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Summary Principles of Animal Nutrition
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The summary covers the theory explained in the lectures, tutorials and practicals and the lecture notes of the course Principles of Animal Nutrition.
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PRINCIPLES OF ANIMAL NUTRITIONLECTURE 1 - Introduction
In animal nutrition, it is important to match the requirements for certain nutrients of an animal at any
moment during its life cycle (growth, pregnancy, lactation, dry period) with the value of those nutrients in a
diet (= the value of nutrients in the diet should be equal to the requirement of the animal for those
nutrients).
Before 1950 mixed farming: animals are “waste converters”, i.e. they were eating the wastes of the farm
e.g. deriving from crops
1950-1990 focus on efficient feed conversion for animals
From 90 onwards focus on functional feeds: insoluble fibres, diurnal feeding, the society emphasizes
more animal welfare and the environment.
Three types of values of feedstuffs:
- Energy value, i.e. how much energy can be derived from a certain feed (net energy value or energy
balance is the most important)
- Protein value, it refers to the amino acid content of the feed
- Physical value, which is the feed form. The form of feed can influence digestion and defecation
process.
A complete diet should combine:
- Palatability: e.g. the taste must be good for the animal
- Feeding value of feedstuff (i values scritti sopra)
- The content of nutrients (protein, fat etc) should be optimal
- Optimising value for money
Nutrient conversion: feed (nutrients) animal food (animal product e.g. milk, meat)
It is a two-step process: 1) digestion and absorption of nutrients, and 2) utilisation of nutrients for
production and maintenance
Protein digestion and utilisation: N is nitrogen, which is one of the main components of proteins
- Nintake (100%) = Ndigested (82-86%) + Nundigested (14-18%)
- Ndigested (82-86%) = Nutilised (45-60%) + Nnon-utilised (22-41%)
- NB: only about half of the digested nitrogen is actually used, so a lot of nitrogen is lost
- The undigested and non-utilised part of nitrogen is lost in faeces and urine.
Conversion of nutrients into animal product is dependent on both the feed and on the animal:
- Feed characteristics: level of nutrients, palatability, volume and structure of feed, digestibility
- Animal characteristics: maximum feed intake capacity (how much feed they can eat) which
depends on age, sex and breed; and level of utilization, i.e. the production level of the animal
influences how much nutrients are converted to be used in production.
,LECTURE 2 – Chemical analysis
Composition of feed:
- Water
- Dry matter: divided into
o Ash (inorganic matter): divided into a soluble fraction and an insoluble fraction. Includes all
minerals and salts
o Organic matter: proteins, fats, carbohydrates, vitamins
- NB: the nutritional value of feed is determined only by the dry matter, not water
- Nutritional value of proteins depends on its amino acid composition
- Nutritional value of carbohydrates in a feed depends on the ratio (rapporto) between structural
(i.e. fibre) and non-structural carbohydrates (I.e. sugars and starch) found in that feed.
- For example, soy bean feed is mainly containing protein; maize feed contains low protein but high
starch content.
Composition of a diet can be known by:
- Calculation from table values for each individual feed ingredient (nutrient), it is easy and cheap but
less accurate (= for each type of feed, there is a table that says what is the content of protein, fat
etc)
- Making analysis of the diet, it is expensive but more accurate
Principles of analytical methods (secondo punto):
- Analyses are made to have more accurate information about the chemical composition of feed
- Table values (calculation from table values) are always an average for specific feed ingredients
- The composition of a diet depends also on the accuracy of mixing different types of feeds, e.g. if
you mix maize and soy bean feed they have to be mixed in optimal quantities to avoid that there is
a disbalance in the content of protein and starch (carbohydrate) in the diet.
Weende analysis for analysis of feed: it does not analyse organic matter!
- It allows to get an accurate picture of the nutritional composition of feed by using simple
procedures and equipment
- Advantage: Easy to perform, cheap and you can compare results of different laboratories and also
different countries.
- It is made by determining the composition of feed in groups of nutrients:
o Water: the water content of a feed depends on how it is stored. The water content is
determined by drying feed at 103°C.
o Ash/inorganic matter: it is determined by removing the organic matter from the sample by
burning it. The inorganic matter that remains includes minerals.
Ash insoluble in HCl it is a part of ash that is indigestible for the animal (because
the HCl of gastric juice cannot degrade it). Therefore, it is used as marker to
estimate the digestibility of other components in the diet.
o Crude protein/N-containing matter: includes all proteins and amino acids. Most proteins
contain on average 16% of N. Usually, the estimate for crude protein in a feed is calculated
as N content*6.25.
Kjeldahl method it determines all components of the feed that contain a NH2
group. It determines the ammonia, so with this method all nitrogen in the feed is
converted to ammonia.
, o Crude fat: it is not soluble in water, so it can be extracted with a non-polar solvent (also fat
is non-polar).
o Carbohydrates: it is a heterogeneous group so carbohydrates must be divided into sugars,
starch etc; carbohydrates in this analysis are divided into:
Crude fibre: the organic fraction that is not soluble in acid and basic solution. It
includes cellulose, hemicellulose and lignin. It has low digestibility.
N-free extractives: include the starch and sugars, and the residue fraction. It can be
determined with organic matter – (crude fibre+crude fat+crude protein).
Alternative analytical methods:
- They are needed to determine specific chemical compounds e.g. a specific mineral or amino acid.
- They require the preparation of a sample and use of sophisticated equipment and analytical skills,
so it is much more expensive.
- Proximate analysis: gives an indication of the chemical composition of feed, but it cannot calculate
the detailed nutritional value of the feed and does not give information on the composition of each
fraction of nutrients.
- Van Soest analysis: gives more information about the composition of the fraction of fibre
(structural carbohydrates).
- NIR: Near Infrared Spectroscopy determines the composition of feed based on the absorption of
infrared light by the solid fraction of feed.
o Advantage you can determine all nutrients at the same time.
o Disadvantage results are not accepted for scientific work and you need to use a different
calibration for different types of feeds.
Pre-treatment procedures in the Weende/proximate analysis:
- Sampling: you collect different samples from the (same) feed and combine them. The total amount
of sample should be around 0.5kg
- Drying: the sample should be dried; some feeds might require chopping or cutting before drying.
- Grinding: samples are ground to around 1 mm and mixed. Samples with a high fat content should
be first defatted.
- Storage: samples are stored in a tightly closed container with a code until analysis. Dried samples
can be stored at room temperature, while fresh samples with high water content must be
refrigerated or frozen.
Chemical calculation
Important: there is a difference between absolute amount and content. For example, an absolute amount
is that 1kg of diet contains 300g of protein. A content is an amount compared to another amount, e.g.
0.3kg/1kg*100%=30%, or 300g/1kg=300g/kg.
In a lab, we only analyse a content, not an absolute amount.
Dry matter correction: the content of dry matter or water can vary during storage of feed depending on the
environmental conditions. For example, 1kg of diet contains 300g of proteins. The content of protein is
300g/kg. However, 1kg of diet and 0.5kg of water contains 300g proteins, so the content of protein is now
200g/kg (=300g/1.5kg) because the total amount of feed has increased (because you consider also water).
So you need to make dry matter correction Nutrient (dm) = nutrient (measured)*1000 (g/kg)/content
of dry matter (g/kg).
, For example:
- diet 1 without water has a content of dry matter of 1000g/kg, therefore
Protein (dm) = 300*1000/1000=300g/kg
- Diet 2 with 0.5kg of water has a content of dry matter of 666.6g/kg (because we have 1000g of dry
matter in 1.5kg of diet so 1000g/1.5kg), therefore
Protein (dm)=200*1000/666.6=300g/kg
Determination of digestibility of a nutrient:
- You have to consider what the animal eats and what is excreted as faeces; all that is not excreted as
faeces was digested and absorbed.
- For example, you feed a chicken with 10kg of feed that contains 50% of protein, so the absolute
amount of protein is 5kg.
The faeces produced are 2kg, so the absolute amount of protein in faeces is 1kg. It means that 4kg
of protein is digested.
- Digestibility coefficient of a nutrient is calculated as
DCnutrient = 100 – [(amount of faeces kg*content of nutrient in faeces g/kg)]/(amount of
diet kg*content nutrient in the diet g/kg)]*100
- Example: DCprotein = 100 – (2kg*500g/kg)/(10kg*500g/kg)*100=80%
- This method to determine digestibility is accurate, but it is difficult to determine the amount of
faeces produced by a single animal if they live all together in a barn.
Alternative method to determine digestibility:
- It is based on the use of a marker: it is a chemical substance that you are sure is not absorbed by
the animal so the amount of this substance you give in the diet must also be found in faeces in the
same quantity.
- For example, this marker can be the ash not soluble in acid HCl. The diet is 10kg, which contains 5kg
of protein and 0.2kg of AsHCl (marker) so you get 2% by doing 0.2/10*100.
The faeces are 2kg, which contain 1kg protein and also 0.2 AsHCl (equal as in the diet) so you get
10% by doing 0.2/2*100.
It means that the content of marker is 5 times higher in the faeces that in the diet.
- In other words, (amount of diet kg*content of marker in the diet)=(amount of faeces kg*content of
marker in the faeces), or (amount of faeces/amount of diet)=(content marker in diet/content
marker in faeces).
- You put this info in the formula of digestibility DC = 100 – (content of marker in the diet*content
nutrient in faeces)/(content of marker in faeces*content nutrient in the diet) *100 this is the
formula of digestibility based on the marker
- Advantage of this method: easier to do, because you do not have to estimate the amount of diet or
faeces
Disadvantage: it is based on assumptions related to the marker used (???)
In case of poultry, there is a problem because the faeces of birds are mixed with urine, which contains
nitrogen. However, nitrogen in urine is not coming from proteins in the diet but is nitrogen that is being
recycled in the body, so you do not have to consider it in your calculation of digestibility of proteins (you
only consider the one in faeces).
- For example, the diet is 4kg. The manure of poultry is made of urine with 1kg of nitrogen and
faeces with 1kg of nitrogen, so in total 2kg of nitrogen.
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