Abstract

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 (R-2 = 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.