Effect of types of non-fiber carbohydrate on in vitro forage fiber digestion of low-quality grass hay

Abstract Two in vitro experiments were conducted to study the effects of supplemental non-fiber carbohydrate (NFC) and fermentation pH on rate and extent of forage neutral detergent fiber (NDF) digestion by mixed ruminal microorganisms. A low-quality grass hay (53 g/kg CP and 761 g/kg NDF) that was finely ground served as the basal forage substrate. In Experiment 1, treatments were arranged as a 4 × 2 factorial in a randomized complete block design. The first factor consisted of supplemental NFC type: none (i.e., control, grass hay only), glucose, maltose, or soluble starch. The second factor was fermentation pH: adjusted (i.e., maintained at approximately pH 6.8) or non-adjusted (i.e., pH allowed to vary from an initial pH of 6.8 without interference). Treatments in Experiment 2 were arranged as a 4 × 2 factorial in a completely randomized design. The first factor consisted of supplemental NFC type: none (i.e., control, grass hay only), glucose, corn starch, and soluble starch. The second factor consisted of fermentation pH: adjusted or non-adjusted (the same as in Experiment 1). In both experiments NFC treatments were set such that the supplemental NFC supplied approximately 200 g/kg of the total substrate dry matter. In Experiment 1, an NFC type × pH interaction ( P P P P  = 0.45). When compared with the control, rate of NDF digestion was depressed ( P P P P  = 0.02) on the type of supplemental NFC included. In conclusion, our results indicate that avoiding the pH depression associated with NFC supplementation enables the maintenance of potential extent of digestion; however, the impact on rate of fiber digestion is not over-ridden by alleviating pH depression and is consistent with the existence of a “carbohydrate effect” on fiber digestion.