To evaluate whether a byproduct is likely to be a suitable feed resource for monogastric animals, the following examination should be undertaken.
The byproduct should be analysed according to the AOAC's Official Methods of Analysis. Emphasis should be put on dry matter (DM), crude protein (CP), crude fibre (CF), ash, acid insoluble ash (AIA), calcium (Ca) and phosphorus (P). This is in order to classify the byproduct into one of INFIC's feedstuff classes. Macroscopic and/or microscopic appraisal of the by-product may be helpful.
The conventional feed ingredient(s) that will be potentially substituted by the byproduct should be identified. Comparing their chemical composition to that of the byproduct always helps in the initial estimation of the level of substitution in isonitrogenous-isocaloric diets for monogastric animal in which the byproduct is to be fed.
In order to evaluate the acceptability, palatability and/or toxicity of the byproduct in monogastric animals, a preliminary feeding trial should be carried out. A statistically sound design should be used with an adequate number (>4) of replication and sufficient numbers of animals in each experimental unit (>35 unsexed broilers, >12 layers or >2 pigs).
Isonitrogenous-isocaloric experimental diets should be formulated including the byproduct to supply a key nutrient (primarily energy or protein) at 4 to 5 levels (eg: 0, 25, 50, 75 and 100%) of that supplied by the substituted ingredient. This is to enable the researcher to detect the optimum range of inclusion from the response curve (Figure 5.1).
A feeding trial on broilers should be carried out through a normal production range (6–8 weeks); for layers it should be through the first half of the laying period (point of lay to 22 weeks). Pigs should be studied over the range 25 to 60 kg liveweight. This is to ensure that the observed responses are real and reliable.
Animal performance can be assessed from the response curve with appropriate statistical analyses of observed weekly records of feed intake and liveweight gain (or egg production).
A precise evaluation of the byproduct, in terms of optimum substitution level, nutrient supplementation/correction and method to improve its nutritive value, can be subsequently undertaken by making use of this preliminary evaluation together with the complete analytical data as will be outlined in the following sections.
Once a byproduct has been defined chemically and tested on animals to know its acceptability level (ie: the maximum concentration in a feed which does not affect too much the health, feed intake or faeces consistency. A digestibility test may provide useful information of its feeding value.
Bio-available energy and amino acids account for over 90% of feed costs. Therefore a priority must be given to determine the digestibility of these nutrients. The following suggestions are proposed as guidelines for such a determination. A comprehensive discussion of the subject was given by Picard et al (1985).
Animals:Adult cockerels of light weight housed in cages with a collecting tray.
Replicates: Minimum 6 birds per treatment.
Preparation of the animals: No adaptation to the diet starvation 24 hr prior to the begining of the test.
Diets: The sample must be homogenized. If acceptability level (x) as determined in (5.3.1) is less than 100% a basal diet (eg: maize grain) is prepared and mixed with the ingredient to reach the desired concentrations (eg:0,0.5x,x and 2x% of the sample in the basal diet). Sampling of individual ingredients for laboratory analysis should be done at the same time as the weighing and mixing of the test diets.
Feed distribution and faeces collection: The birds may be either force-fed (50 g) or fed ad libitum during 24 to 96 hr. The force-feeding will be followed by a 48 hr starvation period with excreta collection. The ad libitum test will be followed by a 24 hr starvation period with excreta collection. Care must be taken to avoid spillage of feed, to record accurately the feed consumption and any variation in moisture content of the feed. The excreta should be collected twice daily and stored at <4°C.
Laboratory assays: DM (2 repetitions per sample), N (3 repetitions), GE (calorimeter) (5 replications). GE values should be made on the ingredients and on the basal diet, separately, and on the pooled excreta samples. Freeze drying of the excreta is recommended but classical methods may be used for drying.
Interpretation: Measurement of the endogenous energy losses on either starved birds or birds fed a 100% absorbable diet (eg: glucose) is highly useful to calculate the energy value with various units (eg: apparent ME, true ME), corrected or not for a given N retention. This should be done for each concentration of the product and by regression to 100% if linearity of the response permits it.
Animals: Growing pigs (liveweight range 25–60 kg), the exact point depending on the type of ingredient being studied.
Replicates: Minimum 4 pigs per treatment if the animal serves as its own control.
Preparation of the animal: Anti-parasite treatment; 10 day adaptation to the cages; 7 day adaptation to the feed; no starvation.
Diets: The sample must be homogeneous. Then all depends on the type of ingredient. Some may be fed pure (eg: wheat bran); some are either high in humidity or difficult to mix; they will be tested by addition to a balanced basal diet or previously tested alone on the same pigs. The level of addition must represent more than 30% of the ingested DM. Some are dry and easy to mix; they will be introduced by dilution as for poultry.
Feed distribution and faeces collection: The digestibility test will last 7 days for each diet after a 7-day period of adaptation. The daily allowance should be given in two meals (morning and afternoon), and the feed should be given wet. The feeding level should be 80–90 g DM/kg (LW) {.75}/d.
Care must be taken in measuring feed refusals, and the moisture content and N content of the wet refusals. Faeces should be collected twice daily and stored at -18°C.
Laboratory analyses: The same procedures recommended for poultry. Freeze drying is proposed as the best method for preparing samples for analysis. DM should be estimated in duplicate; N and GE in triplicate.
Interpretation: Depending on the technique used; digestible energy of the ingredient is calculated either:
Directly (when fed as the sole ingredient) or;
By difference between the two periods (basal diet alone - basal and ingredient); or
For each dilution level and by regression to 100% if linearity permits it.
Metabolizable energy (ME) may be calculated using the following equation:
ME: metabolizable energy (Mcal/kg)
DE: digestible energy (Mcal/kg)
DP: digestible protein (g N × 6.25/kg)
(100 - 1) is the correction for DE lost as heat (1%)
0.07 = Coefficient assuming a 50% N retention and 9 Kcal/g of N
in urine (measured).
For any monogastric animal a precise determination of the amino acid content of the ingredient is essential for feed formulation. However such measurements are costly and difficult. The best way is to send a sample to a specialized laboratory or to refer to a value taken from the literature. In fact, a given protein source shows a relative consistency of its amino acid composition. Therefore a standard value may be adjusted to the actual N content of the sample using the following equation:
Sample AA content = Table AA content (in g/16 g N) × Sample N × 6.25 content
For any new byproduct or, if the chemical analysis value of the sample (moisture, N, ash, lipids, fibre) differs too much from the literature reference values, an analysis of AA is recommended.
Digestibility of N can be worked out together with the energy measurement. It gives useful information to be compared with regular raw materials. For cockerels, a special treatment of the faeces is necessary in order to eliminate the uric acid prior to measurement of the excreted N (Terpstra and Dehart 1974).
Amino acid digestibility per se can be derived from N digestibility using the simplified procedure suggested by Picard et al (1985).
Simple digestibility tests give very useful information on the value of a byproduct if constant attention is given to precision while the test is carried out. Fluctuations in the DM content of the samples, of the diets and of the faeces between weighings and analyses are a major cause of error.
Results must be discussed with regard to the methodology, the level of inclusion, the unit used in order to improve the procedures.
Total collection has been favoured against the use of any indicator. If digestibility cages for pigs are not available, the use of acid insoluble ash is suggested (eg: McCarthy et al 1974, 1977).
This is the last section in the guidelines on methodology for evaluating feeds for monogastric animals. All too often the researcher, having published the preliminary evaluation, is inclined to stop at this point. But the job is not finished!! The research findings must be applied to a practical situation. How can this be done?
Design a practical ration which includes the byproduct taking into account:
Feeding trials should be run using the levels of introduction and the diets suggested above:
At the research farm level: Large numbers of replicates are suggested using practical diets and preparations.
At the commercial farm level: Depending on the target group, this kind of trial will be handled either by a large corporation or directly with small farmers.
Information about feeding trials is important. It should be disseminated in as many ways as possible (eg: local conferences, local publications, in schools and in technical training).
Recommended minimum animal facilities:
20 pens or cages for 3–5 growing chickens
20 pens or cages for 2 pigs each
20 pens or cages each for 1–2 layers
Recommended minimum feed mixing facilities:
A shovel!!
Recommended minimum laboratory equipment:
Oven
Kjeldahl furnace
Muffle furnace
Fibre analysis apparatus
Lipid extraction apparatus
For exact laboratory evaluation:
Calorimeter
Digestibility cages
Minimum of 8 cages for pigs and 30 cages for cockerels
Large scale feeding trial units
24 broiler pens each for 100 birds
100 cages each for 3 laying hens
16 pens each for 2 pigs
However, there was only 2% N in the diet dry matter and the diets were balanced for N by replacing a fermentable-N source (urea) with a protein which largely escapes rumen fermentation (fish meal) and no consideration was given to the level of rumen ammonia. It is equally valid to suggest that rumen ammonia was probably low on the fishmeal diet and that this led to a decrease in the rate of fermentation of the grain carbohydrate; as a result, rumen pH would stay high (see Mehrez et al 1977). The consequence of this is that the pelleted straw and the maize silage might be more fully digested in the rumen. On the urea diet rumen ammonia levels would be higher leading to a faster rate of grain fermentation (Mehrez et al 1977), a lower pH and therefore a reduced rate of cellulolysis (Mould et al 1983).
