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Relationship between urinary energy and N excretion in lactating Jersey cows.

D. L. Morris




Relationship between urinary energy and N excretion in lactating Jersey cows.
D. L. Morris*1, J. L. Firkins2, W. P. Weiss3, P. J. Kononoff1. 1Department of Animal Science, University of Nebraska�Lincoln Lincoln, NE, 2Department of Animal Sciences, The Ohio State University Columbus, OH, 3Ohio Agricultural Research and Development Center, The Ohio State University Wooster, OH.

Estimation of urinary energy (UE) excretion is essential to determine metabolizable energy supply. Because most energy containing compounds in urine also contain N (e.g., urea, allantoin, hippuric acid, creatinine), our objectives were to validate the use of urinary N (UN) to estimate UE. Individual animal data from 4 studies (n = 134) were used. The data set included a 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 period within 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 880 kcal/g of N for the linear model and not different from 0 (P = 0.50) for the quadratic model. Because most energy compounds in urine contain N, an intercept of 880 is biologically unlikely. The intercept was forced through 0. 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 was observed in the linear (P < 0.01), but not in the quadratic regression (P = 0.19). With increasing UN in the quadratic regression, UE increased but at a diminishing rate. Increasing UN from 100 to 150 g and 150 to 200 g increased UE by 599 and 420 kcal, respectively. To better understand the relationship between UE and UN, UE as kcal/g N was regressed against UN (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 excretion was 17.7, 15.6, and 13.6 kcal/g N, respectively. This decrease was likely due to an increase in the proportion of UN that is from urea which has a lower enthalpy per g of N compared with the non-urea components of urine (5.4 vs. 24 kcal/g N). Our analyses show that a quadratic relationship exists between UE and UN excretion from 85 to 220 g/d which is likely due to the dilution of non-urea N with increased UN excretion.

Keywords: metabolizable energy, bomb calorimetry.