Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2007
Publication Date: 6/2/2010
Citation: Bossen, D., Mertens, D.R., Weisbierg, M.R. 2010. Influence of Fermentation Methods on NDF Degradation Parameters. Journal of Dairy Science. 91(4):1464-1476.
Interpretive Summary: In vitro artifical rumen (IV) and in situ fermentation bags in cows (IS) are methods often used to estimate degradation characteristics of feeds. But questions remain about the effects of feed particle size and methods on measuring digestion kinetics (rates and extents) of feeds. Corn silage, grass silage, barley, beet pulp and rapeseed cake were ground through screens with 1, 2, 4 or 8-mm openings and fermented by IS or IV methods. The IV fermentations were done at pH of 6.8 or a lower pH (6.0) that more closely corresponded with that of the cows used for IS determinations. Results indicate that particle size had little impact on degradation regardless of the method. However, the IS method resulted in lower rates of digestion and less degradable and indegradable fiber than the IV method. It appears that both lower pH in the cows' rumens and losses of particles from bags affect IS results. We conclude that the IV method at pH 6.8 provides the most accurate estimate of the maximum protential digestion kinetics of a feed. This is important for evaluating intrinsic feed differences. The IS method may provide estimates of digestion kinetics that have value only if the ruminal condition of the cows used for IS are similar to the cows in practical feeding situations.
Technical Abstract: The effect of three fermentation methods, in situ (IS) in four lactating cows (average pH of 5.8), in vitro (IVn) with media pH of 6.8, or in vitro (IVa) with media pH adjusted to 6.0 using citric acid, on fiber degradation parameters was studied using feeds ground to different particle sizes. Corn silage (CS), grass silage (GS), barley grain (B), sugar beet pulp (BP), and rapeseed cake (RC) were ground using a shear mill. Silages were ground through 8, 4, 2 or 1-mm screens, B and BP through 4, 2 or 1-mm screens, and RC through 2 or 1-mm screens. The amylase-treated NDF (aNDF) content of samples ground using a 1-mm screen was 399, 431, 197, 480, and 251 g kg-1 DM for CS, GS, B, BP, and RC, respectively, but increased with increasing screen size. Materials were incubated for 0, 6, 12, 24, 48 and 96 h either IS, IVn, or IVa. Inoculum for IVn and IVa was prepared as a composite from the cows used for IS. The potentially degradable aNDF (D0), indegradable aNDF (I), lag time (L) and fractional rate of degradation of potential degradable aNDF (kd) were estimated using PROC NLIN in SAS. Except for RC, fermentation methods affected most degradation parameters, especially kd and L. The IVn, IVa and IS methods resulted in kd values of 0.291, 0.105, 0.080 h-1 and 0.262, 0.107, 0.103 h-1 for BP and RC, respectively, demonstrating a decreasing rate of degradation for these feeds when fermented under suboptimal pH conditions. In CS, GS and B, no difference was found in kd between the IVn and IVa methods, which suggests that differences in pH did not alter kd in vitro. The kd values obtained for CS, GS and B were 0.058, 0.109, 0.168 and 0.028, 0.054 and 0.069 h-1 for the IVn and IS methods, respectively, indicating that the IS method using cows fed at production levels can underestimate the potential rate of NDF degradation. Using the IVa method, L was 12.1, 9.1, 7.8 and 2.5 h for CS, GS, BP and RC respectively, which was higher than L obtained from the IVn and IS methods for all feeds except B, where L in all methods were near the parameter boundary of zero hour in NLIN. It was concluded that fermentation methods were more important than grinding screen size on estimates for feed aNDF degradation and that the individual aNDF degradation parameters for the five feeds was affected differently by fermentation methods.