aOMD – apparent organic matter digestibility

Apparent organic matter digestibility is the percent of organic matter digested. This can sometimes appear higher than what may occur in the animal because the FermentricsTM batch system is designed not to run out of protein. If the soluble protein levels are low in the TMR, aOMD may be even lower in the animal than indicated on the FermentricsTM report.

aOMD can be high but microbial biomass production low from too much acetate (which produces gas) and not as much propionate (more ATP for bacterial growth). The 2‐pool total gas production is highlycorrelated with aOMD with a higher 2‐pool gas total typically yielding a higher aOMD value, however, the relationship between gas production and aOMD must be considered. For example, a diet could have a very large slow pool (ml of gas from slow pool) yet exhibit a very slow rate (C:B3 Kd) with the majority of gas produced after 30‐hours. This diet would likely predict a reasonably high aOMD but the nutritional contribution in a high‐producing cow diet (with rapid rumen turnover rate) would be questionable due tothe extended length of the fermentation.