True digestibility of wheat protein and amino acids in broilers

Abstract

Wheat is a major constituent of poultry diets, accounting for up to 0.8 of the metabolizable energy and 0.4 of the protein requirement of broilers. Variation in protein quantity and quality, due to environmental conditions and variety, may influence performance of broilers. Increasingly, digestible rather than total amino acids are being used as bases for diet formulation. However procedures involved in evaluating digestibility often need to distinguish between true and apparent digestibility, necessitating assessments of endogenous losses.

The objective of this study was to assess the digestibility of amino acids from specific wheat varieties, employing a technique based upon the use of an inert dietary marker and ileal sampling of digesta. Three diets per wheat sample were formulated containing 250, 500 and 750 g/kg wheat. The other constant dietary constituents were (per kg) oil (50 g), mineral/vitamin mix (50 g) and 5 g marker (titanium dioxide). Diets were made up to 1000 g/kg with a 50:50 starch/glucose mix. Diets were fed ad libitum for 3 days to 6 cages of 2 male Ross broiler chickens of initial age 18 days, housed in an environmentally-controlled room. Birds had constant access to water.

After 3 days on treatment, samples of ileal digesta were obtained from the birds and analysed for amino acid and titanium dioxide content.

During the study a method for the determination of titanium dioxide, the inert dietary marker, was developed for use with samples of 0.1g. The method relied on the colour change between titanium dioxide and hydrogen peroxide.

The apparent digestibility of each amino acid was calculated by regressing digestible amino acid content in the wheat sample on wheat inclusion rate and extrapolating to 1000g wheat / diet. Regression relationships were usually highly significant. True digestibility was calculated by deducting the ordinate abscissa term.

Six trials in total were carried out. The first 3 used named varieties: Trial 1 used the wheat variety Mercia which had been treated with 3 levels of nitrogen fertilizer. Trial 2 also used the variety Mercia. Trial 3 used two varieties Brigadier and Hussar at 2 crude protein (CP) levels: Brigadier 102, 130 and Hussar 96, 128 g CP/kg.

It was then thought more appropriate to investigate wheat samples with a similar genetic background and minimal genetic differences (ie near-isogenic lines). This was because two varieties may be distantly or closely related and the name alone of the varieties would give no indication of closeness of genetic background. Furthermore, even within a variety, two separate samples could show considerable genetic diversity. Using near-isogenic lines allows differences to be related to the presence of specific genetically defined characteristics against an otherwise comparatively uniform genetic background.

Trial 4 examined two wheat samples, which were isogenic except for endosperm hardness and protein, and their parent varieties Hobbit/Avalon 5A. Trial 5 investigated the differences in digestibility in two of the first varieties to be used in the genetical modification of wheat. Trial 6 evaluated the effects of the introduction of a rye gene (1B/1R) on digestibility. Results revealed that: Significant variation in amino acid digestibilities existed between wheats in individual trials. Apparent digestibility values for individual amino acids within wheats were significantly different. Results suggested that the use of a constant conversion factor to estimate amino acid digestibilities for a particular wheat is invalid. There appeared to be variation in the differences between true and apparent digestibility. Increasing nitrogen fertilizer levels tended to increase the digestibility, as did an increase in crude protein content. The inclusion of the Avalon 5A substitution (thought to affect the protein characteristics) to the Hobbit line appeared to decrease digestibility.

Results revealed that:

-Significant variation in amino acid digestibilities existed between wheats in individual trials.

-Apparent digestibility values for individual amino acids within wheats were significantly different.

-Results suggested that the use of a constant conversion factor to estimate amino acid digestibilities for a particular wheat is invalid.

-There appeared to be variation in the differences between true and apparent digestibility.

-Increasing nitrogen fertiliser levels tended to increase the digestibility, as did an increase in crude protein content.

-The inclusion of the Avalon 5A substitution (thought to affect the protein characteristics) to the Hobbit line appeared to decrease digestibility.

-Insertion of the rye 1 B/1 R gene appeared to decrease digestibility.

-For certain amino acids, as the nitrogen content of the grain increased, the coefficient of apparent digestibility of that particular amino acid increased.

-The data presented have considerable implications for the animal feed industry, plant breeders and, ultimately, growers.