Validación de los n-alcanos como marcadores para estimar la ingestión y digestibilidad en cerdos

With the aim of validating the use of n-alkanes in pigs, an experiment was planned with 12 Landrace x Large White barrows, 15 weeks old and with an average live weight of 44.9 ±0.78 kg. The trial was divided into three consecutive periods to check the hypothesis of an influence of age and feeding level on n-alkane faecal recovery. Within each period, half of the animals were fed ad libitum and the rest at a restricted level. A digestibility trial was performed in each experimental period, in which faecal recovery of natural alkanes was also studied. External (synthetic) n-alkanes (tetracosane (C24), octacosane (C28), dotriacontane (C32) and hexatriacontane (C36)) were dosed in periods 2 and 3 and their faecal recovery also studied. External alkanes were dosed once daily in Period 2 (09:30 h) and twice daily in Period 3 (9:30 and 18:00 h). Spot faecal samples were also taken, twice daily (9:30 and 18:00 h) and directly from the rectum of the animals in periods 2 and 3. Faecal recovery of acid-detergent insoluble ashes (ADIA) was also determined in the three experimental periods. As expected, feed intake increased with the age of the animals, and was higher for those fed ad libitum. With respect to in vivo digestibility, there were differences between periods (except for dry matter –DM- digestibility), with lower values for Period 1. No differences were found between periods 2 and 3. The same trend was observed for ADIA faecal recovery, with lower values for P1 and no differences between P2 and P3. Faecal recovery of the different natural n-alkanes was not affected by the feeding level within each experimental period or the animals’ age, and followed an increasing-decreasing curvilinear evolution. Maximum recovery appeared for alkanes with 28-29 carbon atoms. Recovery of external alkanes was much higher than that of natural hydrocarbons, and was affected by the feeding level and the experimental period. These results indicate that external alkanes might be transported with the liquid phase of the digesta, which rate of passage is highly influenced by the feeding level. Feed intake estimates from recovery-corrected alkane concentration in the whole faeces produced during the digestibility balance period were not different than in vivo values, regardless the odd/even chain pair of alkanes chosen. However, the lower deviations were found for combinations involving C24 and C28 dosed alkanes. Hence estimates of feed intake from recovery-corrected alkane concentration in spot faeces were restricted to combinations with those hydrocarbons. In this latter case, statistical differences were found between animals. The interaction between feeding level, experimental period and form of estimation of feed intake (sampling time and pair of alkanes used) was also significant. Feed intake estimates from spot samples underestimated in vivo values by 16.8 per cent and 20.4 per cent when using morning or evening samples, respectively. Using the average concentrations of the two samples underestimated in vivo values by 6.1 per cent. Important differences in alkane concentrations between morning and evening samples (and then circadian variations) are sought as a possible reason for these discrepancies. Estimates of digestibility from recovery-corrected alkane concentration in the whole faeces produced during the digestibility balance period were not different than in vivo values, regardless the alkane chosen. However, the lower deviations were found when using alkanes C21 (heneicosane), C29 (nonacosane), C30 (triacontane) and C31 (hentriacontane), hence only these paraffins were used to estimate digestibility from recovery-corrected alkane concentration in spot faeces. Values obtained this way, and using individual recoveries for each pig, overestimated in vivo DM digestibility by only 1.0 per cent with morning samples or 1.1 per cent with evening samples. The combined use of both samples produced an underestimation of 0.5 per cent. When average recoveries for all pigs under the same treatment were used, morning sampling overestimated in vivo DM digestibility by 3.9 per cent and evening sampling by 4.5 per cent. Average value of the two samples overestimated by 3.0 per cent. Such small differences between in vivo and alkane-estimated values are a consequence of the unique use of natural alcanes for digestibility estimates, avoiding the effect of the circadian variation in the faecal concentration of external alkanes. Assuming similar recoveries for adjacent odd/even chain alkanes when estimating digestibility, or 100 per cent recovery of individual alkanes for digestibility estimates, is not an advisable practice as the results obtained were always statistically different from those calculated from the in vivo trials. From the results of the present experiment it can be concluded that the use of n-alkanes can be an alternative to estimate individual intake and digestibility in grouped pigs fed concentrate diets. As faecal recovery of natural alkanes was not influenced by animals’ age or feeding level, reference values could be easily obtained, at least for each feed type. Before the use of n-alkanes for feed intake estimate can be recommended, circadian variation of faecal concentration must be studied, mainly that of external hydrocarbons. The low and highly variable ADIA faecal recovery found in the present experiment precludes their utility as intake or digestibility internal markers in pigs