Inclusion of forage standards with known in vivo digestibility in in vitro procedures

Abstract

The objective of this experiment was to establish the in vivo and in vitro digestibility of multiple forages for inclusion in future in vitro dry matter disappearance (IVDMD) procedures thus enabling adjustment to in vivo values. Eight, Angus x Simmental yearling steers (body weight 323±29kg) were used in a 5×5 Latin square design to determine in vivo dry matter (DM) digestibility of five forages. The five forages were immature alfalfa (Medicago sativa), mature alfalfa, immature smooth bromegrass (Bromus inermis), mature smooth bromegrass, and prairie grass hay (17.6%, 16.3%, 9.3%, 7.5%, and 7.9% CP and 60.5%, 67.9%, 66.7%, 69.6%, and 68.3%, NDF respectively). Each of the five samples were included in 21 separate in vitro runs and compared to in vivo digestibility. Ruminal fluid was collected from steers fed 100% smooth bromegrass hay for nine of the 21 in vitro runs and for the remaining 12 in vitro runs ruminal fluid was collected from steers fed 70% forage, 30% concentrate diets. In vitro DM disappearance was used as the dependent variable and regressed on in vivo DM disappearance. Significant differences in IVDMD (P<0.001) occurred among individual in vitro runs; however, there was no difference (P=0.99) in the slopes of linear regression lines fitted to the IVDMD values among runs. The average IVDMD from all runs and in vivo DM digestibility was correlated (R2=0.93 and 0.88 for 100% forage diet inoculum and mixed diet inoculum, respectively) with coefficients of determination within each of the 21 in vitro runs ranging from 0.54 to 0.97. On average, IVDMD was 15% greater (P<0.05) but adjusted IVDMD was not different (P>0.31) than in vivo DM digestibility. Mean square error of prediction was smaller when IVDMD was adjusted to in vivo values using an equation derived from the average of all in vitro runs compared to the average mean square error of prediction from the 21 individual runs. A regression equation derived from standards with known in vivo digestibility included in each individual in vitro run can be used to adjust IVDMD values to in vivo values.