Effect of harvest periods on field productive performance and in vitro digestibility of sweet sorghum

To determine the optimum harvest period for sweet sorghum (Sorghum bicolor) in central Ningxia, sweet sorghum ‘F438’ was used as the research material, and its field productive performance and in vitro digestibility were studied at four harvest periods (jointing, heading, milk, and dough stages). The results showed that the number of stem nodes, stem-leaf ratio, and dry matter yield of sweet sorghum at the dough and milk stages were significantly higher than those of the jointing and heading stages (P < 0.05). The harvest periods had a significant effect on the nutritional quality of sweet sorghum except for acid detergent lignin (P < 0.05). The crude protein content decreased significantly with the delay of the harvest period (P < 0.05), whereas the water-soluble carbohydrates and starch content increased significantly (P < 0.05). The neutral detergent fiber and acid detergent fiber contents increased after the heading stage and then gradually decreased, reaching the lowest levels at the dough stage, with DM values of 58.92% and 35.52%, respectively, significantly lower than those of the jointing and heading stages (P < 0.05). The in vitro dry matter digestibility of sweet sorghum at the dough stage was the highest (53.53%), which was significantly higher than that of other stages (P < 0.05), with an increase of 4.01%～9.33%. The in vitro neutral detergent fiber digestibility at the jointing stage was significantly higher than that of the heading and milk stages (P < 0.05), with an increase of 5.21% and 5.54%, respectively, whereas no significant difference was observed compared with that of the dough stage (P > 0.05). The digestible dry matter yield had significantly positive correlations with dry matter, dry matter yield, water-soluble carbohydrates content, and starch content (P < 0.05). The digestible neutral detergent fiber yield had significantly positive correlations with dry matter and dry matter yield (P < 0.05). The digestible neutral detergent fiber yield and crude protein content showed a significantly negative correlation (P < 0.05). The highest digestible dry matter and neutral detergent fiber yield of sweet sorghum was harvested at the dough stage, at 13.35 and 4.91 t·ha⁻¹, respectively. In conclusion, the optimum harvesting period of sweet sorghum was the dough stage in central Ningxia.