Fecal Measures of Diet Quality in Wild and Domestic Ruminants

Abstract

Fecal indices of diet quality are often used to assess diet quality in free-ranging ungulates, but the nutritional attribute fecal indices measure, the mechanism by which it is measured, and the assumptions involved are seldom considered. I developed an algebraic model relating apparent digestibility (DIG) of diets to endogenous and dietary sources of fecal components. This model predicted that if the ratio of metabolic fecal products to dry matter intake (MFP: DMI) is constant, and a fecal measure indexes MFP, then (1) the fecal index will measure DIG, (2) the relationship between DIG and the fecal index will be curvilinear and asymptotic, and (3) other nutritional characteristics of the diet will be indexed only to the extent that they are correlated with DIG. I used published data from domestic sheep and cattle to test the model assumption that MFP: DMI was constant and to test model predictions for percent fecal nitrogen (FN). Variation in MFP: DMI was partitioned into random variation and that related to apparent indigestibility (INDIG; r 2 = 0.46, P < 0.001). Variation in MFP: DMI related to INDIG slightly altered the expected curvature of the relationship between DIG and fecal measures, whereas remaining random variation influenced the resolution of that relationship. Domestic sheep and cattle on grass diets had the predicted curvilinear relationship between DIG and FN. Also, curvilinear relationships between DIG and FN were similar between domestic sheep fed grass and alfalfa diets, despite different (P < 0.001) intercepts. In contrast, slopes of the relationships between dietary N and FN for domestic sheep differed (P < 0.001) between grass and alfalfa diets, with little overlap between the 2 forage groups, supporting the prediction that dietary N is not indexed directly by FN. Reduced protein digestibility from dietary tannins will confound the relationship between DIG and FN because increased forage N in FN reduces the ability of FN to index MFP. Other potential fecal indices may be less influenced by tannins. I recommend using a natural log transformation of fecal measures that index MFP to linearize their relationship with DIG. The model provides a conceptual framework for additional research relative to fecal measures of diet quality.