No abstract

This study aimed to assess the use of very long-chain fatty acids (VLCFA) as an alternative or a complement to n-alkane markers for estimation of diet composition of goats fed 7 different diets, composed of different proportions of herbaceous (Lolium perenne and Trifolium repens) and heathland woody species (Erica umbellata, Erica cinerea, Calluna vulgaris, Erica arborea, and Ulex gallii), in a metabolism study. Diet composition was estimated from VLCFA (i.e., C(22) to C(34)) and alkane (i.e., C(25) to C(33)) concentrations in diet and feces, by least squares procedures. For all plant species VLCFA concentrations were greater than their alkane concentrations, especially for the herbaceous species and U. gallii. In general, fecal recovery of both markers was incomplete and increased in a curvilinear (P < 0.001) fashion with carbon-chain length. The plants comprising the diets had a significant effect (P < 0.001) on fecal recovery of VLCFA and alkanes. Diet composition estimates based on VLCFA alone were less accurate (P = 0.013) than those obtained using alkanes alone. Combination of VLCFA and alkane data resulted in the most accurate (P < 0.05) estimates, indicating an increase on the discriminatory power among plant species. Use of uncorrected fecal marker concentrations provided the poorest estimates of diet composition, and use of individual recovery data and mean fecal recovery data of the dietary treatment yielded the most accurate ones. Results obtained in this study show that VLCFA have potential to be used as diet composition markers along with alkanes.

Fatty acid (FA) composition of intramuscular (IM, Longissimus thoracis muscle), intermuscular (IT) and subcutaneous (SC) fat of one hundred intensively fed yearling bulls with different propensities to fatten were studied. Meat samples were collected from Asturiana de los Valles bulls with different genotypes with respect to the myostatin gene (mh/mh n = 24, mh/+ n = 26 and +/+ n = 25) and from Asturiana de la Montañ a (n = 25) bulls lacking the mutation responsible for double muscling and characterised by small to mediumframe size adapted to less favoured mountain areas. FA profiles were expressed as percentages of total FA (g/100 g of total FA) and organised into groups (saturated (SFA), branched (BFA), monounsaturated (MUFA), C18:1trans, polyunsaturated (PUFA), n À 6, n À 3, conjugated linoleic acids (CLA), unsaturated (UFA)) and ratios (MUFA/SFA (M/S), PUFA/SFA (P/S), UFA/SFA (U/S), n À 6/n À 3).The IT depot was the most saturated and SC depot contained the most monounsaturated FAs, while IM fat had the most polyunsaturated FAs. IM fat showed the highest P/S ratio and for the n À 6/n À 3 ratio there were no significant differences between adipose tissue depots.In general, genotype effects were more pronounced in IM and SC fat profiles compared to the IT depot, for which no significant differences between genotypes were found in SFA, PUFA (including n À 6 and n À 3), UFA and most of the ratios. IM fat of mh/mh animals had the highest content of PUFA and thus the highest P/S ratio. Accordingly, the presence of the gene causing double muscling influenced the tendency to deposit carcass fat and its FA composition, mainly in IM fat. In general, when carcass fat decreased, SFA content decreased while PUFA and UFA contents increased due to the changes in their percentages.

An experiment was conducted to assess the potential of long-chain alcohols (LCOH), in alternative or combined with alkanes and long-chain fatty acids (LCFA), as faecal markers to estimate the diet composition of goats grazing heathland vegetation with associated improved pastures. A total of seven diets were offered across the grazing season. The diets were composed of mixtures of herb species (Lolium perenne and Trifolium repens) and woody species (Erica spp., Calluna vulgaris and Ulex gallii) in an attempt to simulate diet selection of goats on these complex vegetation areas. The diet composition was estimated using LCOH markers alone or combined with alkanes, LCFA and alkanes+LCFA, by least square optimization procedures. The data showed large differences between plant species in their LCOH profile. Generally, plant species showed higher LCOH concentrations than those of alkanes and lower than LCFA markers. Faecal recovery of LCOH was incomplete and increased in a linear manner (P < 0.001) with the carbon-chain length, and was influenced by diet composition and its digestibility. The diet composition estimates based on LCOH alone were more accurate (P < 0.05) than those using alkanes or LCFA alone. Results showed that the combination of LCOH with alkanes, LCFA and alkanes+LCFA resulted in more accurate (P < 0.05) estimates of diet composition, indicating that LCOH provided different discriminatory information to that of alkanes and LCFA, helping in the discrimination of the plant species used in this experiment. Results indicate that correction of faecal LCOH concentrations to incomplete faecal recovery is necessary to obtain more accurate estimates of diet composition. Nevertheless, it seemed that the use of a less accurate recovery correction on LCOH markers had a lower impact on the accuracy of estimates than that of alkanes and LCFA.

The present study aimed to evaluate the reliability of the n-alkane technique for estimating herbage intake in beef cattle receiving two different feeding levels (low and high) and to test the effect of different faeces sampling procedures (total faeces samples or rectal grab samples at 8 h intervals) on the n-alkane faecal recoveries and hence on intake estimates. Two consecutive experiments were performed with 11 non-lactating beef cows of "Asturiana de los Valles" breed, fed with lucerne hay at two feeding levels: 1.0 or 1.7 kg DM/100 kg body weight (BW). Animals received a daily dose of paper pellets containing C 24 , C 32 and C 36 n-alkanes as external markers with the purpose of using different n-alkane pairs of adjacent chain length (C 23 /C 24 , C 25 /C 24 , C 31 /C 32 , C 33 /C 32 , C 35 /C 36 ) for intake calculations. A 5-day equilibrium period was sufficient for external marker concentrations and n-alkane pair ratios to reach steady state in faeces. There was no effect of different sampling times (every 8 h) on the faecal excretion of n-alkane pairs C 31 /C 32 and C 33 /C 32 . Grab samples obtained at the time of dosing (0830 h) gave the best estimate of n-alkane pair ratios in total faeces collection. There was a general trend of increasing n-alkane faecal recoveries with increased chain length, although n-alkanes C 30 , C 31 and C 33 showed lower faecal recoveries than expected. n-Alkane recoveries were in general lower at the low feeding level, where an increase in individual variability of natural n-alkane recoveries was observed. At both feeding levels, the n-alkane pairs C 23 /C 24 and C 35 /C 36 gave accurate estimates of the treatment average intake, although the natural n-alkane (C 23 or C 35 ) faecal recoveries showed high individual variability, which could be in part due to analytical bias caused by their extremely low concentration in the diet and faeces. However, the n-alkane pairs C 31 /C 32 and C 33 /C 32 gave the greatest deviations of intake estimates, due to the high discrepancy between the faecal recoveries of natural and dosed n-alkanes. These results demonstrate that in beef cattle, feeding level may have some influence on the relative faecal recoveries of the n-alkane pair used for intake calculations. This effect, together with the high individual variability of n-alkane recoveries, especially under low feeding level, may produce significant deviations of the individual intake estimates.