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One of the challenges of modern grassland systems is to minimize nitrogen (N) fertilization without negatively affecting the forage yield. Therefore, critical N dilution curves (Nc = acW−b) have been developed in different species to improve N fertilization management. The aim of this study was to validate a critical N dilution curve for hybrid ryegrasses. Two field experiments were conducted in southern Chile. Treatments were the factorial combination of two hybrid ryegrasses (Shogun and Trojan cultivars) and seven N fertilization rates (0, 50, 100, 200, 350, 525 and 700 kg N/ha). Factors were arranged in a split‐plot design, where forage species were assigned to main plots and N rates to subplots that were randomized into four blocks. A wide range in forage yield and plant N concentration was observed (yield: 0.16 and 3.9 Mg DM/ha and N: 1.6% and 5.1%). The variations in these traits were principally explained by the N levels and harvest times. Relative yield responses of both cultivars were significantly (p < 0.001, R2 = 0.81–0.87) related to the nitrogen nutrition index (NNI) calculated with different critical N dilution curves. However, the NNI calculated with N dilution curves from annual ryegrass best described the relative yield response of hybrid ryegrass. Therefore, this validated critical N dilution curve (%Nc = 4.1W−0.38) will serve as a useful diagnosis tool for improving the N fertilization management of grazing systems for hybrid ryegrasses. [ABSTRACT FROM AUTHOR]

[Display omitted] • Across experiments NUtE ranged between 19 and 70 kg DM kg−1 N uptake. • The variation in NUtE was mostly accounted for by N rates (48 %) and harvest dates (39 %). • NUtE responses were explained by NNI and forage yield. • A NUtE of 29 ± 5.1 kg DM kg−1 N is proposed as a reference value for NNI = 1. Improving nitrogen (N) utilization efficiency (NUtE (kg dry matter (DM) kg−1 N uptake)) is one of the challenges of modern grassland systems in order to improve nitrogen use efficiency. The NUtE is highly variable in response to climate and N supply. Therefore, NUtE must be assessed in relation to the N nutrition index (NNI) and forage yield in pastures growing under different N supplies and environments. The aim of this study was to assess NUtE and NNI in response to different ryegrass genotypes, N supplies and harvest times. Two experiments were conducted in southern Chile for this purpose. In each experiment, treatments were the factorial combination of two hybrid ryegrass cultivars (Shogun and Trojan) and seven N fertilization rates (0, 50, 100, 200, 350, 525 and 700 kg N ha−1 yr−1). A wide range in N uptake, NUtE and NNI were observed (N uptake: 4 – 134 kg N ha−1; NUtE: 20 – 71 kg DM kg−1 N uptake; NNI: 0.23 – 1.43) across experiments. The main sources of variation of the above-mentioned variables were N rates and harvest times. Across experiments, the NUtE was described by a negative power function (R2 = 0.77, P < 0.01) in relation to the NNI and the remaining variability in NUtE was well explained by the forage yield, demonstrating that NUtE depends on the N nutritional status and forage yield. A reference NUtE of 29 ± 5.1 kg DM kg−1 is proposed for grasslands growing at optimal N status (NNI = 1). Therefore, both the NNI and forage yield must be taken into account when interpreting how NUtE responds to genotypes cultivated under different N availabilities and growing conditions. [ABSTRACT FROM AUTHOR]