of the macronutrient, nitrogen (N), from stockpiled tall fescue leaves during winter and spring. The stable isotope 15N was painted on leaves of established tall fescue during early autumn in central Missouri. Above and below ground plant parts were harvested monthly through spring and analyzed for atom%15N, total N, and N fractions of soluble protein, free amino acids, and nitrate. Recovery of 15N indicated that autumn senescing leaves remobilized 45-60% of leaf N by spring. Through mid-winter, remobilized leaf N supported areas of active growth, including new tillers, crowns, new rhizomes, and a proliferation of new roots. In crowns and new roots, free amino acids were a predominant N storage form in mid-winter, with asparagine (~50%) and glutamine (~20%) predominating. During spring regrowth, N from all tissues was remobilized to support new tiller growth. By late April, remobilized leaf N accounted for 70% of new tiller N. These insights into the N dynamics of stockpiled tall fescue may be used to develop more efficient N fertilization and grazing strategies in tall fescue stockpiling systems.