Effects of Degree of Processing and Nitrogen Source and Level on Starch Availability and in Vitro Fermentation of Corn and Sorghum Grain

Ruminal fluid from two heifers (384 ± 2 kg) fed an 85% concentrate diet was used to determine effects of degree of processing and N source and level on in vitro DM disappearance (DMD), starch availability, pH changes, and NH3 concentrations of corn and sorghum. In Exp. 1, treatments (2 × 4 factorial) were unprocessed corn and sorghum (UP) and each grain steam-flaked to bulk densities (BD) of 0.38, 0.33, and 0.28 kg/L (C38, B33, A28, respectively). Treatments in Exp. 2 (2 × 2 × 3 factorial within grain type) were UP and A28 with 0, 10, or 20 mg of N/g provided by either urea (U) or casein hydrolysate (CH). Starch availability of both grains increased linearly (P 0.10) DMD of corn or sorghum beyond 4 h, but DMD increased linearly (P<0.04) as N level increased at 8, 12, 18, and 24 h for corn and at all incubation times for sorghum. At 8 through 24 h, CH and U increased NH3 of UP and A28 corn linearly (P<0.001), but the magnitude was greater (P<0.001) for U than for CH; the same trends were evident (P<0.001) at 12, 18, and 24 h for NH3 of sorghum treatments. Increasing N level increased pH of corn linearly (P<0.05) at 4, 8, 18, and 24 h, with a greater increase for U than for CH. Increasing N level increased pH of sorghum linearly (P<0.03) at 1 through 24 h, but to a greater extent for U than CH at 1, 2, 12, and 18 h. Degree of processing and level of supplemental N were positively related to in vitro DMD, whereas no benefit was evident from supplementing amino acid N compared to U.