Analyzing and predicting the response of the signal grass seed crop to plant nitrogen status

Nitrogen (N) deficiency has detrimental effects on productivity and the profit of producers in areas where signal grass [Urochloa decumbens (Stapf) R.D. Webster (syn. Brachiaria decumbens Stapf.)] cv. Basilisk is grown for seed production. The objective of this paper was to clarify the effects of indicators of signal grass plant N status on seed yield (SY), SY components, yield formation, seed quality, panicle growth parameters, and remobilization of vegetative N on seed growth. Germinable pure SY, harvest index (HI), and N harvest index (NHI) were also measured. Different rates of N fertilizer application (0, 50, 100, and 150 kg ha⁻¹) were applied after the cleaning cut to both the first crop (October - January) and the second crop (February - May) in 2010–2011 and 2011–2012, on a sandy loam soil representative of soils used for seed production in Brazil. Although the N nutrition index (NNI) increased at key developmental stages, the highest values were near to 0.85. This suggests that all crops were maintained under N-limiting conditions. In N-limited crops, a strong relationship was detected between NNI and accumulated N deficit throughout the study period with relative SY. A low NNI after the cleaning cut was found to restrict fertile tiller number (FTN), spikelets per panicle, and spikelet density m⁻² measured at anthesis. In all crops, at harvest, NNI at anthesis increased germinable pure SY, FTN, number of seeds per panicle, HI, NHI, and amount of remobilized N to seeds, but not thousand seed weight (TSW), seed germination, panicle dry matter (DM) accumulation rate, and individual seed growth rate. Regression analyses suggested that the NNI, accumulated N deficit, aboveground plant biomass (AGPB), and N content were better associated with relative SY than with plant N concentration (PNC). The study shows that the NNI quantifies the intensity and duration of N deficiency in signal grass and should be considered in research studies and for application in seed production fields to improve N fertilization recommendations.