|Hall, Mary Beth|
|Nennich, Tamilee - Purdue University|
|Doane, Perry - Archer Daniels Midland|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2015
Publication Date: 6/1/2015
Publication URL: http://handle.nal.usda.gov/10113/62047
Citation: Hall, M., Nennich, T.D., Doane, P.H., Brink, G.E. 2015. Total volatile fatty acid concentrations are unreliable estimators of treatment effects on ruminal fermentation in vivo. Journal of Dairy Science. 98(6):3988-3999.
Interpretive Summary: For scientists working in ruminant nutrition, volatile fatty acid (VFA) concentrations in rumen liquid have long been used to assess how changes in the diet impact rumen function and animal performance. However, a recent analysis of seven separate feeding studies showed that other factors, like rumen liquid volume, make VFA concentrations unreliable by themselves to test treatment effects. To allow valid comparisons of experimental treatments, responses must be on an equivalent basis; concentrations in different amounts of rumen liquid are not on an equivalent basis, and so are not valid for comparing treatment effects. Amounts of VFAs rather than concentrations are more reliable indicators. Based on this information, researchers will need to select responses that more accurately describe the effects of treatments on rumen fermentation so they can more reliably associate them with animal performance.
Technical Abstract: Volatile fatty acid concentrations ([VFA], mM) have long been used to assess impact of dietary treatments on ruminal fermentation in vivo. However, discrepancies in statistical results between VFA and VFA pool size (VFAmol), possibly related to ruminal digesta liquid amount (LIQ, kg), suggest issues with the use of VFA. We investigated relationships among VFA, VFAmol, and LIQ measured 2-h post-feeding using individual lactating cow data (n = 175) from 7 separate feeding studies. Regression analyses were performed using mixed models with “study” as a discrete random variable. The mean across studies and average range of values within studies, respectively, were 151 and 75 for VFA, 11.2 and 9.8 for VFAmol, 73.3 and 41.0 for LIQ, and 289 and 83 mmol/kg for rumen fluid osmolality. LIQ changed with VFAmol (3.76 VFAmol + 31.2; average within-study R2 = 0.69), but the relationship was weak between VFA and LIQ (0.524 LIQ + 112.8; average within-study R2 = 0.12). The relationship between LIQ and VFAmol was likely a function of the osmotic gradient between rumen liquid and blood. VFAmol is a major ruminal solute, and so it affects water entry or absorption from the rumen as similar tonicity of rumen fluid and blood are maintained. This also has a damping effect on ruminal solute concentration, creating the weak relationship between VFA and LIQ. Within studies, similar VFA were found in LIQ differing by 30 kg or more. The difference between minimum and maximum LIQ within cow within study was 12.7 kg (standard deviation = 7.1), so inclusion of “cow” in analyses does not correct for the variation in LIQ. To allow valid comparisons of experimental treatments, responses must be on an equivalent basis; concentrations in different LIQ are not on an equivalent basis, and so are not valid for comparing treatment effects. VFA changed with VFAmol (5.80 VFAmol + 86.3; average within-study R2= 0.56). However, the ratio of VFA/VFAmol ranged from 9.0 to 24.1 as a function of 1000/LIQ; this reflects the inherent calculated relationship among the variables. The varying relationship of VFA to VFAmol further indicates that VFA is not an appropriate measure to evaluate the progress or impact of treatments on VFA production. Predictions of LIQ and VFAmol using cow and ruminal measures were insufficiently precise to be used in research. Previously drawn conclusions based on VFA need to be reevaluated. Alternate evaluations for in vivo ruminal fermentation are needed.