Effects of water soluble carbohydrate content on ensiling characteristics, chemical composition and in vitro gas production of forage millet and forage sorghum silages

A study was conducted to determine effects of water soluble carbohydrate (WSC) concentration on ensiling characteristics, chemical composition and in vitro digestibility of forage millet and forage sorghum. Two cultivars (regular and high WSC content) of each forage were harvested at late vegetative stage and ensiled in mini-silos for 0, 2, 4, 8, 16 and 48d. Ensiled forages went through a rapid fermentation as indicated by a sharp decline in pH during the first 2d of ensiling. Between day 0 and 8 post-ensiling, pH was higher (P<0.05) for ensiled millet than ensiled sorghum forages and was lower for high WSC than regular cultivars. Lactic acid concentration was higher (P<0.05) in high WSC than in regular cultivars and was not affected by forage type. Chemical composition of the 45-d silages showed that forage millet silages contain more (P<0.05) neutral (aNDF) and acid detergent fiber than forage sorghum silages. Fiber fractions were not affected by WSC content except for lignin(sa), which was higher (P<0.05) for regular versus high WSC cultivars. Soluble and non-protein N were higher (P<0.05) in forage millet than forage sorghum silages and were higher for high WSC versus normal cultivars. In vitro gas production, degradation rate and volatile fatty acid production were not affected by forage type but were higher (P<0.05) for high WSC versus normal cultivars. In vitro dry matter and aNDF digestibilities were higher for forage sorghum versus forage millet silages, and were higher for high WSC versus normal cultivars. High WSC content improved ensiling characteristics of both forages, but the effect was more pronounced for forage sorghum than for forage millet. Based on chemical composition and in vitro data, we conclude that the feeding value of forage sorghum silage is superior to that of forage millet silage when harvested at the same stage of physiological development.