Growth, nitrogen partitioning and nutritive value of fodder beet crops grown under different application rates of nitrogen fertiliser

Nitrogen (N) supply needs to be closely matched to crop demand to achieve optimum N use efficiency (NUE). Sub-optimal N supply can lead to poor yields, whereas excess N application may cause nitrate leaching and environmental pollution. An experiment with five rates of N: 0, 25, 50, 100 and 200 kg ha ⁻¹, was carried out at Lincoln, Canterbury, in New Zealand to define the effects of N supply on growth, N uptake and define how derived NUE may influence nutritive value of fodder beet production. Both dry matter (DM) yield and N uptake increased with N supply, by 39% and 129%, respectively, when 200 kg N ha ⁻¹ was applied, compared with the control plots. Leaf area index (LAI) also increased with N supply. However, the control crop did not attain critical LAI (LAI cᵣᵢₜ) and those receiving 25 and 50 kg N ha ⁻¹ attained LAI cᵣᵢₜ for only a short period of time compared with crops supplied with ≥100 kg N ha ⁻¹. This meant that they intercepted less radiation and subsequently had lower DM yield than the high N treatments. The specific leaf nitrogen increased 13% to 2.34 g N m ⁻² leaf when ≥50 kg N ha ⁻¹ was applied, compared with the control plots. The NUE decreased by 64% from 93 kg DM kg ⁻¹ N for the 25 kg N ha ⁻¹ plots compared with the 200 kg N ha ⁻¹ plots. Nitrogen rate had no effect on the nutritive value of fodder beet. The results suggest 100 kg N ha ⁻¹ was adequate for optimum DM production and N was important for canopy development and subsequent DM accumulation.