| A series of trials was conducted to evaluate the effects of supplemental types of non-fiber carbohydrates (NFC) and supplemental sources of rumen degradable protein (RDP) on low-quality grass hay utilization by beef steers. In Trial 1, NFC types (starch and dextrose; fed at 0.16% of initial BW) each offered with an equal amount of RDP (0.087% of initial BW) but with six different proportions of nonprotein nitrogen (NPN; supplied as urea) and true protein (supplied as casein) as sources of RDP were administered intraruminally to beef steers fed ad libitum low-quality grass hay. The proportions of the supplemental N from urea (expressed as a percent of the supplemental RDP) were: 0, 20, 40, 60, 80 and 100%. From this study, it was observed that the NFC types evaluated did not interact with source of supplemental RDP with regard to effects on intake and digestion; however, both factors exerted independent effects on these characteristics. Forage digestion was affected by the provision of sugar versus starch, whereas the relative proportion of true protein versus NPN in the supplemental RDP affected forage intake. Trial 2 was conducted to evaluate the effect of the interaction of NFC type and level of RDP supplementation on low-quality forage utilization. Steers given ad libitum access to a low quality grass hay were intraruminally supplemented with one of two sources of NFC (starch or dextrose) at 0.30% of initial BW in combination with one of seven levels of RDP (casein; 0, 0.015, 0.051, 0.087, 0.123, 0.159, 0.195% of initial BW). From this trial, it was observed that supplemental RDP exerted positive effects on forage intake and digestion and, when supplied in sufficient quantities, overcame the negative effect of supplemental NFC on fiber digestion. In Trial 3, two experiments using an in vitro culture system were conducted to assess the effect of various types of supplemental NFC and media pH on rate and extent of forage neutral detergent fiber (NDF) digestion on low-quality grass hay by ruminal microorganisms. In Experiment I, treatments consisted of the combination of two factors. The first factor consisted of supplemental NFC type: none (i.e., control, grass hay only), glucose, maltose, or soluble starch, and the second factor consisted of incubation media 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 II consisted of supplemental NFC type: none (i.e., control, grass hay only), glucose, corn starch, and soluble starch, and the second factor consisted of incubation media 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). Results from Trial 3 indicate that ameliorating 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-specific effect on fiber digestion. |
