Influence of Rumen Inoculum Source on In Vitro Dry Matter Digestibility of Pasture

The objective of this study was to evaluate the effects of rumen inoculum donor source [pasture diet vs total mixed ration (TMR) or individual cow effects] on in vitro DM digestibility (IVDMD) of pasture. Pasture samples (four levels of pasture diversity from a larger study) were digested in vitro (Tilley and Terry method) using various rumen inoculum sources. Rumen inoculum was collected from four fistulated lactating dairy cows grazing pasture (one cow per pasture diversity treatment). Additionally, rumen inoculum was collected from a confined lactating dairy cow fed a TMR. Samples digested in rumen inoculum from the TMR donor cow diet resulted in lesser (P<0.05) IVDMD values than samples digested in rumen inoculum from pastured cows. Cow-to-cow variation was also significant (P<0.05). When applied to a pasture DMI equation, IVDMD values obtained from a TMR-fed cow resulted in a 19.3% decrease in estimated pasture DMI as opposed to when IVDMD values obtained from pastured cows were inserted into the DMI equation. These results emphasize the importance of researching laboratory and scientific methods used in obtaining IVDMD values for pastures. Use a donor cow that is consuming a diet similar to that being tested whenever possible. Extra caution should be taken when interpreting the IVDMD results obtained on pasture samples from outside labs, especially if the donor cow diet is unknown. Ingredient and nutrient composition of the donor cow diet should be described when reporting IVDMD results in the literature.