Alteration of the carbon and nitrogen stable isotope composition of beef by substitution of grass silage with maize silage

This study investigated the effect of substituting grass silage (C 3 photosynthetic plant product) with maize silage (C 4 photosynthetic plant product) on the natural abundance carbon ( δ 13 C) and nitrogen ( δ 15 N) stable isotope composition of bovine muscle tissue. Forty‐five continental crossbred heifers were assigned to one of three diets consisting of 3 kg of a barley‐based concentrate plus grass silage, maize silage or an equal mixture (dry matter basis) of grass silage and maize silage, fed ad libitum , for 167 days. Substitution resulted in less negative δ 13 C values ( P  < 0.001) in lipid‐free muscle and in lipid, and also a lower δ 15 N ( P  < 0.001) in lipid‐free muscle. Feeding of maize silage was clearly reflected in the δ 13 C of muscle, with each 10% difference in the dietary C 4 carbon intake resulting in a 0.9 to 1.0‰ shift of δ 13 C in lipid‐free muscle and a 1.0 to 1.2‰ in lipid. Minimum detectable mean differences (95% confidence, power 0.80, n  = 15) in this experiment were about 0.5‰ and 1.0‰ for δ 13 C of lipid‐free muscle and lipid, respectively, and about 0.5‰ for δ 15 N of lipid‐free muscle. The power analysis presented here is useful for estimating minimum isotopic differences that can be detected between any two groups of beef samples with a given number of replicates. It is concluded that carbon stable isotope ratio analysis of meat can be used to quantify C 3 /C 4 dietary constituents in beef production. Copyright © 2005 John Wiley & Sons, Ltd.