In Vitro True Digestibility (IVTD) Fact Sheet

Background
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.

Application
To 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 or varieties.

Method

1. Sample required: standard 1 lb. (0.5kg) forage sample.
2. Sample preparation: dried at 60 C for 4 hours and ground through a 1mm UDY Cyclone mill.
3. Sample size for analysis: 250 mg.
4. Incubator: ANKOM Daisy II.
5. Method: ANKOM Filter Bag Technique (FBT). F57 filter bags are prerinsed in acetone and dried prior to filling with sample.
6. Buffer: Van Soest.
7. Incubation period: 24, 30 or 48 hour end point measurement.

Donor Cows

Herd data: 436 cows, 2X milking, 22,852 lbs. milk, 823 lbs. fat (3.6%), 742 lbs. protein (3.3%).

Facilities: 500 cow freestall, double 14 rapid exit parlor.

Donor 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.

• TMR fed 1X/day with frequent push-ups and daily removal of leftover feed.
• Forage: Concentrate, 55:45 DM basis.
• Forages: corn silage/haylage, 70/30 DM basis.
• Concentrates: corn/soy based mixed grain ration including supplemental fat.

For Best Results

1.	FORAGES 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 useful information.
2.	MAXIMIZE 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.
3.	IVTD analyses will add a day or two beyond the normal analyses. Please take this into consideration when submitting samples.

Notes

1.

In vitro 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.

2.

True vs. 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. The first 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.