The objective of this study was to evaluate the pre-grinding superconditioning pelleted corn in comparison with steam-flaked corn on gas production in vitro. An Iranian corn variety (single crass 702) was processed by 1) grinding (G), 2) dry rolling (DR), 3) steam-flaking (SF; moisture 20%, retention time 60 min, rolling temperature 105á´1/4C) and 4) pre-grinding super-conditioning pelleted corn (PSCP; moisture20%, retention time 6 min and conditioning temperature 95á´1/4C). In gas production test, gas pressure and volume were recorded 2, 4, 6, 8, 12, 24, 36 and 48 h after incubation. Gas production kinetics were estimated based on equations without a lag phase. Data were analyzed by GLM procedure of SAS (P < 0.05) using a completely randomized design with 6 replications for every treatment by the model; Y = Î1/4 +Txi+Îμij, were Y is the variable, Txi is the effect due to the treatment, and Îμij is the experimental error. The GP of PSCP in 6 h was significantly increased (P < 0.05) (163.53, 98.15, 56.24 and 64.50 with SEM 12.76 mL/gDM in PSCP, SF, DR and G, respectively), but in 48 h GP in PSCP and SF was similar (298.7 vis 297.5 mL/gDM). The pH and N-NH3 in 24 h were significantly lower (P < 0.05) in PSCP than other processing methods (P < 0.05). Total VFA after 24 h incubation was significantly increased (P < 0.05) in PSCP in than other processing methods (73.91, 70.74, 70.21 and 68.42 with SEM 0.756 Mmol/ml in PSCP, SF, DR and G, respectively). The SF corn showed higher (P < 0.05) acetate (62.94%), lower (P < 0.05) propionate (24.24%) and higher (P < 0.05) acetate to propionate ratio (2.60) between treatments. However, lower (P < 0.05) acetate (58.23%), higher (P < 0.05) propionate (28.88%) and lower (P < 0.05) acetate to propionate ratio (2.02) were observed in PSCP between treatments. Asymptote gas production (A) was not affected by different processing methods, but time to produce half of A (K; 5.74 h), time for ferment of 25% (2.45 h) and 75% (13.42 h) of substrate were significantly reduced (P < 0.05) by PSCP processing. The in vitro gas production fermentation rate (c) was higher (P < 0.05) for PSCP (0.113, 0.056, 0.046 and 0.041 /h with SEM 0.008 in PSCP, SF, DR and G, respectively). It seems that PSCP in comparison to SF increased in gas productivity and starch fermentability. Short time for achieving maximum gas production in PSCP corn indicated that PSCP was an effective way for enhancing of corn fermentability in the rumen.

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