Essential in calculating digestibility of nutrients is the feces to food ratio.

You can do that by measuring total feed intake (usually somewhat restricted to avoid spillage  into the manure) and total feces output, or by measuring chromic oxide in both (dry) food and (dry) feces.

You must be sure that the sample in which you measure the chromium content is representative for the total output! Otherwise all your efforts might be worthless.

Simply calculate total amount of nutrient in the feces and that in the feed by using the feces/food ratio and the their ratio is the digestibility coefficient (see also attached paper).

Dr. Kan, thank you very much for your nice and concise answer. Could you please tell me, how can I add the greem color chromium oxide powder with feed? just mixing or anyothwer system?

We usually prepared up to 50 kg batches in a feed mixer. We first would make a small amount of premix to dilute the chromium oxide about 1 : 10 and then a second one in some kg of feed and then the final mix. Then mechanical mixer had an intense green color afterwards! Take your time (really minutes of mixing) to achieve a homogeneous product at every step. Remember that these steps are essential to obtain a reliable result.

Nevertheless you will always observe (unexplainable) differences in feces/to food ratios between the replicates in one treatment group. That is unavoidable in doing animal experiments.

If you find difficulty in estimating the Chromium Oxide, it is better to go for total collection method. You have to collect faecal output for at least 3days. You must have a record of feed intake during that period. Analyze the nutrients both in diet and faeces, which digestibility you want to evaluate. The following formula can be used

Apparent digestibility (%) = (Nutrient intake - Nutrient outgo)X100/ Nutrient intake 

For details please see the article

Dear Manirul I hope you have the formula you need, given the support offered by the colleagues already. I can only add that you should choose TiO2 instead, if you have the possibility. The reason being that Cr2O3 do separate into the iquid phase causing accuracy problems. It is also a proven carcinogen, so not recommended nowadays. You may read Sales and Janssens (2003) for a review in this regard. Titanium dioxide works very well, giving you minimal variability in the determinations- because you get a very high recovery rate of this material especially if you are sampling at the ileal level. When you analyse with spec just ensure that CV among the readings do not exceed 3%. There is also an atomic spectrophotometrical approach to determine TiO2 which is the current gold standard in mineral determinations. But the old reliable spec  still work wonders, absolutely.

By the way are you interested in ileal digestibility or you want to follow the excreta approach? If you are interested I could mail you a copy of my thesis for free. It contains details of some of the related methodologies.

Digestibility trial in broiler using chromic oxide:

A broiler chick bioassay was used to measure the effect of two inert digestibility markers on the determination of dietary AME. Diets contained 80% of either wheat or barley (with or without enzyme) and either chromic oxide at 0.5% or one of three levels of insoluble ash (0.5, 1.0, or 1.5%) as markers. The various cereal and marker diet combinations were consumed ad libitum (0 to 21 d) by two groups of 10 male broilers in each of two trials. The AME of each diet was determined by measuring the respective marker ratios between diet and excreta (collected for 24 h at 7 or 21 d) or ileal digesta collected at 21 d. Growth and feed conversion were measured on each group of birds between 0 and 21 d. There was no effect of marker on growth or feed efficiency. However, determination of AME of wheat- or barley-based diets with or without enzymes were affected by choice of marker and whether markers were measured in excreta (7 or 21 d) or ileal digesta. Chromic oxide was viewed as the least accurate method for determining AME, based on chronic oxide's inability to define AME differences between barley-based diets with and without enzymes, whereas insoluble ash clearly demonstrated improved AME of wheat- and barley-based diets with an enzyme. The optimum levels of insoluble ash for accuracy and repeatability were between 0.5 and 1.0%. The AME of the diets were, on average, 5% lower when determined with 7 vs 21 d excreta and 2.5% lower for ileal digesta than excreta collected at 21 d. It was concluded that identification of components that result in variability in AME levels of diets will be improved if a bioassay uses insoluble ash as a marker.

I am so grateful to you all for your nice explanation. I'm a bit confuse. I have mixed .2% of chromium oxide with specific amount of feed and started to feed the birds. I'm cheching the feces color this time but not started to collect the feces. It will continue for 3 days and then 1 day (24 hrs) I will off fed the birds. And then Again, weigh the feed and start to feed the birds and will collect all feces for 3 days. Finally, will calculate total intake and total feces outgo and calculate direct method of digestibility by the above formula provided by Dr. Subrat Kumar Vhanja. In addition, I will analyse the feed and feces chromium content by atomic absorption spectrometry to measure the digestibility (indirect method) of nutrient using the formula,

Digestibility (%) = [1-{(Nf × Cd) /(Nd × Cf)}] × 100

Where,

Nf = Nutrient concentration in feces (%DM)

Nd = Nutrient concentration in diet (%DM)

Cf = Chromium concentration in feces (%DM)

Cd = Chromium concentration in diet (%DM)

So, could you please comment on it, whether my procedure is right or any suggestion from you?

I think using Titanium dioxide as a marker is easier and reliable than  chromium oxide, but my concern is to gave the animal two days (48 hour) adaption period before go the main period where you need to measure the total feed intake and extract voided and express them on a dry matter basis. 

According to my literature search, I agree with colleagues that currently Titanium dioxide should be considered for digestibility studies especially in non-ruminants like chickens, see review attached. However i have failed to find the clear protocol for using and analyzing Titanium Dioxide.

Dear David Akinde and any other who can provide information help out? I am also interested in the information

Determination of titanium dioxide added as an inert marker in chicken digestibility studies

F.J. Short, P. Gorton, J. Wiseman, K.N. Boorman

DOI: http://dx.doi.org/10.1016/0377-8401(95)00916-7

From my readings I agree with colleagues that  Titanium dioxide should be used for  chickens