Context. Dairy pasture production is reliant on fertiliser to supply nitrogen (N); however, fertiliser N-use efficiency (FNUE) is low and N can be lost to the environment.

Aims. The aim of this study was to track the fate of N fertiliser applied in a pasture system of ryegrass (Lolium multiflorum, temperate grass) oversown into kikuyu (Pennisetum clandestinum, tropical grass).

Methods. We used ¹⁵N-labelled urea to track the residual plant uptake of a one-off application of ¹⁵N over three pasture cuts subsequent to the first cut in the kikuyu growing season from February 2018 to April 2018 (Experiment 1), followed by total soil and plant recoveries of ¹⁵N over a 12-month period (Experiment 2). Total N treatment rates were 0, 120, 240 and 480 kg N ha⁻¹ year⁻¹, consisting of application events of 40 kg N ha⁻¹. In Experiment 1, ¹⁵N was applied only at the first fertilisation, whereas in Experiment 2, ¹⁵N-labelled urea was applied at each fertilisation event.

Key results. In Experiment 1, uptake of residual ¹⁵N fertiliser in the pasture biomass was <6%. In Experiment 2, FNUE was 29–32% and unaccounted ¹⁵N fertiliser was 22–142 kg N ha⁻¹, across the 120, 240 and 480 kg N ha⁻¹ year⁻¹ treatments.

Conclusions. Recovery of ¹⁵N residual fertiliser did not increase with N rate and was attributed to the mass increase in soil ¹⁵N recovery. FNUE in the pasture did not decrease with N rate. Unaccounted ¹⁵N increased with N rate.

Implications. Existing and alternative N and pasture management strategies such as clover and multi-species pasture need to be implemented and explored to reduce the amount of unaccounted N in dairy pasture production.