Nitrogen Utilization and Predictive Modeling of Nitrogen Efficiency and Excretion in Lactating Dairy Cows

A key factor in reducing nitrogen (N) emissions from dairy farms is understanding the interdependencies between N intake (NI), N excretion (Nex), and N use efficiency (NEff) in cows. This study analyzed these parameters and their interrelationships to develop predictive models. Data were collected over two years, during six feeding periods with different rations (N: 20.5–25.6 g per kilogram dry matter [g/kg DM]; metabolizable energy (ME): 10.8–12.1 megajoules per kilogram dry matter [MJ/kg DM]; net energy for lactation (NEL): 6.5–7.5 MJ/kg DM). Each period included 48 German Holstein cows (620 ± 65 kg body weight [kg BW]; first to fifth lactation). Blood, urine, feces, milk, and feed samples were collected, and individual feed intake and milk yield were recorded. NI was positively related to nitrogen in milk (NM; R2 = 0.68 **) and feces (NF; R2 = 0.49 ***). Several feeding, milk, and blood parameters showed significant correlations with NEff. These variables were used to develop two mixed-effects models. One model is based on variables available under practical feeding conditions. It includes dietary gross energy (GE), dietary crude protein (CP), milk yield, and milk protein concentration. This model shows good statistical performance in comparison to the calculated NEff (CCC = 0.78). The best explanatory model added blood parameters and further increased prediction accuracy (CCC = 0.90). In conclusion, NEff can be reliably predicted using moderately complex models. These models combine routine feeding and milk data with selected blood indicators.