Effect of compound feed pelleting and die diameter on rumen fermentation in beef cattle fed high concentrate diets

The effect of feeding a compound feed as meal (M) or pelleted at 3.5 (P3.5) or 10 (P10) mm i.d. on feed intake pattern and rumen fermentation in growing calves was investigated. Compound feeds were fed ad libitum with barley straw to six 3-mo-old rumen cannulated calves in a double 3 × 3 Latin square arrangement. In a first 9-wk phase (I) free access to concentrate was allowed, whereas in a second 6-wk phase (II) access was restricted to 09:00 h to 11:30 h and 17:00 h to 19:30 h. Rumen samples were collected on two non-consecutive days at 08:30 h, 13:00 h and 17:00 h. Pelleting increased the degree of starch gelatinization from 0.113 in M to 0.205 and 0.154, in P3.5 and P10. In both phases, P10 slowed consumption of concentrate during morning feeding (P<0.01), although treatments did not affect total daily concentrate intake (97 g DM/kg LW0.75 and 82 g DM/kg LW0.75) in phases I and II. In phase I, pH before morning distribution was lower than in phase II (6.5 versus 7.2). Four hours after feeding, there was a pH drop (P<0.001), which was higher in phase II (1.5 units) than in phase I (0.7 units), in line with a higher concentrate intake (4.10 kg versus 1.89 kg). Calves fed P3.5 tended (P=0.052) to a lower pH than those on M in phase I, and those fed P10 had the least decrease in pH after feeding. No pH differences occurred between treatments in phase II, although pH recovery from 4 to 8 h after feeding tended to be more (P=0.073) for P10. Rumen lactic acid concentration increased after feeding (P<0.05 and P<0.001) in phases I and II, reaching 45.7 mg/l and 39.6 mg/l, respectively. In phase II, P3.5 animals reached a higher (P<0.01) concentration than M and P10 animals. Rumen ammonia concentration decreased (P<0.001) after feeding, reaching the lowest values 4 h later (23.3 mg/l and 13.6 mg/l in phases I and II). P3.5 showed lower ammonia concentration than M in phase I (P<0.05). Total volatile fatty acid (VFA) concentration increased (P<0.001) after feeding in both phases, decreasing the acetic to propionic ratio (P<0.001). In phase I, calves fed M tended (P=0.074) to have lower total VFA rumen concentration and higher acetic to propionic ratio (P<0.01) than those fed pellets. Results suggest that with pelleting at 3.5 mm, increased ruminal fermentation rate dominates regulation of substrate delivery and results in lower pH values that could eventually translate into higher risk of acidosis compared with unprocessed meal. Increasing the pellet diameter to 10 mm may decrease the rate of fermentation through a homogeneous daily intake pattern, without affecting total intake, even under conditions of restricted feeding.