- Chakwizira*, E.I. Teixeira, J.M. de Ruiter, S. Maley and M.J. George
The New Zealand Institute for Plant & Food Research Limited, Private Bag 4704, Christchurch, 8140, New Zealand. Phone: +64 3 3256400, Fax: +64 3 3252074
* Corresponding author: Email: Emmanuel.Chakwizira@plantandfood.co.nz
Nitrogen (N) is one of the major yield-limiting nutrients for crop production. At high application rates the efficiency of N use is reduced and the risk of N loss in soil-plant systems is increased. The N taken up by maize crops is partitioned between vegetative (e.g. leaves and stems) and reproductive organs (e.g. grains) that have economic value. The ratio of grain N to total crop N, defined as the nitrogen harvest index (NHI), provides an indication of how efficiently the plant converts absorbed N into grain. Two field experiments with the maize hybrid ‘Pioneer 39G12’ were undertaken to investigate how N rate and irrigation affected NHI and grain quality of maize grown. Harvest index (HI) and NHI increased with increasing water supply, from 0.47 to 0.53 (HI) and 0.43 to 0.60 (NHI), for the dryland and irrigated crops, respectively. However, neither HI nor NHI was significantly affected by N rate. The grain N concentration (Ng%) increased from 0.97% to 1.1% with water supply, and from 0.92% for the N control to 1.25% for the 200–250 kg N/ha crops in both experiments. However, Ng% did not significantly increase at the higher rates of fertiliser N. The NHI was closely related to HI, which suggests that management options to improve the HI of maize crops would also improve the crops’ ability to utilise N. The response of both HI and NHI to moisture stress, but not fertiliser N, highlights the importance of soil moisture in crop production in this environment, due to its influence on N uptake. Treatments with high water availability caused higher NHI values in crops and therefore we conclude that water management was of more value than N fertiliser rates for increasing NHI up to reported critical thresholds up to ±0.65.