In Vitro True Digestibility (IVTD)
IVTD is an anaerobic
fermentation performed in the laboratory to simulate digestion as
it occurs in the rumen. Rumen fluid is collected from ruminally
cannulated high producing dairy cows consuming a typical TMR diet.
Forage samples are incubated in rumen fluid and buffer for a specified
period of time at 39 C. During this time, the microbial population
in the rumen fluid digests the sample as would occur in the rumen.
The end result of the IVTD procedure is the undigested fibrous residue.
This value is the undigested NDF of the forage which in turn, can
be used to determine the digestible NDF (NDFD). Both the IVTD and
dNDF can be used to relatively rank and compare forages. The NDFD
can be substituted into our energy prediction scheme to derive a
biological estimate of energy.
derive more precise estimates of forage digestibility than can be
predicted from compositional data. Most useful:
as a diagnostic tool when herds are not performing as predicted
using conventional analyses.
- fine tuning rations for exceptional herds.
- relative ranking of forage digestibilities for comparing hybrids
required: standard 1 lb. (0.5kg) forage sample.
preparation: dried at 60 C for 4 hours and ground through a
1mm UDY Cyclone mill.
size for analysis: 250 mg.
ANKOM Daisy II.
ANKOM Filter Bag Technique (FBT). F57 filter bags are prerinsed
in acetone and dried prior to filling with sample.
period: 24, 30 or 48 hour end point measurement.
data: 436 cows, 2X milking, 22,852 lbs. milk, 823 lbs. fat (3.6%),
742 lbs. protein (3.3%).
500 cow freestall, double 14 rapid exit parlor.
cows: Five cows averaging 80 lbs. milk/day. Cows are housed
and managed as part of the herd. No special treatment. Cows cease
being donors once they drop below 60 lbs. milk/day. Fluid is collected
from one cow on collection day.
fed 1X/day with frequent push-ups and daily removal of leftover
55:45 DM basis.
corn silage/haylage, 70/30 DM basis.
corn/soy based mixed grain ration including supplemental fat.
ARE THE KEY. The procedure was developed for and works best
on forages. If cows are not milking well, analyzing a TMR will
tell you what you already know - that it is less digestible
than predicted. The primary culprit is probably one of the forages.
Analyzing the most suspect or all forages will yield the most
ACCURACY. With all biological measurements, variation can occur
from run to run. All steps are taken to minimize this variation.
For best results, submit all samples for IVTD evaluation at
the same time. This is particularly pertinent for research or
field trial samples involving comparisons between multiple treatments
or hybrids. Call ahead and we will make arrangements to process
your samples as a batch.
will add a day or two beyond the normal analyses. Please take
this into consideration when submitting samples.
analyses are not standardized. Factors including buffer, particle
size, source of rumen fluid, incubation time, etc. can influence
the results. It is inappropriate to directly compare results
across in vitro systems or with in situ results. The best that
you can hope for is that the relative ranking of samples between
systems will be similar.
Apparent Digestibility In Vitro Dry Matter Digestibility (IVDMD)
or apparent digestibility is the classic two stage Tilley
& Terry procedure. The first stage is a 48 hour incubation
in rumen fluid and buffer followed by a second 48 hour digestion
in pepsin and HCl.
stage of the In Vitro True Digestibility (IVTD) is a 24, 30
or 48 hour incubation in rumen fluid and buffer. The second
stage substitutes an NDF extraction for the pepsin and HCl.
The NDF extraction more completely removes bacterial residues
and other pepsin insoluble material yielding a residue free
of microbial contamination. Additionally, it shortens the
analysis time by two days. In general, IVDMD can be estimated
by subtracting 11.9 form the IVTD.