AbstractIn 1981 field studies at the lsabela Experiment Farm in northwestern Puerto Rico, we evaluated agronomic performance, hydrocyanic acid potential (HCNp) and heterosis in forage sorghum hybrids (Sorghum bicolor (L.) Moench). Three cuttings were made at 60-day interval. The combined analyses showed significant differences in yields of green forage (GF), dry forage (OF) and crude protein (CP); contents of dry matter (OM) and crude protein; tillers/plant; height; leaf area; and HCN-p at 45 days. Harvests were not a significant source of variation in leaf/stem ratio or in HCN-p at 60 days. The F1 hybrids of CK-60 x Fa357, CK-60 x Common and CK-60 x Lahoma produced the highest OF and CP yields across the three harvests. The OF and CP yields of these top sorghum-sudangrass hybrids-about 20 and 2 metric tons, respectively in 180 days-and their relatively low HCN-p values, establish these genotypes among the most promising forage material for use under irrigation. The HCN-p values of all genotypes were lower at the 60-day harvest than at the 45-day sample. There were some instances of HCN-p value exceeding the "threshold of danger" of 200 p/m. Midparent heterosis was manifested for GF, OF and CP yields, height, tillers/plant and leaf area, but not for OF and CP content. High parent heterosis was less than midparent heterosis for most traits studied and was not observed for OM and CP content or for tillers/plant. The F1 hybrid of CK-60 x Common exhibited the greatest heterosis, producing 146 and 125% more DF than the midparent and high parent, respectively. Fa357 was superior to all other lines in having the lowest HCN-p among the male parents at 60 days. Lahoma had the highest yield potential of the male parent lines and an intermediate level of HCN-p compared to the other lines.
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