5.5. Added dietary tannin expressed in excreted nitrogen and tannin content

Galliform birds produce two different kinds of excreta, or ”droppings”. Intestinal droppings are fibrous and cylindrical, and they are excreted regularly throughout the day. Caecal droppings, in turn, are soft, shapeless and semi-liquid, and they are usually excreted once a day.

The wet composition of both caecal and intestinal excreta was obvious in grey ­partridges fed natural food (III, V) or tannin (V). The appearance was different in other ways as well. In the natural food group, fibre strands from shoots and grain were visible (III, V). The quebracho-tannin powder tainted the excreta of birds fed tannin dark ­reddish-brownish. It is noteworthy, that the tannin-fed birds emptied their caeca several times per day. Increased water elimination may indicate a decreased capacity to ­concentrate urine (i. e. nephrotoxic effect). It may also indicate a disruption in gastro­intestinal absorption. A larger volume of water may also be needed to void the high concentration of secondary compounds (Jakubas et al. 1993).

Ingested phenolics are reported to significantly increase fecal nitrogen excretion in the American elk Cervus elaphus nelsoni (Mould & Robbins 1981), thus reflecting the amount of tannin-protein complexes in excreta (Mould & Robbins 1981, Bernays et al. 1989, Hewitt & Kirkpatrick 1997). In contrary to this, in this study birds fed natural food had the highest nitrogen content of intestinal excreta (V), not the birds fed tannin. The high nitrogen concentration immediately after the change in diet may have resulted from the inhibiting effect of the high fibre content on the assimilation of nitrogen. Nitrogen assimilation improved during the feeding trial, and nitrogen content of intestinal excreta decreased to the initial level.

The nitrogen content of caecal droppings lowered in both tannin and natural food groups (V). It is possible that more material was actively transported to the caeca in natural and tannin groups, which could then be seen in decreased nitrogen content in the excreta. The excreta were not weighed, so only the relative amount of nitrogen in the excreta is known. Since tannin did not increase the caecal excreta nitrogen level, it may be assumed that the food was sufficient to inhibit the effects of tannin. In the western scrub-jay (Aphelocoma californica) effects of tannins were eliminated in high-protein (30 %) diets (Fleck & Tomback 1996). According to Blytt et al. (1988) the antinutritional effects of dietary tannins did not result from the direct inhibition or binding of digestive enzymes with tannins, but other membrane-associated processes, such as absorption, may have been slightly affected.

Highly water-soluble tannins, like quebracho, probably have low bonding capacity to gut proteins (Haslam 1989), and are difficult to degrade to low molecular weight phenolics (McArthur et al. 1991). The high tannin concentration of caecal excreta (V) also supported the idea that more material was transported to caeca for processing. The low nitrogen concentration of caecal excreta could then be explained by the high tannin concentration. According to Hewitt et al. (1997) northern bobwhites Colinus virginianus excreted 56 % of dietary tannins in caecal excreta.

In ruffed grouse, tannins had the highest impact on caecal function (Hewitt et al. 1997). Transportation of tannin to the caeca could have explained the increased proportion of caecal excreta of total excreta. Although in the northern bobwhite increased filling of caeca could not be explained only by the effects of tannin, caecal volume may also have increased because of abnormal amounts of digested food received. In addition, more uric acid may be transported to the caeca for nitrogen recycling (Karasawa 1989, Karasawa et al. 1997, Stevens 1996). Symbiotic micro-organisms of the caeca (Suomalainen & Arhimo 1945, Barnes 1977, Hanssen 1979b) may metabolise proteins and tannins of tannin-protein-complexes, which may be facilitated by an effective filling of the caeca. The excreta were not weighed, but the caeca of the birds fed tannin were full of material in contrast to other birds. The frequent emptying of the caeca may have been a reflection of rapid filling.