17 May 1996 Solving the fertiliser use – crop yield equation

Efficient use of fertiliser nitrogen (N) in grass seed crops is essential for optimising yields whilst minimising environmental risk. This has been the focus of research for Dr Jacqueline Rowarth, of Lincoln University's Plant Science Department and AgResearch Lincoln, for the past five years. Nitrogen is generally the most limiting nutrient in cropping soils and there has been considerable research on what rate of N to use and when it should be applied. Results have been conflicting, mostly because of differences in N supply from the soil and atmosphere, plus soil moisture and temperature differences. Recognising these problems, Dr Rowarth has been working on finding a predictive yield indicator.

An ideal indicator would reflect potential crop yield at harvest sufficiently early in the season to be able to make changes if necessary. In a series of pot and field trials Dr Rowarth was able to establish that herbage N % measured at stem elongation was highly correlated with seed yield at harvest. "Herbage N is a measure of the plant N status", said Dr Rowarth. "The plant takes up nitrogen from several sources; measuring the fertiliser input alone is far less sensitive than measuring the integrated effect of all sources in the plant. The relationship is so good that I hardly believed the results the first time I saw them." Repeated testing has, however, confirmed the relationship and seed growers are now using it to assist fertiliser planning.

Herbage N % can be measured sufficiently early to be able to make adjustments in fertiliser strategy, but the 'window of opportunity' for efficient fertiliser uptake is narrow. To refine the process Dr Rowarth is now working on an instant indicator of N status. Grower interest is such that she will be able to calibrate the meter against ryegrass crops around the Plains – which is much more satisfactory than simply using research crops.

Nitrogen uptake is not the only part in the equation; what happens to N fertiliser not used by the plant? Unused N might be available to a subsequent crop, but it might also be susceptible to leaching during rainfall or irrigation. This is the subject of N recovery work with Dr Prue Williams of Crop and Food Research, involving ryegrass, fescue, and browntop. Further detailed studies involving lysimeters is being done with Professor Keith Cameron of Lincoln University's Soil Science Department.

"Fertiliser use efficiency will become increasingly important as growers strive for increased yields by using more N, while society requires less N to be used because of concerns about water portability", said Dr Rowarth. "As scientists our task is to find a compromise acceptable to all; the fact that so many different groups are prepared to become involved is an indication of the importance of the issue."

Dr Rowarth's research has been funded by the New Zealand Plant Breeder's Research Association, the New Zealand Herbage Seed Subsection of Federated Farmers, the Foundation for Arable Research, the Foundation of Research Science and Technology, AgResearch Lincoln and Lincoln University – another indication of the importance of fertiliser use efficiency to the seed grower and to the environment.

Ian Collins, Journalist, Lincoln University, Canterbury, New Zealand.