Variation in apparent and true ileal amino acid digestibility in barley using a rat model

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Abstract

The study aimed to determine ileal amino acid and total tract energy digestibility coefficients for a range of barleys. The laboratory rat served as a model nonruminant animal and digesta samples were obtained using the slaughter method with chromic oxide being used as a dietary marker. In a preliminary study the effect of time interval after the feeding of a single meal of ground barley and before sampling of ileal digesta on the apparent ileal digestibility of total nitrogen (N) and of the effect of the length of terminal ileum sampled were investigated. There was no significant effect of time of sampling but there was an effect (p<0.001) of site in the ileum. The final 10 cm of ileum was considered appropriate for the sampling of ileal digesta for barley-fed rats. In the main study 20 barley samples comprising 14 different cultivars which had been grown in the same year but in three distinct geographical locations in New Zealand were given as sole sources of protein to growing rats (n=120). Digesta were collected (final 10 cm of ileum) 4 h after the commencement of a single meal. The barley samples had variable contents of crude protein, amino acids, neutral detergent fibre, acid detergent fibre, acid detergent lignin and gross energy. The mean (n=6) apparent ileal N digestibility ranged from 0.74 to 0.85 while the equivalent range for true N digestibility was 0.84 to 0.94. Apparent and true ileal amino acid digestibility showed considerable between-sample variation. Digestible energy ranged from 13.0 to 15.1 MJ/kg dry matter. There was a significant effect of the geographical locality where the barleys were grown on amino acid digestibility for most of the cultivars studied.

Introduction

There is a considerable amount of information in the literature on the apparent ileal digestibility of amino acids in barley for the growing pig (Heartland Lysine, 1992) and on its apparent total tract digestible energy content. There is less reliable information, however, on true ileal amino acid digestibility in barley. True ileal amino acid digestibility is a preferred measure for dietary formulation (Boisen and Moughan, 1996). Moreover, there have been few controlled studies on the variability of barley nutritive value.

The aim of the present study was to determine the apparent and true ileal digestibility of amino acids and the apparent total tract digestibility of energy for a range of New Zealand barleys to provide information on the variation in nutritive value for this cereal. The work was conducted using the growing rat as a generalised model animal for simple-stomached species (Moughan et al., 1984; Picard et al., 1984; Moughan et al., 1987; Smith et al., 1987, Smith et al., 1988). With small animals such as the growing rat, digesta are normally collected at slaughter. Preliminary work was undertaken to define an optimal time (after the commencement of feeding) and site in the ileum, for collecting digesta from barley-fed rats.

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Animals, housing and diet

Male and female Sprague Dawley rats of 150 g liveweight (Small Animal Production Unit, Massey University) were caged individually in a room with controlled temperature (22±2°C) and lighting (12 hL/12 hD). The rats were fed a diet comprising ground (1 mm screen size) barley, minerals and vitamins (Table 1). Chromic oxide was included as an indigestible marker.

Preliminary Trial 1: The effect of time of sampling of the digesta on apparent ileal digestibility

Thirty rats were housed in separate cages with mesh flooring designed so that faeces could not be reached by the rats. The rats were equally

Results

In the two preliminary trials and in the main study, the rats remained healthy, became rapidly accustomed to the feeding procedure and consumed the diets readily.

Discussion

In the first preliminary trial of the present study it was observed that the ileal N digestibility was relatively constant over the sampling times of 3–6 h following a meal but was observed to be least variable 4 h after feeding. This is in accordance with the results reported by Donkoh et al. (1994), that the nitrogen digestibility of meat and bone meal in the rat was least variable with a 4 h sampling time.

Despite the fact that Donkoh et al. (1994)reported no significant differences in ileal

Acknowledgements

Thanks to Dr's G. Gort (Department of Mathematics, Wageningen Agricultural University) for helpful discussions on the statistical data analysis and to Dr Ir M.W. Bosch (Department of Animal Nutrition, Wageningen Agricultural University) for her advice during the preparation of this article.

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