Comparison of long-chain fatty acids and alkanes as markers to estimate diet composition of equines and cattle consuming heathland vegetation species

Abstract

This study aimed to evaluate the application of long-chain fatty acids (LCFA) as an alternative or as a complement to alkanes to estimate diet composition in equines and cattle. Twelve mature crossbreed mares (385 ± 47 kg live weight – LW) and 6 adult non-lactating cows (499 ± 36 kg LW) of Asturiana de los Valles breed were divided in groups of 3 animals and housed in individual stalls. Animals received a daily total amount of 1.0 kg DM/100 kg LW of different diets composed of herbaceous ( Lolium perenne) and woody species ( Ulex gallii and heather). Diet composition was estimated from LCFA (i.e., C 22–C 34) and alkane (i.e., C 25–C 31 and C 33) concentrations in diet and faeces by least-squares procedures, using marker faecal concentrations uncorrected for incomplete faecal recovery (FC0), corrected using mean recovery rate of the dietary treatment that the animal belonged to (FC1), or corrected using mean recovery rate across all experimental diets (FC2). For all diet components, LCFA concentrations were higher than the alkane ones, with even-chain LCFA accounting for more than 0.80 of total LCFA. In general, faecal recovery (FR) of the markers was incomplete and related to their carbon-chain length. In equines, the FR of both markers tended to decrease with carbon-chain length in a curvilinear fashion ( P < 0.001), whereas in cattle LCFA and alkane FR tended to increase with carbon-chain length in a curvilinear ( P < 0.001) and linear ( P < 0.001) fashion, respectively. Diet composition had an effect ( P < 0.001) on the LCFA and alkane FR in both animal species, and, in equines, seemed to be related with diet digestibility, i.e., decrease of marker disappearance from the digestive tract with a decrease of diet digestibility. In general, diet composition estimates based on LCFA alone were more accurate than those obtained using alkanes alone. Combination of LCFA and alkane markers resulted in an increase ( P < 0.05) in the accuracy of diet estimates in both animal species, indicating a higher discriminatory power among plant species. The use of FC0 and FC2 resulted in the worst ( P < 0.05) estimates of diet composition, while high levels of accuracy were observed when using more accurate faecal corrections (FC1). Results showed that LCFA can be useful markers for studying diet selection of equines and cattle grazing on heathland vegetation communities, if a proper adjustment of their faecal concentrations prior to their application is applied. It is also concluded that use of LCFA in combination with alkanes can increase the accuracy of diet composition estimates.