Abstract

Hydrocyanic acid potentials (HCN-p) of grain sorghums [Sorghum bicolor (L.) Moench] need not be considered in grain production but become important when fresh forage is fed to livestock. This study involved determination of seasonal leaf blade HCN-p levels of 11 field-grown grain sorghum hybrids, relationships between HCN-p levels in growth chamber-grown seedlings and older fieldgrown plants, and effects of N fertilization on seasonal leaf blade HCN-p levels.

Growth chamber plants were grown in vermiculite, subirrigated daily with nutrient solution. Plants were harvested and assayed for HCN-p 10, 14, and 18 days after planting. In 1972, the 11 hybrids were field-grown on N fertilized soil. In 1973, ‘DeKalb E-59’ was grown on N fertilized and on N-deficient soil. Field-grown hybrids were assayed for HCN-p weekly from 27 (1972) or (1973) days after planting until physiological maturity. HCN-p was determined potentiometrically on homogenized, β-glucosidase hydrolized leaf blade tissue.

Relationships between HCN-p levels at sampling dates during the season and seasonal average HCN-p levels indicated that, for purposes of ranking cultivars, approximately equal precision would be attained at any time from growing point differentiation through soft dough. The correlation between HCN-p levels in 18-day-old growth-chamber plants and seasonal average HCN-p levels of field plants was significant at the 10% level (r = 0.56) but HCN-p levels in 10- and 14-day-old plants were not significantly related to seasonal average HCN-p levels of field plants. HCN-p in leaf blade tissue was highly dependent on N availability. At a sustained adequate level of N, HCN-p levels in sorghum leaf blade tissue declined from the seedling stage to about flag leaf and then remained fairly constant until they declined at the approach of maturity. HCN-p levels exceeded the “threshold of danger” (200 ppm) until physiological maturity. On N-deficient soil, leaf blade HCN-p levels declined to a low level (<100 pm) at soft dough.

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