Effects of test weight, precision processing and processing index on in situ ruminal digestibility of barley grain in beef heifers
The objective of this study was to evaluate the effects of test weight (TW; g/L), precision processing (PP) and processing index (PI) of barley grain on kinetics of in situ ruminal digestion. The study was designed as a 2×2×2 factorial arrangement with treatments: TW (low vs. high), PP (control vs. PP) and PI (75 vs. 85%). Ten barley samples with 5 low (574 g/L) and 5 high (632 g/L) TW were either dry-rolled with single roller setting (control) or sieved into small and large kernels, then dry-rolled based on kernel size of each fraction (i.e., PP). Each sample was dry-rolled moderately or coarsely with PI of 75 or 85%, which was calculated as TW after rolling/TW before rolling × 100%. Three beef heifers (650 ± 25 kg BW) fitted with rumen cannulas and fed diet consisting of 70% barley silage and 30% barley grain concentrate were used for in situ incubation. Kinetics of DM digestibility in situ after 0, 3, 6, 12, 24, and 48 h of incubation was estimated using the model: y=a+b (1-e-ct). Effective ruminal degradability (ED) of DM was estimated using equation ED = a + bc /(c + k) with k = 6%/h. There was no interaction between TW and PI on kinetic parameters, however, there was interaction between TW and PP on ED of DM (P < 0.01), and between PP and PI on potential degradable fraction, b (P < 0.01) and rate of degradation, c (P < 0.04). Overall, the TW of barley grain did not affect the kinetic parameters except that the ED was greater (P < 0.01) with high (43.4%) than low (41.8%) TW in control, but not in PP (low vs. high TW; 44.5 vs. 44.1%). Compared to control, PP reduced (P < 0.01) soluble fraction a (PP vs. control; 3.0 vs. 4.6%) and the b (73.6 vs. 75.4%), but increased (P < 0.01) the c (7.8 vs. 6.4%/h) and ED (44.3 vs. 42.6%). Decreasing PI from 85 to 75% considerably increased (P < 0.01) kinetics parameters, a (3.0 vs. 4.5%), b (71.7 vs. 77.3%), c (5.7 vs. 8.6%/h), and ED (37.2 vs. 49.7%). These results indicated that manipulating processing method such as PP and PI could effectively alter ruminal digestion kinetics of barley grain, whereas the effect of varying TW is limited.
Keywords: barley grain processing, in situ DM digestibility, beef heifers