Abstract: This experiment studied the nutritional value and gas production performance, when incubated with sheep rumen fluid, of sweet sorghum forage harvested at different crop growth stages. The forage sweet sorghum cultivar Dajiang 3180 was harvested in August (S₁), September (S₂), October (S₃) and November (S₄), the herbage nutritive value analyzed. And the relative feeding value (RFV), total digestible nutrient (TDN), dry matter intake (DMI), digestible dry matter (DDM) and relative forage quality (RFQ) were evaluated. Herbage digestibility and the gas production at 3, 6, 9, 12, 24, 36 and 48 h were evaluated by in vitro simulated fermentation. It was found that: 1) There were significant differences in dry matter, crude protein, neutral detergent fiber, acid detergent fiber, ethanol extract and crude ash levels at the different harvest dates (P<0.05). 2) There were significant differences across the four harvest periods in RFV, DMI and RFQ (S₁>S₄>S₃>S₂) (P<0.001). The TDN and DDM of S₁ and S₄ were higher than the S₂ and S₃ harvests (P<0.001). 3) For the first 12 h of fermentation, there were gas production (GP) differences between the four harvest periods (P<0.001), with the GP of S₃ being the highest. After 24 h, the GP of S₄ was higher than S₂ (P<0.05); There were no significant differences in GP between the four harvest periods after 36 h incubation (P>0.05). 4) There were no significant difference in DMD between the four harvest periods at 12, 24 and 48 h of incubation (P>0.05). In summary, the feeding value of sweet sorghum in the ‘hill and gully’ region of the Loess Plateau, China, is highest in August.

Key words: sweet sorghum, harvest period, nutritional value, in vitro fermentation characteristics