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 (× 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 × 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<0.001) as BD decreased. In Exp. 1, DMD of sorghum was greater (P<0.10) than that of corn at 24 and 48 h of incubation. For both grains, DMD increased linearly (P<0.02) as BD decreased at 4 and 8 h. Corn culture NH₃ was less (P<0.10) than sorghum at 8 h, and as BD decreased at 4, 8, and 12 h, NH₃ decreased linearly (P<0.05). Culture pH was less for corn than for sorghum at 8 h and decreased linearly (P<0.04) with decreasing BD at 12, 18, and 24 h. In Exp. 2, source of N did not affect (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.