TTNDFD: A new approach to evaluate forage fiber digestibility

Abstract Text:

We hypothesize that an estimate of total tract NDF digestibility (TTNDFD) can provide useful information about fiber utilization.  Our objective is to present an approach for predicting in vivo total tract NDF digestibility from in vitro NDF digestibility measurements. The parameters needed to estimate TTNDFD include the proportion of potentially digestible fiber (pdNDF), the digestion rate of the pdNDF (kd), the ruminal passage rate of pdNDF (kp), and post-ruminal digestion of NDF. The in vitro TTNDFD approach accounts for ruminal and post-ruminal fiber digestion and can be adjusted for changes in fiber passage as size or intake of the animal changes.  The TTNDFD method has been validated with in vivo experiments.  In one study Lopes et al., (2013) compared estimates of TTNDFD as predicted by the in vitro model to in vivo measurements in lactating dairy cows.  Cows were fed diets that varied in proportions of corn silage and alfalfa. The diets contained  55% forage and the dietary NDF concentration was similar across treatments. Milk yields were similar amongst diets.  The observed (in vivo) TTNDFD values were calculated from feed and fecal samples.  Cows consuming the diet with alfalfa as the only forage had higher NDF digestibility than cows on the diets that contained corn silage. The NDF digestibility coefficients predicted by the in vitro TTNDFD method were similar to the in vivo values.  The TTNDFD analysis can provide important insights into fiber utilization by dairy cattle.  The rates of fiber degradation determined from the in vitro NDFD assays appear to be consistent with what has been measured in vivo feeding studies. The pdNDF, kd, and kp and parameters predicted by the TTNDFD model appear to be consistent with in vivo measures.

Effect of changing ratios of corn silage to alfalfa on intake, production and fiber digestion in dairy cows.

Corn silage (CS):alfalfa (AS) ratio		100CS  0AS	67CS 33AS	33CS 67AS	0CS 100AS
Corn silage, % of TMR		56	37	18	0
Alfalfa silage, % of TMR		0	19	37	55
Concentrate mix, % of TMR		44	44	45	45
Diet NDF, % of DM		24.9	25.5	24.6	25.5
	SE
DMI, kg/d		25.2ab	25.3a	24.3b	21.9c	0.8
4% FCM, kg/d		36.2	35.4	35.2	35.6	0.9
Observed TTNDFD, in vivo		38.3a	40.9ab	39.4ab	43.8c	1.9
Predicted TTNDFD, in vitro		38.0	41.0	41.0	45.0	2.1

^a,b,cValues within row with different supercripts differ, P<0.05.

Keywords: NDF digestibility, Fiber, Dairy