Effects of type of supplemental carbohydrate and source of supplemental rumen degradable protein on low quality forage utilization by beef steers

This study was conducted to assess the impact of type of supplemental non-fiber carbohydrate (NFC) and source of supplemental rumen degradable protein (RDP) on low quality forage intake, ruminal fermentation, and total tract digestion. Twelve ruminally fistulated beef steers (average initial body weight (BW) 500±22 kg) given ad libitum access to a low quality grass hay (53 g/kg crude protein; 748 g/kg neutral detergent fiber (NDF)) were used in a two period, two treatment crossover design nested within a six treatment completely randomized design. Periods consisted of an 11 day adaptation to treatments, a 5 day interval for intake and digestibility determination, and a 2 day ruminal sampling interval. Treatments were arranged as a 2×6 factorial and consisted of two different NFC types (corn starch and dextrose; fed at 1.6 g/kg of initial BW) each offered with an equal amount of RDP (0.87 g/kg of initial BW) but with six different proportions of non-protein nitrogen (NPN; supplied as urea) and true protein (supplied as casein) as sources of RDP. Proportions of supplemental N from urea, expressed as a percent of the supplemental RDP, were: 0, 20, 40, 60, 80 and 100. Treatments were dosed ruminally once daily. There were no NFC type×RDP source interactions for forage intake, digestion, or passage rate. Forage organic matter (OM) and total digestible OM intakes decreased linearly (P<0.05) in response to increased supplemental N from urea. However, NFC type did not affect intake. Digestibilities of OM and NDF were higher (P=0.05 and 0.06, respectively) when dextrose was provided compared with starch. Rumen solids passage rate initially decreased but then plateaued (quadratic; P=0.06) in response to increasing supplemental N from urea, and it was slower (P=0.02) for dextrose versus starch fed steers. The NFC types evaluated did not interact with source of supplemental RDP with regard to intake and digestion, although both 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.