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Nitrogen Dilution Curves and Nitrogen Use Efficiency During Winter–Spring Growth of Annual Ryegrass

^a Facultad de Ciencias Agrarias (UNMdP), CC 276, (7620) Balcarce, Buenos Aires, Argentina
^b Estación Experimental Agropecuaria Balcarce (INTA), Balcarce, Buenos Aires, Argentina
^c Unité de Recherche Écophysiologie des Plantes Fourragères–INRA, 86600 Lusignan, France

View larger version (13K): [in a new window]   Fig. 1. Diagram representing tissue N concentration vs. accumulated forage dry matter (+). N1–N5 = values measured in different N fertilization treatments at a given date. • = calculated critical point at this date (adapted from Justes et al., 1994).

View larger version (22K): [in a new window]   Fig. 2. Annual ryegrass yield [forage dry matter (DM) accumulated, solid line] and forage N concentration (each observation corresponds to a pooled sample of three blocks, dashed line) for 1994 and 1995 in response to N fertilization at the end of the experimental period. Bars indicate standard errors of the means (SEM).

View larger version (25K): [in a new window]   Fig. 3. Annual ryegrass N accumulation in forage for each N fertilization rate during winter–spring growth in (a) 1994 and (b) 1995. Bars indicate standard errors of means for sampling dates.

View larger version (20K): [in a new window]   Fig. 4. Annual ryegrass forage N concentration in relation to forage yield for each harvest date of 1994 and 1995, reference N dilution curve (Nref), critical N dilution curve (Ncr), and minimal (Nmin) and maximum (Nmax) forage N concentration. SE = standard error estimated from the Ncr regression.

View larger version (25K): [in a new window]   Fig. 5. Evolution of the N nutrition index (NNI) for each N fertilization level during winter–spring growth in (a) 1994 and (b) 1995.

View larger version (15K): [in a new window]   Fig. 6. Relationship between N nutrition index (NNI) and relative accumulated DM (DMac/DMcr) for different N fertilization levels during annual ryegrass winter–spring growth in (a) 1994 and (b) 1995.

View larger version (25K): [in a new window]   Fig. 7. (a) Nitrogen apparent uptake efficiency (NUpE) and (b) N apparent conversion efficiency (NCE) in 1994 (solid line) and 1995 (dotted line). Bars indicate standard errors of means (SEM).