Nitrogen rate and timing effects on growth and yield of drill-sown rice

Abstract

Eight rice experiments were established at two sites in the Riverina district of south-eastern Australia in the 2012–13 and 2013–14 seasons. Two semi-dwarf rice varieties were drill-sown and nitrogen (N) fertiliser (urea) was applied at different rates at the 4-leaf stage before permanent water (pre-PW) and at panicle initiation (PI). The research assessed the impact of timing of N application on grain yield, compared the apparent N recovery of N fertiliser applied at the two stages, and determined an application strategy for N to obtain consistently high grain yields for current, semi-dwarf rice varieties when drill-sown. The apparent N recoveries achieved were 59% for N applied pre-PW and 25% for N applied at PI, averaged across years, sites, varieties and N rates. Grain yield increased significantly with increased rate of N applied at both stages, but the rate of increase from N applied at PI decreased as the rate of N applied pre-PW increased. The grain yield increase for N applied pre-PW was due to increased number of panicles at maturity and increased number of florets per panicle. Nitrogen applied at PI increased dry matter at maturity and number of florets per panicle. Application of N at PI increased grain yield over that when no N was applied; however, at low PI N-uptake levels, application of N at PI is not enough to achieve high grain yields. Therefore, sufficient N should be available to the crop from a combination of soil- and pre-PW-applied N for the crop to reach a level of N uptake at PI whereby high yields can be achieved. Nitrogen applied at PI did not appear to increase the potential for cold-induced floret sterility as much as pre-PW-applied N. Further research is required to confirm this in other seasons and for other rice varieties.

Modeling Mid-Season Rice Nitrogen Uptake Using Multispectral Satellite Data

“…Following determination of N uptake at PI, recommendations of optimal N topdressing rates are needed. These recommendations may be generated using studies such as the one described in [15], where plots were sampled for PI N uptake, and various topdressing rates were applied across plots. Then final grain yield results are used to generate tables of optimal PI N application as a function of PI N uptake.…”

“…The most effective time for a mid-season application is at the panicle initiation (PI) stage [13], which occurs at the start of the reproductive phase of rice development, when the panicle begins to form in the base of the stem [14]. Typically, a base N rate is applied at the start of the season, then tissue tests are done mid-season to determine additional N top-dressing rates [15].…”

“…Pre-season soil tests have not proven to be a reliable way to determine paddy soil N requirements [16,17]. Detailed recommendations of N top-dressing rates as a function of PI N uptake and rice variety have been developed, utilising physical samples of plants at PI [15,18]. Site-specific management of rice N application has been shown to both improve yield [19] and reduce total N applied [20].…”

“…There are a number of methods to determine N uptake at PI, with varying degrees of cost-effectiveness, practicability, accuracy and spatial resolution. Plant tissue samples can be collected and analyzed, however growers have little time and motivation to perform this task [15]. When they do take samples, they are often only at a few points and so may not encompass the variability in their paddies.…”

“…Therefore, developing an operational remote sensing based method to determine N uptake is of great interest, as it captures spatial variation and may eliminate the need for field sampling. Typically industry desires variable N prescription maps with rates separated into 30 kg/ha classes (as aerial spreaders are probably not much more accurate than this and there is minimal impact on yield at PI topdressing rates less than 30 kg/ha [15]). Therefore, N uptake prediction errors of less than 30 kg/ha are required [18].…”