Nitrogen uptake and leaching from urine and fertilizer applications during simulated grazing rotations of a perennial grass pasture

Context

The productivity of grazing-based dairy systems is driven in large part by availability of nitrogen (N) as it cycles though the soil, plant and animal. However, N use efficiency (NUE) is generally less than 40 % with significant N losses attributed to animal excreted N, especially urinary N, due to high N concentrations deposited.

Objectives

This experiment aimed to quantify N uptake, N transformations, and N leaching losses in a grazing-based dairy pasture and recommend strategies to enhance farm-system NUE.

Methods

A detailed experimental field site was established on a perennial ryegrass dominated pasture to measure the fate of N under contrasting fertilizer forms (urea or calcium nitrate) and artificial urine applications (400 or 800 kg N/ha), which aimed to reflect differing cow crude protein intakes. N uptake, use efficiency and leaching losses were determined over an 18-month period during typical grazing rotations and contrasting seasonal conditions.

Results and conclusions

Pasture yield response and N concentration reflected N rates applied. NUE from urine and fertilizer applications was highly variable and strongly influenced by soil moisture and temperature conditions. N applied increased pasture yields up to 8 months after application. Soil water N concentrations peaked and then decayed to background levels over a 3 to 4-month period, corresponding to N loads applied and soil moisture conditions. Regular urea and Ca nitrate fertilizer applications resulted in nitrate-N leaching losses of 92 and 97 kg N/ha, respectively. N leaching losses from urine treatments ranged from 168 to 735 kg N/ha, representing 21 to 46 % of total urinary N applied. The timing of urine applications was crucial, with over 40 % of urinary N leached when applied during the first half of the year (summer to autumn), compared to less than 21 % when applied in the second half of the year (late winter to spring).

Significance

Grazing-based dairy production systems result in high N loads from urine and fertilizer with highly variable pasture NUE in different seasons, and substantial N leaching. Improving the amount and timing of urinary and fertilizer N inputs is critical to increasing N use efficiency. Optimizing dairy cow N intakes through better balanced diets and restricting grazing and fertilizer applications during periods of low pasture growth are proposed management approaches to increase whole-farm NUE and reduce N losses.