Does choice of estimators influence conclusions from true metabolizable energy feeding trials?

Abstract

True metabolizable energy (TME) is a measure of avian dietary quality that accounts for metabolic fecal and endogenous urinary energy losses (EL) of non-dietary origin. The TME is calculated using a bird fed the test diet and an estimate of EL derived from another bird (Paired Bird Correction), the same bird (Self Correction), or several other birds (Group Mean Correction). We evaluated precision of these estimators by using each to calculate TME of three seed diets in bluewinged teal (Anas discors). The TME varied by <2% among estimators for all three diets, and Self Correction produced the least variable TMEs for each. The TME did not differ between estimators in nine paired comparisons within diets, but variation between estimators within individual birds was sufficient to be of practical consequence. Although differences in precision among methods were slight, Self Correction required the lowest sample size to achieve a given precision. Feeding trial methods that minimize variation among individuals have several desirable properties, including higher precision of TME estimates and more rigorous experimental control. Consequently, we believe that Self Correction is most likely to accurately represent nutritional value of food items and should be considered the standard method for TME feeding trials.

Assessment of excreta collection methods to estimate true metabolizable energy of waterfowl foods in wild ducks

“…Although inherently large variability in processing efficiency of aquatic vegetation among individual mallards contributed to our inability to claim practical equivalency between collection methods, the means and ranges of TME N values reported herein resemble those of other species of aquatic vegetation (McClain , Gross ). In their comparison among methods of estimating endogenous energy losses during TME trials, Sherfy et al () postulated a priori that a mean difference in TME between individuals relative to the mean TME for the 2 methods exceeding 20% warranted biological significance. It is unclear from Sherfy et al () how this threshold was derived, but they justified the value based on ‘very high precision’ among published TME estimates, which were primarily seeds from moist‐soil plants, acorns, and agricultural grains (Petrie et al , Checkett et al , Kaminski et al ).…”

“…In their comparison among methods of estimating endogenous energy losses during TME trials, Sherfy et al () postulated a priori that a mean difference in TME between individuals relative to the mean TME for the 2 methods exceeding 20% warranted biological significance. It is unclear from Sherfy et al () how this threshold was derived, but they justified the value based on ‘very high precision’ among published TME estimates, which were primarily seeds from moist‐soil plants, acorns, and agricultural grains (Petrie et al , Checkett et al , Kaminski et al ). Indeed, previously reported means for these food items have been precise (i.e., CV = 0.2–16.9%, Checkett et al ) relative to TME N of submersed aquatic vegetation for mallards (CV > 89%; McClain ) despite similar or lower sample sizes.…”

“…Sequences were randomized; therefore, when basin collection followed harness collection, we removed the harness material and allowed the retaining ring to slough naturally before collecting excreta using the basin method. We corrected for metabolic and endogenous energy of each individual bird and each collection method using the self‐control method (Sherfy et al ).…”

Variation in True Metabolizable Energy Among Aquatic Vegetation and Ducks

“…We set an a priori goal of obtaining 16–20 (8–10 males, 8–10 females) TME N samples per vegetation species from both mallards ( n = 35 individuals) and gadwall ( n = 51 individuals) based on expected levels of variation among samples and sample sizes from previous studies (Checkett et al 2002, Kaminski et al 2003, Sherfy et al 2005, Dugger et al 2007, Coluccy et al 2015). We used the self‐control method advocated by Sherfy et al (2005) to reduce intraspecific process variation, but we substituted group‐control estimates calculated from the like duck species and year if a self‐control sample was unavailable, likely biased (i.e., apparent error in excreta collection or analysis resulting from loss or contamination of excreta), or produced an energy assimilation efficiency exceeding 100% (i.e., TME N > gross energy). Vegetation species and control trials were dispersed throughout each autumn trial period to account for potential temporal variation associated with digestion efficiency (Gross 2018).…”

“…Avian diet quality is typically assessed using true metabolizable energy (TME N ; Sibbald 1976), which is a measure of assimilable energy accounting for innate endogenous losses (i.e., fecal and urinary energy of non‐food origin; Sherfy et al 2005). True metabolizable energy assays were developed to refine poultry diets but have been adapted to determine the value of natural wetland foods (i.e., moist‐soil seeds, aquatic invertebrates, and submersed aquatic vegetation) for wild waterfowl (Sibbald 1976, Kaminski et al 2003).…”