Nitrogen and Carbohydrate Partitioning in 'Caucasian' and 'WW-Spar' Old World Bluestems

Total nonstructural carbohydrates (TNC) and nitrogen (N) in 'WW-Spar' (Bothriochloa ischaemum) and 'Caucasian' (B. caucasica) Old World bluestems were monitored in field experiments during spring-summer and summer-fall growth cycles. The objectives were to determine seasonal partitioning of TNC and N among biomass compartments and to determine relationships between concentrations and pool sizes of TNC and N in the perennating storage organs (stem bases and roots). Average TNC concentrations during the growing season were highest in the leaf sheaths and enclosed stems followed by leaf blades>stem bases>roots. Total TNC was highest in the roots>stem bases>stem plus sheaths>leaf blades. Average N concentrations were highest in the leaf blades>roots>stems plus sheaths>stem bases while the rank for total N was root>stem base>leaf blade>stems plus sheaths. Thus, the perennating organs (stem bases, roots) represented the largest reservoirs for both TNC and N reserves. Reserve cycles were similar in both grass species. Differences were primarily in the perennating organs as WW-Spar bluestem stored more TNC and N in stem bases and Caucasian bluestem stored more of both constituents in roots It is often assumed that plant vigor is related to TNC and N reserves and that management of forage-based livestock production systems can be keyed to reserve cycles. Therefore, we sought to answer the question of whether concentrations alone could adequately predict relative vigor or whether pool sizes must be known. Our analysis showed that concentrations tracked pool sizes of TNC extremely well in the roots, but that the relationship was not as strong in the stem bases. The relationship for N concentration and total N was highly significant for roots, but not as good as for TNC. Concentrations of N were not good predictors of total N in stem bases. Fluctuations in total N were much greater than for concentration. Although nitrogen-use efficiency increased linearly with season, N investment per unit leaf blade area declined. This suggested that nitrogen limitation was the main cause for reduced rates of increase in TNC-use efficiency during the last third of the first growing cycle. This was a time when TNC investment per unit leaf blade area was increasing. In these Old World bluestems, management decisions related to plant vigor can apparently be keyed to TNC concentrations thereby eliminating the more laborious tasks required to determine TNC pool sizes. Further study is necessary to determine the feasibility and economics of using nitrogen fertilizer applications in the final third of a growing cycle to reverse the loss in leaf TNC-use efficiency observed in this study.