Ruminant Nutrition General

摘要

Estimation of urinary energy (UE) excretion is essential to determine metabolizableenergy supply. Because most energy containing compounds inurine also contain N (e.g., urea, allantoin, hippuric acid, creatinine), ourobjectives were to validate the use of urinary N (UN) to estimate UE. Individualanimal data from 4 studies (n = 134) were used. The data set includeda wide range (min to max) in d in milk (88 to 346), dry matter intake(11.6 to 24.6 kg/d), N intake (282 to 642 g/d), energy-corrected milk yield(14.8 to 48.2 kg/d), UE excretion (1390 to 3160 kcal/d), and UN excretion(85 to 220 g/d). All models included the random effect of cow and periodwithin study. The relationship between UE (kcal/g) and UN (g/100 g) was(parameter estimate ± standard error) UE = 0.128 ± 0.0074 × UN + 0.0165± 0.0054. In the initial models between UE and UN, the intercept was 880kcal/g of N for the linear model and not different from 0 (P = 0.50) for thequadratic model. Because most energy compounds in urine contain N, anintercept of 880 is biologically unlikely. The intercept was forced through0. The regressions of UE (kcal/d) on UN (g/d) were UE = 14.6 ± 0.32 ×UN and UE = 20.9 ± 1.0 × UN – 0.0357 ± 0.0056 × UN2. Slope bias wasobserved in the linear (P < 0.01), but not in the quadratic regression (P =0.19). With increasing UN in the quadratic regression, UE increased butat a diminishing rate. Increasing UN from 100 to 150 g and 150 to 200 gincreased UE by 599 and 420 kcal, respectively. To better understand therelationship between UE and UN, UE as kcal/g N was regressed againstUN (g/d). The regression was UE (kcal/g N) = −0.0412 ± 0.0060 × UN(g/d) + 21.8 ± 1.0. The ratio of UE to UN at 100, 150, and 200 g of UN excretionwas 17.7, 15.6, and 13.6 kcal/g N, respectively. This decrease waslikely due to an increase in the proportion of UN that is from urea whichhas a lower enthalpy per g of N compared with the non-urea componentsof urine (5.4 vs. 24 kcal/g N). Our analyses show that a quadratic relationshipexists between UE and UN excretion from 85 to 220 g/d whichis likely due to the dilution of non-urea N with increased UN excretion.