Effects of corn kernel hardness and grain drying temperature on particle size and pellet durability when grinding using a roller mill or hammermill

Corn kernel hardness and drying temperature may affect particle size and distribution after grinding, impacting feed mill productivity and animal performance. In the present study, the effects of two corn hybrids with different kernel hardness (average and hard respectively) and three-grain drying temperatures (35, 80, and 120 degrees C) on the geometric mean (d(gw)) and standard deviation (S-gw) of particle size were assessed. For roller-milling, four combinations of roller gap settings were utilized (0-0, 25-20, 30-30, and 35-35), whereas, for hammermilling, three tip speeds were used (1508; 4207 and 6320 m/min) in each case grinding through a 12/64'', 4.76 mm (12-12) screen combination. This evaluation was conducted in corn stored for less than 2 weeks (wk) for new crop and 30 wk. Additionally, at 2, 4, 8, 12, 17, and 20 wk of storage, treatments were ground by a hammermill to study the effects of storage on dgw and Sgw. After that, these ground batches of corn were included in feeds to investigate pellet durability. For evaluations performed in new crop corn, and at 2, 4, and 8 wk of storage, treatments were arranged in a 2 x 3 factorial design with two kernel hardness and three drying temperatures. In contrast, evaluations at 12, 17, and 20 wk, treatments consisted of a 2 x 2 factorial design with two-particle sizes and two drying temperatures. For all evaluations, three replicates per treatment combination were considered. Data were analyzed in a completely randomized design, and mean separation was done using Tukey's or student's t-tests. Results in new crop corn showed that at 35 degrees C level, harder kernel corn yielded larger (P < 0.001) dgw compared to the hybrid with average kernel hardness when roller milling with 0-0 and 35-35 roll gap. Hammermilling (4207 and 6320 m/min) corn with hard kernel and dried at 120 degrees C resulted in greater dgw (P < 0.001) than similar corn but dried at 80 and 35 degrees C. At 30 wk of storage, average kernel corn and dried at 80 degrees C yielded larger (P < 0.01) dgw and lower (P < 0.001) Sgw in comparison to similar corn but dried at 35 degrees C when hammermilling at 6320 m/min. Regression analysis showed that storage time decreased (P < 0.05) the dgw in corn with average and hard kernel hardness, when hammermilling corn at 6320 m/min. The pellet durability index (PDI) was better for diets with average kernel hardness and corn dried at 35 degrees C than at higher temperatures. The particle size and moisture of corn affected PDI. This study shows the impacts of kernel hardness and drying temperature on PDI, dgw, and Sgw regardless of the grinding method and confirms that dgw is reduced as storage time increases.