Recent Trends in Feed Assessment

 Professor Dr. Ravi Ravindran* says that “Feed assessment is the basis for feed formulation and routine feed evaluation offers critical highlights”. Matrix values need to be updated and to bring nutrient supply closer to target requirements, lower nutrient wastage into environment and improve sustainability.

Introduction:

Routine and proper ingredient evaluation is central to precise and cost-effective feed formulations. The matrix values used in formulations, whether from tabular values, prediction equations or research data, are based on ingredient evaluation research. The aim of feed evaluation is to provide the nutritionists with reliable data on the variability in digestible and metabolisable nutrient contents in feed ingredients, so that the variation within ingredients could be incorporated into matrix values. Furthermore, the predicted future growth of the poultry industry will have a profound effect on the demand for ingredients and search for ‘alternative raw materials’. When using such poorly digested alternative ingredients, formulation based on digestible nutrients is a requisite and, better feed evaluation practices become even more pertinent.

Basic principles of poultry feed evaluation:

Two fundamental features from the basis of feed evaluation in poultry.

Ileal digestibility:

First, it is well accepted that determination of digestibility of nutrients in poultry should be based on the analysis of ileal digesta. The only exception is the determination of apparent metabolizable energy (AME), which involves total collection of excreta and the metabolisability is calculated as the difference between dietary energy input and excreta energy input. 

Composition of assay diets:

Second, three different methodologies (direct method, difference method and regression), varying in the composition of assay diets, are used in digestibility assessments of a nutrient. None of these methods are prefect and, each methodology has its own strengths and weaknesses.

Protein evaluation:

During the past 30 years, the basis of feed formulation has slowly shifted from total amino acids (AA) to the digestible AA system, which has enabled us to meet AA requirements more precisely and to increase the range and inclusion levels of alternative ingredients, while maintaining performance levels. Initially, AA digestibility measurement was based on excreta analysis. During the 1980’s, the precision-fed rooster assay was popular but this assay has since lost global acceptance because of ethical issues. 

Currently, use of ileal-based broiler digestible AA is widely accepted. Considerable published data have now become available on the ileal AA digestibility of raw materials. A major issue with these data however, is the wide variability reported in digestibility estimates. There are two sides to the observed variation namely, (i) inherent variability expected in raw materials and (ii) the differences arising from methodological differences employed in different research stations. The latter is a real concern. Results from a collaborative study, involving three research stations, exemplifies the variability that may occur due to differences in assay procedures (Table 1).

Table 1: Apparent ileal digestibility coefficients of crude protein and select amino acids in maize determined at three research stations, using station protocols.

	Station 1	Station 2	Station 3	SEM	p≤
Crude protein	0.73	0.85	0.80	0.027	0.05
Lysine	0.63	0.83	0.79	0.036	0.05
Methionine	0.84	0.92	0.86	0.029	0.16
Threonine	0.62	0.77	0.73	0.023	0.05
Average1	0.76	0.87	0.83	0.030	0.05

1Average of 17 amino acids

In a follow-up study, which used a common agreed protocol, the between station variation was eliminated. These results highlight the need for a consensus protocol for use not only in the measurement of digestibility of AA, but of all nutrients to enable better comparison of data generated across research stations working in ingredient evaluation.

Confusion about the terminology used to describe the AA digestibility estimates becomes clear to anyone perusing the available digestibility data. For each AA, there are at least six possible values, and combinations thereof, to describe the digestibility for poultry: apparent or true or standardized; rooster or broiler; and excreta or ileal. Many end-users often do not know which values are being used in their matrixes. Currently, the term ‘standardized ileal digestibility (SID)’is being increasingly used in the poultry industry. Compared to other terms, SID is the one relevant to the way we formulate diets. It is additive and aligned with the ideal protein concept.

Energy evaluation:

The use of appropriate energy system is another critical issue because of the importance of energy to bird performance and diet cost. An ideal energy system must be easy to measure, predictive of bird performance, additive in feed formulations and independent of bird factors. However, energy metabolism is too complex to meet all these ideals. 

Since the 1950’s, apparent metabolizable energy (AME) has been the system of choice of describing available energy for poultry. True metabolizable energy became popular in the 1980’s but has since lost favor owing to ethical issues. Currently, the AME is the widely accepted system to describe favoured system in the foreseeable future. It is not a perfect system, with several limitations (Table 2). However, it is familiar and universal, and its limitations are overlooked.

Table 2: Limitations of AME

Ingredient values may not be additive in formulations

Variations in published data due to: Methodology differences, including bird factors (age, gender, production stage) Ingredient factors

Does not take account of energy lost as heat

Net energy (NE) system, a refinement of the AME concept, has received attention from time to time. In theory, NE will more closely describe the energy available in an ingredient for bird’s metabolic functions and is more predictive of animal performance. It is, however, difficult to assay, costly and time consuming. To be acceptable, its economic advantage over the AME system needs to be demonstrated.

Phosphorus evaluation:

Globally there are growing concerns about phosphorus (P) excretion from intensive animal operations into the environment. Gradual depletion of global feed phosphates deposits is another concern. These issues are driving research into P digestibility in order to efficiently use and conserve the finite P resources.

A related issue is the considerable confusion that exists regarding the terminology to describe P that is available to the bird (e.g. available P, non-phytate P and retain-able P). Use of a sound criterion to assess P availability. One key finding from recent research is that, contrary to common premise, non-phytate P does not equate to digestible P suggesting that broilers are able to utilize a portion of phytate-bound P in feed ingredients.

Calcium evaluation:

Measurement of Ca digestibility in poultry has received relatively little attention in the past due to the cheap availability of limestone, the major source of Ca in poultry diets. However, the move towards a digestible P system necessitates a closer look at digestible Ca because of the close relationship between Ca and P during and after absorption.

It is widely assumed that Ca in common Ca sources (limestone, meat and bone meal, dicalcium phosphate) is highly available, but recent studies have shown that this is not true (Table 3).

Table 3: True digestibility of calcium of feed ingredients (%).

Feed ingredient	Calcium digestibility (%)
Meat and bone meal	50 (range, 41-60)
Limestone	55 (range, 43-71)
Dicalcium phosphate	35 (range 28-45)
Monocalcium Phosphate	35 (range, 32-45)
Canola meal	30
Fish meal	25
Poultry by-product	30

 Sources: Naveed Anwar (2017), Phd Thesis; Laura David (unpublished PhD thesis)

In Vitro methods:

Simple in vitro digestion assays have the potential to yield useful indication of nutrient digestibility (Table 4).

Table 4: Pros and cons of in vitro assays:

Limitations	Value
Not possible to accurately simulate the complex in vivo biochemical and physiological processes	Screening of feed additivies
Anti-nutritional factors, dietary dry matter and fiber, endogenous protein secretions, activity of gut enzymes, and gut bacteria not mimicked in vitro	Ranking of feedstuffs by digestibility
Assays should include lipases, carbohydrases etc. as these affect release of proteins from food matrix	Predicting in vivo nutritive value directly or in combination with measures of chemical constituents

Digestion and absorption processes in the animal, however, are too complex and simulation in laboratory is not possible. Nevertheless, the use of in vitro techniques is attractive because such assays are relatively simple, rapid and reproducible, and avoid the use of animals. While in vitro data are useful scrrening samples, they cannot be used in practical feed formulations.

Near-infrared relectance spectroscopy (NIRS):

Many feed mills routinely use NIRS technology to predict the protein moisture, fat, and ash contents of feed ingredients on an on-going basis. The investment on rapid tests, however, should be extended to the measurement of AME or digestible nutrients (especially digestible AA) to formulate the diets precisely. The success of NIRS in ruminant nutrition suggests that the NIRS is capable of predicting energy values for poultry, but it remains to be seen if this is practically feasible.

By:

*Dr. Ravi Ravindran (V.Ravindran@massey.ac.nz) is Professor of Poultry science with the Monogastric Research Centre, Massey University New Zealand.