The effect of housing system on apparent digestibility in pigs, using the classical and marker (chromic oxide, acid-insoluble ash) techniques, in relation to dietary composition

The present study examined differences in faecal digestibilities of organic matter (dOM) and crude protein (dXP), between growing pigs housed either in pens as groups or individually in metabolism cages. In addition, a study was made of the influence of dietary composition on these observed differences. Four experimental diets were composed by iso-energetic exchange of maize starch in the control diet with purified cellulose (260 g kg-1), toasted soyabean hulls (280 g kg-1) or renderers fat (67 g kg-1), respectively. Both in pens and in metabolism cages, dOM and dXP were measured, using both chromic oxide (Cr2O3) and acid-insoluble ash (AIA) as markers. In metabolism cages, the results of the marker method were compared to those of the classical method, where dOM and dXP were measured by collecting faeces quantitatively. Recoveries of both markers were measured, after a period of 10 and 3 days. With Cr2O3 as marker, pen-housing resulted in a dOM which was on average 2.5 (1.7-4.5) units lower and a dXP averaging 4.5 (3.0-5.8) units lower than measured in the metabolism cages (P < 0.001). No significant interactions were demonstrated between housing system and dietary composition. In contrast, with AIA as marker significantly higher digestibility values were observed for pigs in pens, when fed the cellulose diet. With this diet, the dOM was on average 14.7 units higher and the dXP was on average 10.9 units higher for the penned pigs. For the pigs fed one of the other three diets, the dOM was on average 1.5 (0.1-2.7) units lower and the dXP was on average 3.7 (1.9-5.7) units lower. Thus, with AIA as a marker, the effect of housing system on digestibility interacted with the type of diet (P < 0.05 on dOM and P < 0.01 on dXP). Comparison between the marker method with the classical method, showed that Cr2O3 provided similar results. In contrast, AIA displayed significantly higher dOM and dXP, except with the cellulose diet. This could be explained by differences in the recoveries of both markers. For Cr2O3 this was close to 100% and for AIA it varied from 97% on the cellulose diet to 183% on the control diet. The analytical procedure for AIA requires more research. Shortening the measuring period from 10 to 3 days did not prove to be significant, but increased the standard deviations. It was concluded that for practical application, faecal digestibilities should be measured with penned pigs. According to the findings, under such conditions Cr2O3 a good marker. AIA was found to be unsuitable.