The purpose of this paper is to assess the impacts of changes in herd average urine genetic traits on nitrogen (N) leaching per hectare per year. Two potential urine traits being considered for genetic selection are total volume of urine per cow per day (at constant urinary N output), and average volume per urination (at a constant daily urinary volume and N output). To understand the potential reduction in N leaching that would arise if these traits could be changed using genetic selection, a model of urine patches was developed to derive the necessary relationships between urine genetic traits, patch characteristics, and N leaching. To simulate the effect on N leaching of genetic selection the mean for each urine trait was then changed from base model values by ±10%. Increasing total urine volume per cow per day by 10% was more effective (4.5% less N leaching) than decreasing volume per urination by 10% (2.9% less N leaching). Also, selection for urine traits at higher stocking rates (four cows/ha) and higher N intake (650 g/N/day) had a lower impact on N leaching compared with lower stocking rates (two cows/ha) and lower N intakes (450 g N/day). This study showed that genetic selection for urine traits could have a positive, but limited, impact on N leaching and would be a useful tool for New Zealand dairy farmers. Keywords: Urine traits; urine patches; nitrate leaching
Proceedings of the New Zealand Society of Animal Production, Volume 80, Online, 60-64, 2020
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