Carbon and nitrogen stable isotope composition of cattle hair: Ecological fingerprints of production systems?

Societal interest in food safety, animal welfare, and environmental quality attributes of food production is increasing, creating a need for reliable indicators of such factors. Here we test the hypothesis that cattle farming systems create unique and meaningful isotopic fingerprints, which can be characterized by analysing cattle tail switch hair. To this end we analysed feeding practices and nutrient fluxes, and sampled hair, feed components and fertilizers from 13 different farms in Upper Bavaria, Germany. The farms represented the range of cattle farming types present in the region and included: conventional confinement dairy, pasture based organic and conventional dairy, suckler cow, and bull and steer and heifer fattening enterprises. Samples were analysed for their carbon (C) and nitrogen (N) stable isotope composition (δ¹³C and δ¹⁵N). Feed samples could be assigned to one of three groups with characteristic δ¹³C, which varied very little between and within farms: C ₃ forages (including fresh forage, hay or silage from grassland and clover-grass mixtures) with -28.4‰ (±0.5‰ S.D.), maize (Zea mays L.) with -12.5‰ (±0.4‰), and C₃-derived concentrates (including mainly cereal grain and legume seeds) with -26.8 (±1.1‰). The dry matter fraction of maize in the diet explained 96% of the farm average δ¹³C of hair. Hair was approx. 2.7‰ enriched in ¹³C relative to the diet (trophic level shift), and this effect was very similar for growing animals and cows, and seemingly independent of the fraction of maize in the diet. In contrast to δ¹³C, the δ¹⁵N of individual feed types differed very strongly between - and also within - farms. Only legume seeds had relatively constant δ¹⁵N (1.2 ± 0.5‰). δ¹⁵N of cow hair was correlated with stocking rate (r ² = 0.55) and N input surplus (farm gate) (r² = 0.78), respectively. This correlation was probably caused by increasing losses of ¹⁵N-depleted N via ammonia volatilisation, nitrate leaching and denitrification with increasing farm-level N surplus. Heterogeneity of feed ¹⁵N signatures indicated within-farm heterogeneity of N fluxes and cycling that was at (least partially) integrated in cattle hair. Thus, cattle hair ¹⁵N signature appears to indicate the 'leakiness' of cattle production systems for N. Conversely, the ¹³C signatures reliably indicates maize feeding and, thus, the type of land use (arable forage cropping versus grassland farming) on which cattle production in the region is based.