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

Two in vitro experiments were conducted to study the effects of supplemental non-fiber carbohydrate (NFQ 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 x 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 x 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 x pH interaction (P < 0.01) was observed for extent of NDF digestion. The extent of NDF digestion was greater (P < 0.01) when pH was adjusted, regardless of the NFC treatment. However, when pH was not adjusted, extent of NDF digestion was depressed (P < 0.01) for treatments receiving supplemental NFC compared with the control. Rate of NDF digestion estimated by non-linear regression did not exhibit an NFC type x pH interaction (P = 0.45). When compared with the control, rate of NDF digestion was depressed (P < 0.01) regardless of pH treatment when supplemental NFC was provided. No significant differences were observed among the three sources of supplemental NFC with regard to either rate or extent of NDF digestion. In Experiment 2, an NFC type x pH interaction was observed (P < 0.01) for extent of NDF digestion. In this experiment, no difference was observed among treatments for the extent of fiber digestion when pH was adjusted. However, similar to Experiment 1, when pH was not adjusted the extent of NDF digestion was depressed relative to the control when supplemental NFC was provided. Overall, rate of NDF digestion calculated from non-linear regression was depressed (P < 0.01), regardless of pH treatment, in response to providing a supplemental source of NFC. However, the magnitude of depression was dependent (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 overridden by alleviating pH depression and is consistent with the existence of a "carbohydrate effect" on fiber digestion. (c) 2005 Elsevier B.V. All rights reserved.