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Urochloa (syn. Brachiaria) cultivars are widely used as forage for ruminants in tropical countries and also in Thailand. Two new Urochloa cultivars, OKI-1 and Br-203, have high forage yield and digestibility compared with other cultivated Urochloa grasses. Seed production is a fundamental requirement for widespread sowing, and it can be increased by optimizing agronomic practices, such as changing plant spacing. Two field trials were conducted in 2018-2020 at Lampang Animal Nutrition Research and Development Center, Northern Thailand, to determine the optimal plant spacing for seed production of these Urochloa cultivars. In both trials, four plant-spacing regimes (50 Â 100, 75 Â 100, 100 Â 100 and 125 Â 100 cm) were used in a randomized complete block design with four replicates. In two consecutive harvests of both trials, plant spacing did not significantly affect seed yield and quality but affected tiller and inflorescence densities. Specifically, significant higher numbers of tillers and inflorescences were produced per unit area at the narrowest plant spacing (i.e., 50 Â 100 cm). OKI-1 and Br-203 tended to have higher pure seed yields (PSY) at plant spacings of 100 Â 100 (136.46 kg/ha) and 75 Â 100 cm (79.59 kg/ha), respectively. Both cultivars showed similar trends in PSY, filled seed percentage and a thousand seed weight, which tended to be higher in the first-year crops than the secondyear crops. This difference could be attributed to a reduction in available soil N resulting from a large amount of N utilization during vegetative growth, combined with inadequate and erratic rainfall. In addition to the recommended optimum plant spacing, sufficient fertilizer and suitable environmental factors could increase seed yield.

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Two field trials were conducted in Thailand to determine an appropriate closing cut date (CCD) and rate of nitrogen application (N-rate) to maximize seed yield and seed quality of the two novel cultivars (cv.) of Urochloa spp. (Synonym Brachiaria spp.), cv. ‘OKI-1’ (an open-pollinated tetraploid Urochloa ruziziensis (R. Germ. and C.M. Evrard) Crins originated from cv. ‘Miyaokikoku-ichigou’) and cv. ‘Br-203’ (U. ruziziensis cv. ‘Miyaokikoku-ichigou’ × U. hybrid cv. ‘Mulato’). The following treatments were evaluated in this study: four CCDs (uncut, 15 June, 1 July, and 15 July) and four N-rates (0, 50, 100, and 150 kg/ha). The cv. ‘OKI-1’ showed somewhat differences in tiller number/m2 (TN), fertile tiller percentage (FTP), inflorescence number/tiller (IN/T) and spikelet number/raceme (SN/R) with the CCD, while the cv. ‘Br-203’ showed only in SN/R. However, TN and SN/R were highest for 15 June, and FTP and IN/T were highest for 1 July in cv. ‘OKI-1’. The cv. ‘OKI-1’ showed the highest total seed yield (TSY), pure seed yield (PSY), and pure germinated seed yield (PGSY) for 1 July, followed by 15 June, and the cv. ‘Br-203’ showed the highest TSY, PSY, and PGSY for 15 July, followed by 1 July. Nitrogen (N) fertilization showed a negative effect on TSY for both the cultivars due to the higher N content in the soil. Withholding N fertilizer, a CCD in late-June to early-July and early-July to mid-July is recommended for cv. ‘OKI-1’ and cv. ‘Br-203’, respectively.

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