Evaluation of three collection protocols to determine urine output and urinary urea nitrogen excretion in dairy cows in response to dietary salt supplementation

Authors: LETELIER, PAULINA - University Of Wisconsin; Zanton, Geoffrey; WATTIAUX, MICHEL - University Of Wisconsin

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: This study evaluated three protocols to determine urine output and urinary urea-N (nitrogen) excretion. Urinary urea-N excretion, the end-product of N metabolism in dairy cows, was lower when determined by spot sampling compared to bladder catheterization or via an external collection cup device. Urea-N and creatinine concentration were greater for spot sampling than for bladder catheterization. Urine specific gravity explained 66.5, 73.2, and 32.1% of the variation in urine output for bladder catheterization, collection cup, and spot sampling, respectively which could lead to reasonably good predictions of urine output for the former two methods. Increasing salt in the diet tended to increase urine output, decrease urinary urea-N concentration, and milk protein concentration and yield. Determining and predicting urine and urinary nitrogen output for on-farm application and nutrient management require robust methods for data generation and validation. This experiment provides a test and recommendations for existing and common methods of urine collection and estimation.

Technical Abstract: Urine output and urinary urea-N (UUN) excretion is a critical measure to accurately evaluate N metabolism in lactating dairy cows and environmental concerns related to manure N. The main objectives of this study were to: (a) compare estimates of UUN, urine output, and related variables from three pre-established measurement protocols (bladder catheterization, external collection cup, and spot sampling) and from dietary salt supplementation, (b) study within-day and between-day variation in UUN, urine output, creatinine concentration (mg/dL), and excretion (mg/kg body weight) as affected by measurement protocol, and (c) to evaluate urine specific gravity as a predictor of urine output. Twelve multiparous Holstein cows were used in a split-plot, Latin square design. Cows were randomly assigned to a diet (main plot) containing either 0.7 or 1.7% NaCl (dry matter basis). Cows were then assigned to a sequence of three measurement protocols (sub-plot) in a balanced 3 × 3 Latin square with 14-d period. Urine was collected and measurements were conducted every 4 h in a 24-h period for 3 consecutive days. Urine specific gravity was measured by refractometry. Urine output was calculated directly for total collection procedures (bladder catheterization and external collection cup) or by estimation using cow body weight and urinary creatinine concentration for spot sampling (assuming 29 mg creatinine excretion/kg body weight per d). Compared to bladder catheterization, the spot sampling protocol predicted 26 g/d lower UUN excretion and 6.8 kg/d lower urine output. However, UUN concentration was 58 mg/dL greater for the spot sampling compared to the bladder catheterization protocol. Urine output tended lower for the collection cup procedure, although no difference in UUN concentration or excretion was observed when compared to bladder catheterization. Creatinine concentration was greater for spot sampling (88.1 ± 3.19 mg/dL) compared to the other two protocols (64.7 mg/dL). Salt supplementation tended to increase urine output and decrease urinary urea concentration and milk protein concentration and yield. Urine output exhibited greater within-day and between-day variation for the collection cup compared to bladder catheterization protocol. The collection cup protocol resulted in greater coefficient of variation (CV) for UUN excretion than bladder catheter for d1 and d2 of sampling (27.0 versus 10.2%, respectively) but not for d3 of sampling (10.4 versus 9.25%, respectively) suggesting that an adaptation may improve data quality from the collection cup protocol. Thus, collection cup device may be used as a less invasive protocol than bladder catheterization to determine UUN excretion. Urine specific gravity explained 66.5, 73.2, and 32.1% of the variation in urine output for bladder catheterization, collection cup, and spot sampling, respectively, which may result in improved urine volume predictions.