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18 Cards in this Set

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Digestible Feed

The portion of a feed that is not excreted in feces and is assumed to be absorbed by the animal. Expressed as a coefficient or percentage

A cow eats 15 kg of hay DM and excretes 6 kg of DM in the feces. What is the digestibility?

DM digestibility = (consumed - excreted)/consumed



DM digestibility = (15-6)/15 = 0.60 or 60%

In vivo method: total collection

feed a known quantity of feed to the animal. Collect total decal output for 7 - 14 days after a minimum of one week of feeding. Analyze the feed and feces for the nutrient under consideration (could be DM, CP, NDF, EE, etc.)

In vivo method: indicator method

feed a known quantity of feed with a known concentration of an indigestible indicator. This is your "marker". Collect fecal samples for at least 5 days after a minimum of one week of feeding the marker. Use indicator concentration in the feces to estimate fecal output:


Fecal output = marker dose (g/day)/marker concentration in feces (g/kg)

Marker/indicator types

may be a natural part of the feed (ex. lignin) or a chemical such as chromic oxide

Ex. An animal is given 10g of chromic oxide/day. Concentration in feces is 4g/kg decal DM. What is your fecal output?

Fecal Output = marker dose (g/day)/marker concentration in feces DM (g/kg)



Fecal Output = 10/4 = 2.5 kg DM/day

In vitro method

Lab method that simulates animal digestion.


Non-ruminants - protein digestion determined by digesting feed in pepsin and HCl


Ruminants - feed is incubated for 48hr in rumen fluid under anaerobic conditions. Bacteria are then killed by acidifying with HCl and digested with pepsin for 48 hr.

In sacco method for ruminants

Ground feed samples (3-5g DM, 2mm in size) are placed in small nylon bags and put in the rumen of cannulated cows and incubated for 24-48 hrs. After incubation, bags are washed in ice water, dried, weighed, and contents analyzed. Digestibility determined using how much feed has disappeared inside the bag

In sacco method for pigs and dogs

Animals are cannulated at ileum and fed a known amount of feed, the "ileal digesta" volume is calculated by using an indigestible marker. Digestibility up to the ileum is determined.

How feed composition affects digestibility

- grains show less variability in digestibility than forages because of a more consistent composition


- fibre fraction has the greatest influence on digestibility (more lignin = less digestible)


- in ruminants, excess lipids inhibit rumen bacteria, thereby decreasing digestibility


- low protein feeds cause low rumen ammonia. this causes poor microbial growth and, in turn, low digestibility

How diet composition affects digestibility

- digestibility is influenced by a feed's composition, as well as the composition of feeds consumed with it


- this effect is greatest when low quality forage is fed with a starchy ingredient like grain. Rapid fermentation of starch to VFA lowers rumen pH, decreasing fibre digestibility.

How feed preparation affects digestibility

- excessive grinding of forages reduces digestibility of fibre due to rapid passage through the rumen


- heat treatment of grains gelatinizes the starch, increasing digestibility


- B-glucanase (an enzyme treatment) is added to barley diets for poultry

How does the level of feed intake affect digestibility

Increased intake = increased rate of passage = decreased digestibility

Digestibility formula

Digestibility = (nutrient consumed - nutrient in feces)/nutrient consumed

What can you do if fecal output can't be measured directly?

use an indigestible marker to help estimate output

Digestibility can be measure using these 3 methods

in vivo, in vitro, or in sacco methods

Why are digestibility studies a measure of apparent digestibility, not true digestibility?

Measuring digestibility at the ileum is more true than at the feces which is apparent. Bacteria and sloughing that occurs in the hindgut doesn't get re-absorbed and contributes to the nitrogen content in the feces, which could be falsely contributed to the feed.

Digestibility varies with what 3 parameters?

1. feed composition


2. diet composition


3. level of feed intake