Tracking Livestock Diets Using Stable Isotopes in Complex Mixed Farming Systems

Stable isotope analysis of ¹³C and ¹⁵N has been widely used to provide information regarding short- or long-term animal dietary composition as affected by changes in land or ecological system use. In complex mixed-crop−livestock systems, rangeland biomass, crop residues, and feedstuff contribute to livestock diets with high seasonal variations. Particularly in the Limpopo province, South Africa, livestock feed supply varies considerably throughout the year leading to regular feed gaps. Hence, to estimate livestock diet composition in relation to these variations, we analyzed the stable isotopes of C and N of tail hair and feces samples across free-ranging beef cattle in distinct agroecological zones (AEZs) and farm types. Here, farm types were structured according to mixed-crop−livestock farms and livestock-only farms. We found that farm types did not influence the $\delta$¹³C of feces and hair (P > 0.05), but AEZ had a strong effect on $\delta$¹³C patterns (P < 0.01). The contribution of C₃-C₄ plants in the diet varied considerably across farm types and AEZ (P < 0.01). The $\delta$¹⁵N values indicated nutritional stress, probably due to low protein concentrations in feed (e.g., rangeland biomass, crop residues) irrespective of AEZ and farm types. In general, cattle exposed to arid conditions (drier AEZ) had significantly decreased $\delta$¹³C values and increased $\delta$¹⁵N values that could potentially be linked to feed gaps. Moreover, the results from the present study indicate that $\delta$¹³C of feces and hair samples are more meaningful predictors of cattle diet than $\delta$¹⁵N as affected by different farm practices. Therefore, stable isotope analyses potentially have an important role in helping improve the understanding of seasonal shifts in livestock diet intake in diverse production systems.