Effect of form and of quality of feed on the concentrations of purine derivatives in urinary spot samples, daily microbial N supply and predictability of intake
Introduction
Low intake, particularly during the dry season when feed availability is low, is one of the major factors limiting animal production from native pasture in developing countries in the tropical and subtropical regions. Consequently, a rough guide on how much the animal has consumed from grazed pasture is necessary in order to intervene strategically with supplementary feeding. Prediction of intake, and for that matter of grazing stock, is a long-standing problem, and although numerous attempts have been made, it is but fair to add that few of the methods had integrated intake prediction with some indices of nutrient, particularly N, status.
Consumed feed undergoes ruminal fermentation which is coupled, to varying degrees, with the synthesis of microbial protein. Since microbial protein synthesis is dependent on fermentable OM and ammonia-N supply, it is hoped that if the efficiency of synthesis does not vary enormously, digestible OM intake can be predicted given daily ruminal output of microbial N. Microbial N is assimilated principally as amino acids and nucleic acids, both of which undergo a series of metabolic processes. The catabolism of purine bases usually yields purine derivatives (PD), which are principally, xanthine, hypoxanthine, uric acid and allantoin. Although allantoin is quantitatively the most abundant, the former three in the urine of ruminants vary from one species to another. It is, therefore, not surprising that a close relationship exists between urinary excretion of PD and duodenal supply of purines (Giesecke et al., 1984, Fujihara et al., 1987, Barcells et al., 1993). Others (Antoniewicz et al., 1980, Chen et al., 1990a, Verbic et al., 1990) have demonstrated that the response of PD in the blood and urine to any changes in the uptake of exogenous purines from the intestine occurs very rapidly, though Balcells et al. (1991) have added that allantoin is the only PD that consistently responds to changes in exogenous purine supply.
These studies indicate that PD measured in urine or in plasma are quantitatively related to absorbed purine; thus suggesting that the concentration of PD in the urine would be proportional to exogenous purines when urination is invariant. Osuji et al. (1993) demonstrated the potential of PD concentration in total urine as a predictor of intake and nutrient status in sheep. However, a logical step forward would be to use the concentrations of PD in plasma or urinary spot samples as an index of intake or N status. Unfortunately, the concentration may vary as a function of the plasma pool size or urine volume, both of which may vary considerably as a function of dietary and environmental factors. Others have suggested a cautionary use of the creatinine, which is excreted in a fixed proportion relative to the metabolic body weight and is less affected by dietary factors (Fujihara et al., 1987, Balcells et al., 1991, Chen et al., 1992a), to adjust for variation in the urine or plasma volume. Chen et al. (1992a) have added that the variation of the ratios of the concentrations of allantoin or PD to creatinine in spot urine may be quantitatively small. Consequently, this study examined the concentrations of PD in urinary spot samples as predictor of intake as such a method would lend itself quite usefully to the prediction of intake of grazing animals.
Section snippets
Materials and methods
These studies were conducted at the Debre Zeit Research Station of the International Livestock Research Institute (ILRI) in 1995.
Chemical and statistical analyses
Feeds, faeces and urine were analyzed for DM, OM and/or N according to standard procedures (AOAC, 1980). NDF was determined according to Van Soest et al. (1991), urinary allantoin according to Young and Conway (1942), but the ice bath was replaced with an icy alcohol bath (0.40 alcohol, −10°C). Urinary uric acid was analyzed by auto-analyzer according to Chen et al. (1990b).
PD of microbial origin (X; mmol) were estimated according to Chen et al. (1990c)where PD (mmol) is the
Results
Dietary compositions are given in Table 1 and indicate that teff straw had marginally inferior N content relative to napier grass; both of which had large differences in the NDF content. Supplementation increased dietary N content and reduced the fibber content.
Effect of drying, roughage type and of supplementation
The roughage diets used in this study had similar contents of crude protein, which were within the range 37.0–55.4 g/kg. This range is far lower than 90 g/kg which is perceived as the minimum at which microbial requirement for N could be met (Nsahlai, 1991). The quality of teff straw was, however, worse than that of napier grass as indicated by the fibber content (670 versus 556 g/kg). This could, thus, explain why napier grass tended to be superior to teff straw in terms of efficiencies of
Acknowledgements
The authors happily acknowledge the assistance of (1) staff in the cattle barn at DZ Station during data collection, (2) Ato Berhanu Wodajo for sample organization and (3) staff at the ILRI Nutrition Laboratory in Addis Ababa.
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