Effects of nitrogen fertilizer rate on dry matter yield, nutritive values and nitrate nitrogen of Guineagrass (Panicum maximum 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/m², 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/m² of DMY with 10 g N/m² fertilizer application and at least 800 g/m² of DMY with 20 g N/m² 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.