Concepts on feed & feed ingredients

• Daily Growth Coefficient
• Food Conversion Efficiency
• Nitrogen retention

Daily Growth Coefficient (DGC)

A growth coefficient based on the one-third exponent of an animals weight gain. For specific validation and comparison of measures of growth in fish. see: Kaushik (1998a). DGC is calculated as:

Where Wt represents the animal's weight at time period t, and Wi represents the animal's weight at the beginning of the time period.

Food Conversion Efficiency (FCE)

The efficiency of a feed is normally measured by the amount necessary to produce a unit weight of fish. This is called the feed conversion ration or FCR. The FCR is the unit weight of feed given, divided by the live weight or wet weight of the animal produced.

The higher the FCR is, the less efficient the feed is. Moisture levels in the animal or in the feed are not normally taken into account when the FCR of a feed is calculated. This means that wet or moist feeds have a higher FCR than dry feeds because so much of the feed consists of water. This does not mean they are less efficient. Strictly it is only possible to compare the FCR of a moist diet with that of another diet of similar moisture content unless a further calculation is done. To compare moist feeds with dry diets the FCR's must be reduced to a common level, either to a dry matter basis or to an assumed 10% moisture basis.

Nitrogen retention

The amount of nitrogen (or energy or phosphorus) retained by an animal when fed a specific diet. This parameter provides an indication of the potential a diet has to support biosynthetic activity. Also often referred to as PPV (Productive Protein Value) or ERV (Energy Retention Value).

• Nt is the nitrogen content of the animal at time t
• Ni is the initial nitrogen content of the animal
• Nc is the amount of nitrogen consumed by the animal from initial assessment to time t.

Determination of Energy and Phosphorus retention is achieved the same way, but with the substitution of the relevant energy and phosphorus criteria where the corresponding nitrogen criteria are indicated in the equation. Typically, this figure is determined based on gross nitrogen / energy / phosphorus intake, though more accurate assessments will determine it based on digestible intake. Notably this figure will be influenced by animal size, therefore it is important that comparisons are made only between animals of similar initial size.