To investigate the effect of harvest time associated with P and S fertilization on yield and quality of forage sorghum (Sorghum bicolor (L.) Moench), grown as a rainfed crop in Pakistan, a field study was carried in summer 2009. Three levels of phosphorus (0, 30 and 60 kg P2O5 ha-1), in combination with three levels of sulphur (0, 20 and 40 kg SO4 ha-1), in combination with three harvest times (35, 45 and 55 days after emergence; DAE), were tested in a factorial randomized block design. At each harvest, morphological, functional, yield and quality traits were assessed on plant samples. Extending the growing season from DAE 35 to 55 enhanced almost all the traits and greatly benefited dry biomass yield (ca. +100%). The longer growth also determined a higher efficiency in the accumulation of dry biomass per unit time and land surface (crop growth rate ca. +30%), as well as per unit time and leaf surface (net assimilation rate ca. +100%). Quality traits also improved when plants were harvested at a more advanced maturity: leaf hydrocyanic acid content, a toxic component to livestock, decreased by ca. 30%; stalk soluble-solid content, an indicator of forage juiciness and palatability, increased by ca. 50%. On concluding, the combined effects of harvest delay, P and S fertilization on sorghum are deemed able to significantly increase forage production in warm, relatively dry areas of the world.
Key words: Forage sorghum, phosphorus, sulphur, harvest time, crop growth rate (CGR), net assimilation rate (NAR), hydrocyanic acid (HCN).
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