Animal and digestibility marker variation influence predictions of dry matter intake and dry matter digestibility

The current literature shows that there is animal and marker variability in digestibility marker recovery, but does not address the effect of this variability on individual animal DMI predictions. The objective of this study was to test the use of various markers and administration methodologies to predict DMI in grazing systems and determine the main contributors to variability. Eight rumen cannulated Nellore steers were randomly assigned to two 4 x 4 Latin squares. Steers had ad libitum access to pasture of Brachiaria brizantha. Steers were either not supplemented or individually supplemented with a mixture of fine ground corn and sodium monensin at 0.3, 0.6 or 0.9% of body weight. Steers were each administered 3 external markers via rumen cannula: a C32 controlled release capsule (CRC) on d 3; LIPE® (purified lignin), once daily on d 7 to 15; and Cr2O3 once daily on d 1 to 15 of each period. Fecal grab samples were collected twice daily for the last 5 d of the 15-d periods. Intake predictions were calculated using the Large Ruminant Nutrition System (LRNS), C31:C32 ratio, C33:C32 ratio, LIPE®, and Cr2O3. Indigestible DM (iDM), indigestible NDF (iNDF) and indigestible ADF (iADF) were used as internal markers. LIPE® was determined by infrared spectroscopy, Cr2O3 by inductively coupled plasma optical emission spectroscopy and alkanes by gas chromatography. A 240-h rumen incubation was used for the internal markers. A mixed model with the fixed effect of supplementation level and random effects of Latin square, animal(Latin square), period, and animal x period was used to determine treatment effects. LIPE® and Cr2O3 detected a treatment effect on pasture intake (P < 0.10) except for the Cr2O3/iADF combination (P = 0.43). Only LIPE® detected a treatment effect on total intake (P < 0.03). The alkanes provided greater and variable intake predictions. The final 4 d in each period demonstrated decreased CRC release rate compared with the previous 8 (mm/d = -0.012*d2 + 0.091*d + 3.93, R2 = 0.79). A completely random model containing internal and external markers, and their interactions with animal, period, and supplementation level determined which variables contributed to intake prediction variability. Variability was mainly due to internal and external markers (11-66% of variation), residual error (11-24% of variation), and external marker x animal interactions (5-16% of variation). Digestibility markers should be used cautiously to predict individual intakes i