HARVEST DATE EFFECTS ON YIELD AND COMPOSITION OF FORAGE SORGHUM AND SORGHUM X SUDANGRASS HYBRIDS [SORGHUM BICOLOR (L.) MOENCH.]

Information on yield and nutritive value of forage sorghums planted on Vertisols is limited. Forage sorghum (FS) and sorghum χ sudangrass hybrid (Sudax) were planted in 0.5- ha plots to determine date of harvest effects on dry mater yield (DMY) and chemical composition. The experimental design consisted of four replicates in a split-plot arrangement (sorghums as the main plots and harvest dates of 70 and 90 d as subplots). Soil type was a fine, smectitic, isohyperthermic Typic Haplusterts (of the Fraternidad series). At 70 and 90 d post planting, plant height was measured from 10 randomly selected plants in a 2-m area. Plants in the 2-m2 quadrat were clipped to 2.5-cm height and weighed. Subsamples (500 g) were dried in a 60° C air-forced oven, ground and analyzed for CP and Ρ concentrations. Plant height was greater (P<0.05) for Sudax (1.2 m) than for FS (0.7 m) at the 70 day harvest and was also different at the 90-d harvest. There were cultivar effects and harvest date effects on DMY (PC0.05). Forage yield was greater for Sudax than for FS on d 70 and d 90. At 70 d, DMY of Sudax and FS averaged 4,134 and 2,832 kg/ha, whereas at 90 d the average was 6,791 and 4,724 kg/ha, respectively. But CP was greater (P<0.05) for FS than for Sudax only at the 70 d harvest (86.9 vs. 80.7 g/kg). The Ρ concentrations were similar for both sorghums and did not differ by harvest date. There was a 62% increase in DMY of Sudax when harvested at d 90 and a 60% increase for FS. Considering their high CP at d 90, both sorghums are good alternatives for use in forage conservation systems in Puerto Rico.