Effects of nitrogen fertilizer rate on dry matter yield, nutritive values and nitrate nitrogen of Guineagrass ( <scp> <i>Panicum maximum</i> </scp> Jacq.) extremely late cultivar “Umaku” as a summer annual grass in a warm temperate zone

Abstract Cultivation as a summer annual grass in temperate regions of new extremely late‐maturing guineagrass cultivar “Umaku” without heading with fertilizer application entails risks of toxic nitrate nitrogen (nitrate‐N) concentration in the grass. No report has described a study of nitrate‐N in guineagrass without heading cultivated as an annual grass in temperate regions. Therefore, for two years, we clarified nitrogen fertilizer effects on dry matter yield (DMY), nutritional values and nitrate‐N concentration in Umaku without heading cultivated as a summer annual grass. Subsequently, we evaluated management strategies aimed at optimizing nitrogen application to enhance DMY and crude protein (CP) content without increasing nitrate‐N. Treatments were factorial combinations of two basal fertilizer applications (10 or 20 g N/m 2 , respectively, as S or D treatment) and two grasses (“Umaku” and “OKI‐1” of Urochloa hybrid as a control) in a randomized complete block design with three replicates. Although differences between years in all measurements were significant, CP and nitrate‐N for the D treatment were significantly higher than for the S treatment. Nitrate‐N values of both grasses exceeded the toxic thresholds (0.2% DM) in all except for the S treatment of 2020. Delaying harvest until after a sufficient yield has been attained is suggested as an effective strategy for suppressing nitrate‐N concentrations. For Umaku, approximately 620 g/m 2 of DMY with 10 g N/m 2 fertilizer application and at least 800 g/m 2 of DMY with 20 g N/m 2 fertilizer application are recommended. The derived relation between total nitrogen and nitrate‐N well demonstrated that cultivation as annual summer grass without heading carries a risk of toxic nitrate‐N concentration (0.2% DM) when the supplied nitrogen fertilizer is intended to achieve a CP content exceeding 9%. However, because of our limited dataset, further data accumulation is necessary. Furthermore, measuring nitrate‐N concentrations at the time of harvesting on farm is required for preventing nitrate‐N poisoning.