Nutritive value of coarse and fine wheat brans for rabbits

Abstract

The objective of this study was to compare the nutritive value of two wheat brans obtained at different stages of milling, described commercially as coarse and fine wheat bran. Digestibility trials of four samples of coarse and four of fine wheat bran from four commercial Spanish firms were carried out in 90 New Zealand White×Californian rabbits fed ad libitum. Fine brans were characterised by a higher content of particles of less than 1 mm, while coarse brans had a higher content of particles greater than 2 mm. There was no significant differences in the chemical composition of the brans, but fine brans showed, on average, a significantly higher (P<0.05) content of crude protein bound to acid-detergent fibre (2.94 vs. 1.90 g CP–ADF kg⁻¹ dry matter). The digestibility of the chemical components did not vary widely except for crude fibre and crude protein (CP) which were, on average, significantly higher in coarse than fine bran. Animals on coarse bran consumed more feed (126.2 g/day) than those on fine bran (111.7 g/day). The best single predictor of digestible energy (DE) and gross energy digestibility (dGE) was ADF ($\text{R}{}^2\text{/}\text{R}{}_{\mathrm{<mtext>max</mtext>}}{}^{\mathrm{<mtext>2</mtext>}}\text{=0.81}\text{and}\text{0.79}$, respectively; P<0.001). A significant improvement in the accuracy of prediction was obtained when CP–ADF (P<0.01) was introduced into the model ($\text{R}{}^2\text{/}\text{R}{}_{\mathrm{<mtext>max</mtext>}}{}^2\text{=0.88}\text{and}\text{0.89}$ for DE and dGE, respectively). The best prediction for crude protein digestibility (dCP) was obtained by the inclusion in the model of both ADF and CP–ADF (R²/R_max²=0.64; P<0.001). For digestible protein, the best single predictor was acid-detergent lignin (R²/R_max²=0.63; P<0.001), although accuracy of digestible protein prediction was improved with crude protein and CP–ADF as independent variables (R²/R_max²=0.69; P<0.001). Results suggest that the classification of wheat bran into coarse and fine brans is difficult to justify from their chemical composition and that their nutritive value seem to be related to the digestibility of its fibrous or protein fractions and less to other nutritive components.

Introduction

The fibre requirement of rabbits is usually met in Spain by including in their diets 250–350 g lucerne hay kg⁻¹ and 150–250 g cereal by-products kg⁻¹ (De Blas and Mateo, 1998), mainly wheat bran that also provides a substantial amount of protein. Traditionally, by-products of wheat from the milling industry have been standardised in every country: wheat bran, wheat middlings and wheat shorts (wheat feed in the United Kingdom) are the American names for products varying in their crude fibre contents and are approximately equivalent to “son de blé”, “remoulage bis”, “remoulage blanc” in France and “salvado”, “tercerilla” and “harinilla de trigo” in Spain.

However, the different objectives and technologies used by millers lead to the supply of many products with the generic name wheat bran, that could have various contents of endosperm and, consequently, could be different from the standard product. Moreover, two types of wheat bran are usually obtained at different stages of milling at present: (1) coarse wheat bran, obtained during the initial stages of sieving, and called “son fin” in France and “salvado grueso” or “hoja” in Spain; (2) fine wheat bran, which is characterised by smaller size, obtained during the final stages of sieving, and called “gros de blé” in France, and “fino” or “cuarta” in Spain. These brans are usually named and sold as different products, but they do not seem to have any specific standard.

The nutritional value of wheat bran for rabbits has been evaluated in several experimental works (Robinson et al., 1986; Fekete and Gippert, 1986; Villamide et al., 1989; Fernández-Carmona et al., 1996), which are far from being consistent, showing great variability in their chemical and digestible composition, probably due to the very different products actually studied. Values for the main digestible components of wheat bran used in the formulation of diets, energy and protein are available in some tables of feeding standards published by individual workers or institutions (NRC, 1977; INRA, 1989; Maertens et al., 1990; Villamide et al., 1998), but they cannot easily be linked to the experimental studies from which they come.

It seems from the above, that “wheat bran” includes a range of products which do not have a unique nutritive value which can be assigned with sufficient accuracy. Therefore, the present work attempted to characterise the two types of wheat bran usually sold at present, comparing the nutritive value of different coarse and fine brans and trying to relate this to their chemical composition.