Use of acid-insoluble ash and n-alkanes as markers of soil and plant ingestion by chickens

Soil ingestion is recognized as the main source of environmental contaminants in food-producing animals reared outdoors. Therefore, its consumption should be quantified for proper risk assessment under practical circumstances. A quantitative method, involving the determination of acid-insoluble ash (AIA) in feces, was previously evaluated in several animal species. Nevertheless, this method is highly sensitive to feed digestibility, which may be influenced by the unknown amount of soil actually consumed. Besides, animals may ingest plants in addition to soil. Therefore the amount and the digestibility of plants ingested should also be known. The purpose of the current study was to assess a method for determining soil and plants ingested by free range chickens using AIA and odd-numbered n-alkanes (C25 to C31) as markers of soil and of plants, respectively. Ninety slow-growing chickens aged 38 days were placed in individual cages and given nine pelleted diets containing, in addition to standard feed, graded levels of soil and plants alone, or in combination (0 to 30% and 0 to 15% of diet dry matter (DM), respectively). After a 5-day adaptation period, excreta from each bird were collected in their entirety for three days. The energy retention coefficient (ERC) of feed alone and of plants was estimated to be 0.70 and 0.14, respectively. The presence of soil linearly depressed feed ERC but did not influence plant ERC. The effect of soil on feed ERC was quite low, reaching 0.02 for 30% of soil incorporation in diet DM. The average recovery rate of ingested n-alkanes in excreta was low (43%) and variable between n-alkanes and between diets, while AIA was totally recovered in all diets except in feed alone. After the concentration of each n-alkane in excreta was corrected by its mean recovery rate, comparison of the feed, plant and excreta profiles elicited an estimate of plant contribution to DM ingested biased by 0 to 2% of plant contribution to DM. Despite these biases and the slight influence of soil on feed ERC, introducing the ERC of feed alone into the soil-ingestion equation provided a good estimate of soil contribution to DM ingested. The sequential current methodology is a promising tool for evaluating soil ingestion by broilers reared outdoors under practical circumstances. However, further work is required to alleviate uncertainties concerning the impact of different types of soil on the ERC of feed given to animals. (C) 2013 Elsevier B.V. All rights reserved.