Sampling and Statistical Analysis of Snow-Urine Allantoin: Creatinine Ratios

Pils, Andrew; Garrott, Robert; Borkowski, John

Investigations with captive and free-ranging cow elk (Cervus elaphus) have demonstrated that urinary allantoin:creatinine (A:C) ratios can be used as an index of metabolizable energy intake. Practical management applications of the technique, however, would require random collections of snow-urine, resulting in unknown proportions of samples from various sex-age classes. The goals of this research were to investigate sampling protocols and statistical methods for analyzing A:C ratios from random collections of snow-urine to develop an index of cow elk nutrition, and to evaluate these methods by comparing the winter nutrition of cow elk in several free-ranging herds. Snow-urine samples (n = 2,051) were collected from cow, calf, and bull elk in the Madison-Firehole Drainage of Yellowstone National Park during 5 consecutive winters from 1991-92 through 1995-96. Significant differences were detected in mean A:C ratios among sex-age classes. Computer simulations using these data demonstrated that mean A:C ratios derived from random sample collections were highly variable and consistently overestimated the true mean A:C ratio of cow elk. These problems were alleviated by the use of trimmed means, which we calculated by trimming 15% off the right tail and 20% off the left tail of the ordered sample distribution and calculating the mean of the remaining samples. Trimmed estimates of mean A:C ratios were consistently less biased with lower variability of sample means, and hence smaller confidence interval widths than were untrimmed means. Additional simulations and field experience suggested that a realistic sampling goal would be the collection of 20 samples at 2-week intervals throughout the winter. We tested this sampling protocol and the use of trimmed means during the winter of 1996-97 by collecting 1,008 samples from 6 elk herds that were managed for 3 different goals and were expected to have different nutrition. One difference in average overwinter A:C ratios was detected with trimmed means that was not detected with untrimmed means. Several differences in average overwinter A:C ratios were also detected that were consistent with expected differences in nutrition among the 6 herds, indicating that the sampling protocol and statistical methods detailed in this paper for urinary A:C ratios may provide a useful technique for indexing winter nutrition of ungulate herds.

Sampling and Statistical Analysis of Snow-Urine Allantoin: Creatinine Ratios

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