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Optimizing Nitrogen use on the farm

When aiming to optimize yield and protein of wheat grown under rainfed conditions, growers must supply sufficient, not excessive, nitrogen. What works one year may not necessarily be right the next year. The variability of rainfall amount and distribution results in highly variable responses to a set rate of N-fertilizer.

Growers should aim to match the supply of nitrogen, mineralized from soil organic matter or from a bag of fertilizer, with the requirements of the crop. Crop requirements constantly change depending on available soil water and rainfall.

This chapter details trials that help identify if nitrogen is a prime limitation to yield on a farm. Secondarily, it explains how to target and change grain protein percentage in the crops. It explains how to calculate how much N should be applied and when it should be applied to ensure that yield responses are more consistent from year to year.

For introductory methodologies you should check chapters Constraints to cereal-based rainfed cropping in Mediterranean environments and methods to measure and minimize their effects, Why do any of your research on-farm?, Optimizing variety x sowing date for the farm. Check the chapter on cropping sequences for effects of rotations, using legumes and non legumes, on nitrogen.

Which farms could benefit from these nitrogen trials?

Too little nitrogen and low yield

Many farms in the Mediterranean region have low wheat yields of around 1 t/ha. For a high proportion of these farms yield could be substantially boosted by the application of nitrogen either from a sack or through use of legume crops. One possible response to N is shown in the figure, other responses appear later.

How grain yield and protein can change with applied N on a low-N farm

Nitrogen is not commonly applied at all or applied spasmodically at one or two sacks per hectare. Fertility is seemingly maintained by the use of fallows.

It is on these farms that basic nitrogen trials paralleled by economic assessments (cost-benefit or gross margins analyses) are most needed.

Too much nitrogen and haying-off

Farmers growing cereals where rainfall is both limited and variable in distribution must also be very careful not to apply too much nitrogen.

When water is adequate, nitrogen stimulates the crop to grow faster and accumulate more biomass and set a potentially high yield. If water then becomes inadequate to support that increased biomass as the season progresses, the crop responds by shedding leaf and other tissues solely to survive. This is called ‘haying-off’.

Too much N: Normal grain (left) and grain shrivelled due to over application of nitrogen leading to haying-off (right)


Depending on its severity it can cause serious economic loss to growers on three fronts; it decreases yield, the harvested crop is poor quality because of pinched or shrivelled grain and the applied nitrogen was wasted. Indeed, the costly applied nitrogen was more than wasted because it was the cause of the yield losses.

At the time of flowering, crops of very high-N status contain fewer reserves of water-soluble carbohydrates (stem sugars) than crops of low-N status. Shortage of these stem sugars, normally transferred to the ear to fill the grain, is responsible for the yield reduction and pinched grain of very high N crops.

Very high levels of nitrogen can over-stimulate tillering which locks up the carbohydrates in structural materials rather than leaving them in storage to be used later to fill the grains.

Balancing nitrogen to water availability

In those seasons when there is sufficient rain during spring to ensure there is little or no drought stress during the later stages of stem elongation through to grain filling, a higher nitrogen crop has a higher yield potential.

It can retain more green leaves for longer and use them to fill the grain from current photosynthesis. It only needs to call on stored stem sugars during short periods of very high requirement.

By contrast, in the event of post-flowering drought, these crops of high-N status suffer the combined stresses of the drought (haying-off) and shortage of stem sugar reserves.

Crops of low nitrogen status grow less before flowering thus having relatively fewer grains to fill per unit land area and less above ground biomass to supply with water after flowering.

Consequently, during a late drought they lose relatively little of their green leaf. This green leaf maintains current photosynthesis and this assimilate, when combined with the high reserves of stem sugars, fills the grains satisfactorily through the drought. If the drought is particularly severe causing green leaves to die, the stem reserves alone can go a long way towards filling the grains.

When there is adequate water throughout grain filling, applied nitrogen boosts yield. However, when a drought occurs, then extra N may diminish yield.

How should farmers decide what level of nitrogen they should add to boost yield in good years but not cause haying-off in bad years?

The solution is a series of collaborative on-farm trials with inputs measured and matched by calculation to requirements. Trials should span a region and be carried out preferably over two or more years. Spanning a region through time enables questions to be answered about the impact of soil type on nitrogen requirements and conjointly the impact of different environmental factors.

Crop management to maximize the benefit of supplementary nitrogen

Wheat can be managed to produce grain at or close to the yield ceiling set by water availability in each season.

It requires knowledge of what factors may be affecting yield other than nitrogen. It requires nitrogen budgeting before sowing and further budgeting as the season progresses. Each decision about whether or not to add N with time depends on cumulative and current rainfall and what targets have been set for yield and protein.

Nitrogen will have little impact on crop yield if other factors present a greater limitation. For a good yield response to nitrogen attend first to the following

Criteria to satisfy first to realize benefits from N applications

Discussions with collaborating farmers

A good place to start your discussions with collaborating farmers is to find out, in general terms, to what extent the above-mentioned conditions have been met over recent years.

As the proposed trials are about nitrogen, ask about direct applications of nitrogen to wheat in the last three seasons.

Even without details you will soon be able to categorize a farmer as a non-user, a light haphazard user, a heavy user, a user who monitors the crop’s requirements and matches N applications to them. Your aim is to persuade the associated farmer group into that last category.

Ask about yields and whether farmers are happy with their farms’ output. If yields approximate 1 t/ha, the farm is likely to be one of low input and a good candidate for a very basic N study, perhaps broadcasting different levels of N in strips within the farmer’s crop (described later). If the farm produces higher yields, you might have to contemplate the more complex N trials also described later.

Expand your discussions to the other criteria above. It is likely that the haphazard N-user will have paid little attention to other nutrients and other constraints. You might need to help with a soil test.

Find out whether farmers use crop sequences that include a legume for harvesting or a legume for ploughing back in to improve soil organic matter and fertility.

Response to N applications at two farms contrasting with the farm in the earlier figure

You may have discussed the earlier figure of N-response to demonstrate how yield and protein can change with N application, but it might be worth explaining that this is not the only possible response. The shape and level of the curves is dependent on many other interacting factors that may or may not be present on the farm. Data from two other farms contrasting with that in the earlier figure are shown here for comparison.

You might like to use these figures to point out how important it is to do a basic low-effort study to categorize the likely response to N on that farm as no exact pattern can be assumed.

Note also the different responses in protein because an input for that will be required when you do your N budgeting.

Nitrogen budgeting for a farm

When you have made your assessment of the nutrition state of the farm and have some idea of the likely main constraints to yield, the next step is to construct your nitrogen budget for the farm.

If you can arrange a N soil test before planting and complete the following budget, you will be able to calculate N fertilizer requirements at sowing that will be accurate enough to meet targets of 70-80 percent of average yield assuming 11 percent grain protein.

More accurate updated assessments of nitrogen requirements can be made as the season progresses. Actions to rebalance N applications to crop requirements might, for example, be taken when the stems of the crop begin to elongate. This should be done to increase yield and protein but only as a tactical response to favourable soil water supply and low crop N status. It should not be done as a matter of course. A second tactical application to further increase protein can be made up to flowering, but this will also depend on water supply.

How much N does the crop require?

Start by calculating overall crop nitrogen requirements assuming a target for yield that is the average for recent years on the farm.

The following example calculation estimates the nitrogen requirement for a wheat crop yielding 2 t ha-1 of grain at 11 percent protein. It uses a correction factor of 2.34 that converts percent protein to kilograms of nitrogen and assumes that 25 percent of crop nitrogen is held in the straw.

1) Target crop yield x target protein percent x correction factor = nitrogen required over the season

e.g. 2 t ha-1 x 11 percent x correction factor = 52 kg N ha-1 required.

Similarly, a 1 t ha-1 crop would require 1 t ha-1 x 11 percent x 2.34 = 25.7 kg N ha-1 from somewhere.

How much N is already in the soil?

The above-mentioned calculation might tell how much nitrogen the crop requires but a part of that may not need to be applied as fertilizer, as it may already be available in the soil.

To test for existing soil mineral nitrogen, take several soil cores to 60 cm from around the field. This is the extent to which many roots penetrate. Tests to 10 cm are of limited use but if it has been dry since the last harvest, 30 cm may be deep enough to allow for most mineral N.

Send the samples to a testing agency or use the method of Wetselaar and colleagues for estimating soil nitrate levels in the field (see Further Reading). For the example calculation it is assumed that the tests indicated 20 kg N ha-1 of mineral nitrogen in the soil to a depth of 60 cm.

Remember that mineralization is a continuing process so will make additional nitrogen available throughout the growing season. Nitrogen mineralization varies between 60 kg ha-1 for infertile soils with less than 0.9 percent organic carbon to 100 kg ha-1 on fertile soils (more than 1.8 percent organic carbon). The calculation assumes the soil is infertile.

2) Existing soil mineral N + N mineralized during the season = total season supply in the soil

e.g. 20 kg N ha-1 + 60 kg N ha-1 = 80 kg N ha-1 potentially available to crop.

3) However, actual recovery of nitrogen from soil supplies is often closer to 50 percent than 100 percent

e.g. 0.5 x 80 kg N ha-1 = 40 kg N ha-1 actually available to the crop during the season.

The deficit in soil N available for the crop is (required N - available soil N) or ((1) - (3)). In the example for a crop of 2 t ha-1 this is 52 kg N ha-1 - 40 kg N ha-1 = 12 kg N ha-1.

As only about 50 percent of applied fertilizer is available in the first season, for our example, the estimated requirement for fertilizer to be applied is 12 kg N ha-1 x 2 = 24 kg N ha-1.

For a 1 t ha-1 crop the entire requirement could be met by soil mineralization. In fact, mineralization alone could support a crop of 1.5 t ha-1 providing all other management options were optimized (e.g. correct seedbed preparation leading to good crop establishment, see chapters on tillage and establishment for guidance).

Setting up a high yield potential by splitting N applications

Managing nitrogen fertilizer during the crop’s growth can be done so that the proportions of the crop yield components are changed.

For example, the later in crop growth that N is applied the greater is its impact on increasing protein and the less its proportional impact on biomass and yield.

Budget to work out how much nitrogen the crop needs and its sources



Crop N needs


target yield for crop

2 t ha-1


target protein %

11 %


correction factor of 2.34


crop N needs

(A x B x C)

52 kg N ha-1

Soil N supply


mineral N at sowing to 60 cm

20 kg N ha-1


mineralization of soil N during season

60 kg N ha-1


gross N supply

(E + F)

80 kg N ha-1


net N supply assuming 50% available to crop

(G) x 0.5

40 kg N ha-1

N the farmer should add


extra N needed

(D - H)

12 kg N ha-1


only 50% applied to crop so N to add is -

1 x 2

24 kg N ha-1

If the proposed N-budgeting method is used to determine N-needs of a crop of 3 t ha-1 at 13.5 percent protein and apply all that nitrogen at sowing, a yield potential of approximately 4 t ha-1 at 10 percent protein is actually set up.

This higher yield potential, when set early, has the detrimental effects of increasing crop height, reducing stem strength and using more soil water up to flowering.

This approach can increase the bulk of a crop in wet years to the extent that it lodges and in dry years leads to haying-off. The consequence in both cases is that the potential is lost.

Actually achieving the potential of 3 t ha-1 at 13.5 percent protein without lodging or haying-off requires the budgeted amount of nitrogen to be applied through time, with only part applied before or at sowing.

In effect this fools the crop into initially underestimating its potential so it then sets up its first yield components conservatively (i.e. fewer tillers, less leaf area and spikes per unit area).

Later in the season when conditions improve (by the remaining budgeted nitrogen being applied), these yield components are no longer plastic so the crop is restricted to using the later yield components of kernels per spike and kernel weight to express its yield potential.

When kernel weight is the only plastic component left, any late nitrogen available or applied has to be used up by the crop in increasing protein.

Encouraging the crop to arrange its form into the yield components that the farmer wants while ensuring growth is balanced to the environment, is a continuing exercise throughout the season.

[Read “Constraints to cereal-based rainfed cropping in Mediterranean environments and methods to measure and minimize their effects” for an explanation of when yield components are set during the crop’s development.]

At the time when a decision needs to be made whether to apply supplementary N-fertilizer, available soil water should be assessed.

As a guide, if rainfall from sowing up to this time exceeds the potential evaporation then the storage of water in the soil is likely to have increased and the chance of getting a positive response to N-fertilizer applied at this time is high.

If rainfall has been low or absent then response will probably be small. It should be borne in mind that nitrogen is used much more efficiently if applied just before a rainfall event.

If management decisions or the environment seriously limited early growth, N applied late may not compensate for low yield potential. An adequate plant stand must first be established to make it worthwhile applying N during later growth.

Interpreting the effects of N fertilizer applied at sowing: A Case Study

The following trial shows that nitrogen applied at sowing can have positive or negative effects on yield depending on the availability of water at different stages of development. It shows that you have to be very careful with nitrogen. It was carried out in a relatively dry region.

The six wheat cultivars used differed very little in duration, all reaching anthesis over an eight-day period (anthesis dates 1 to 8 on the figure), but this turned out to be very important to yield.

On average across the six cultivars, an application of 37 kg N ha-1 before sowing increased yield by about 200 kg ha-1. This ranged from a 350 kg ha-1 yield increase for the two earliest flowering cultivars to a yield decrease of 200 kg ha-1 for the two latest cultivars (check the green up and down arrows showing response to N in the figure).

Response to nitrogen by 6 cultivars of wheat that reached anthesis on different dates. The red line is the high N treatment and the blue line low N. It shows a positive response to N by those cultivars flowering early (cv 1 to cv 4), and a negative response by the 2 latest cultivars (cv 5 and cv 6).

Why did N not always increase yield?

A yield increase would normally be expected in all six cultivars from this small amount of N fertilizer, but the weather complicated the response. All crops were exposed to a hot, very dry period from immediately preceding anthesis and into early grain filling so their soil water reserves were significantly depleted. Water, not N, became the major constraint to yield.

The cultivars that flowered first and very early in the drought had, in effect, six more days of water to use on filling grain than the latest flowering wheats. They could fill the extra grain sites that had resulted from the 37 kg N ha-1 applied before sowing.

By contrast, the latest two cultivars ran out of water for completing grain filling and ran out earlier in the treatments that had received N at sowing.

They ran out earlier because the added N had stimulated tillering and leaf area production. This in turn had led to faster and greater total use of stored soil moisture before anthesis.

This trial was chosen to demonstrate that there are many complex interactions occurring in crops. In this case, a failure to respond to N was nothing to do with the genotype’s inherent responsiveness to N, but due to a third factor that was dominant in that particular season.

Always be alert for such additional variables. Repeating trials over several seasons is often the only way to average out their impact when optimizing a farming package for a location.

Setting up your on-farm trials

A nominal N response curve

If you are dealing with collaborating farmers who have not applied N previously, or used it sporadically, you need to determine a nominal response curve to N for the location.

Make this a low effort study within the farmers’ crops and use areas within different planting dates and different varieties if those are available.

Use a strip design with 0, 25, 50, 75, 100 kg N ha-1 applied in bands like stripes on a scarf in each of the areas. Mark out the plots and areas clearly with handfuls of lime. A single replicate using large plots of say 10m x 10m, should be sufficient for this starter trial.

Measure only grain yield at maturity but encourage farmers to measure rainfall throughout the season. This will affect the outcome as in the case study. You will be able to show the costs of the additional nitrogen against the value of the increments in crop yield. On that basis you will together be able to decide whether further, more complex, optimizing studies are worth doing.

Layout of a basic trial using a strip design but with three replicates. N added at sowing at 0, 25, 50 or 75 kg N ha-1 plots 2.5 m x 10 m

Beyond the nominal response curve

For farmers who have used N and have some idea that it may be beneficial to crops, you might like to consider suggesting one of the following three trials or some variant.

The three trials have the same basic design but add increasing levels of complexity.

Complexity increases as more possible limitations to crop growth are considered (that might interact with the nitrogen response). These are added as subtreatments. Though water is a critical determinant of response, it is not included as a treatment except in that it is recommended that the trials be conducted at several sites differing in water availability.

By comparing sites, water can be regarded as a variable. However, it is essential that rainfall is measured throughout the season and if possible that soil water availability is estimated at sowing. For this you will need to dry the soil samples at the research station in some form of oven. If soils are reasonably uniform, then three replicates of treatments will be good enough to give reliable results. If the soil is variable for various reasons,then consider four replicates.


Single cultivar nitrogen response trials, to determine if crops in the region will benefit from supplementary nitrogen fertilization, are ideally suited to integrating into farmer-sown fields. This has the time saving benefit of not having to sow the trials, the research will answer the question as to whether nitrogen is limiting yield, and a larger number of trials encompassing greater environmental diversity can be set up.

Your choice will depend on your aims but the assumption within this chapter is that the trial(s) will be on farms and the farmers will be taking an active part in the project.

Nitrogen treatments to use

All N applied at sowing: the basic trial

The basic nitrogen fertilizer trial aims to determine if profitable responses to N fertilizer are possible in the farmers’ crops if N is applied only at sowing. It has a similar aim to the starting point trial but has a more refined design.

A control plot and two or three nitrogen treatments all replicated three times, are necessary.

To determine the rates of N-fertilizer to be used (above that currently used by the farmer) multiply the average yield and protein of crops in the region by the correction factor as in the earlier budget table and round off the result to the nearest 10 kg N ha-1. Amounts of nitrogen to be used as N-treatments can be this amount, half this amount and 50 percent more than this amount.

An example calculation for a crop of 2 t ha-1 grain with 11 percent protein would be: 2 t ha-1 grain x 11 percent protein x 2.34 = 52 kg N ha-1.

This would be the pivotal treatment rounded to 50 kg N ha-1 and N amounts for all treatments become 0, 25, 50 and 75 kg N ha-1 additional to the farmer’s normal application (so the 0 treatment is the farmer’s normal rate). A further step below the farmer’s normal rate can be introduced if nitrogen studies have not been carried out previously in the area. This will check whether current rates are economic.

If urea (46percent N) is the source of N then amounts of fertilizer to be applied are: 0, 54, 108 and 163 kg urea ha-1 (i.e. 0, 25, 50 and 75 divided by 0.46).

If ammonium nitrate (34 percent N) is the source of N then amounts of fertilizer to be applied are: 0, 74, 147 and 220 kg ammonium nitrate ha-1 (i.e. 0, 25, 50 and 75 divided by 0.34).

A complex research trial marked out in a farmer’s crop. The trial considers the would response of a wheat to four levels of N applied at be the sowing (black numbers) or pivotal split between sowing and stem elongation (red numbers). A ninth treatment with a trace element spray is included (yellow numbers). Reps 1 and 2 of three replicates can be seen. Plants in rep 2 are artificially tinted on the photo. Only the centre rows of each plot would be harvested and the plot ends would be discarded. The paths between the plots are made by spraying with herbicide or weeding out rows at the crop five-leaf stage.

If topsoil has a neutral to alkaline pH then urea should not be applied to the surface, as there is a high chance of losses due to ammonia volatilization. Prior to sowing, urea can be drilled into the soil. When topdressing, nitrate-containing forms of nitrogen fertilizer should be used to minimize losses.

Layout of the basic trial

Below are a few rules to follow with this trial:

Timing of N application: a more complex trial

The second level of trial complexity adds timing of N to amount applied. This level aims to manipulate the yield components to a more yield-efficient combination, but still without the farmer having to make decisions during the season.

The basic amounts of N applied remain as calculated above but they become three subtreatments: all applied at sowing as above; 50 percent applied at sowing and 50percent delayed until later in the season (beginning of stem elongation); and finally 100 percent delayed till later. The proportions and timing can be altered in line with local knowledge.

If the trials are being carried out at only one location and there is no consequent necessity to standardize the treatments for several farmers across a region, the crop stage for the delayed application can be decided to suit the season. In addition, more or less nitrogen can be applied based on how the season is progressing. This may be an additional subtreatment.

As a general rule, if the season is dry the application of supplementary nitrogen at stem elongation may result in haying-off and poor quality grain. However, if the season has been wet and there is adequate soil moisture, a positive response to more supplementary nitrogen is likely.

If N is top dressed during the season then the highest efficiencies of crop uptake are achieved if the nitrogen is applied just prior to rainfall. Rain takes the N into the soil quickly for root uptake.

Allowing for the interactions of N with other nutrients: a very complex study

If it is suspected because of crop symptoms or tissue analysis or the use of indicator crops that other nutrients such as phosphorus (P), potassium (K) or sulphur (S) are limiting yield, then these can be added as separate treatments. In the simplest study they would only be included as a subtreatment within the lowest N treatment (25 kg N ha-1 in the previous example).

A complex trial with N added at 0, 25, 50 or 75 kg N ha-1 (on top of farmer’s usual application) either at sowing (S), at early stem elongation (E) or split between S and E (split) for three replicates. In treatment 25+t trace elements are added to a 25 kg N ha-1 treatment

Application of N-fertilizer may induce a nutrient deficiency through its stimulation of growth. If it seems that a trace element deficiency may be co-limiting yield then test this by including a treatment of a foliar application of a complete trace element mix, again within the lowest N treatment (25 kg N ha-1 in previous example).

If you get a response to the complete trace element spray then in later trials you can apply treatments with single elements to determine which of the trace elements is needed.

Measurements during your study

As emphasized earlier, the impact of nitrogen on crop growth and yield is dependent on the amount of water available as the crop progresses through its developmental stages. Consequently, to be able to interpret the effects of nitrogen correctly requires knowledge of soil water status.

Measurements and calculations to determine crop water use

A rain gauge

Details of where to position this and how to measure are in the chapter Constraints to cereal-based rainfed cropping in Mediterranean environ-ments and methods to measure and minimize their effects.


Soil samples at sowing

If time and resources permit, take soil samples to at least 0.6 m and preferably to 1 m deep for determination of soil water content at sowing, in conjunction with soil mineral N testing, and then again at harvest.

Details of how to do this and ways to calculate soil water content are in the chapter Constraints to cereal-based rainfed cropping in Mediterranean environments and methods to measure and minimize their effects.

Measurements of crop material to determine yield and yield components

Most of the measurements that follow are designed to help interpret the results of the trials. If you and the farmers are concerned solely with recording the responses to nitrogen without understanding in detail why they happened, the two critical measurements to take at maturity are plot grain yield and harvest index. Also take 100-grain weights from the grain threshed from the harvest index sample, for comparing kernel weights across treatments. Record this information for each plot, together with the plot identifier and date, on a table that was prepared in advance of harvest time (for designs of tables see the chapter dealing with Optimizing variety x sowing date for the farm).

For these less detailed measurements it is still vital to measure and record the plot areas actually harvested to work out production per unit area. Otherwise the comparisons between treatments become nonsense. It is also vital to bring all crop samples to similar water content before making comparisons, as water is very heavy. Quickly drying all samples on a hot day in the sun at the same time before weighing may achieve this satisfactorily. The sooner this is done after harvest the better, as any losses of material to respiration or accidents will negate the work of the studies.

You will need to take:

1. A hand harvest at anthesis if you are doing an in depth analysis of crop response;

2. A similar hand harvest will be required at maturity; and

3. Afull plot machine harvest at maturity.

Details of how to carry out all these procedures are in Constraints to cereal-based rainfed cropping in Mediterranean environments and methods to measure and minimize their effects.

When you have the numbers, how do you interpret them?

The following imaginary study assumes you have applied four nitrogen treatments (0, 25, 50 and 75 kg ha-1, 0 is the farmer’s normal practice) at sowing, following the design of the basic trial, and have duplicated the study over two farms. The primary aim is to determine whether farmers would gain an economic benefit from applying more nitrogen and if so, how that benefit had arisen.

Crop grab samples were taken at anthesis and again at maturity for yield component analysis (see Constraints to cereal-based rainfed cropping in Mediterranean environments and methods to measure and minimize their effects) and each complete plot without its end and edge border rows was harvested at maturity for plot yield. For the purposes of illustration, and to demonstrate trends, the average data from the three replicate plots is presented.

The anthesis harvest

N-fertilizer kg/ha





Farm 1

Biomass (t/ha)





Crop height (cm)





Farm 2

Biomass (t/ha)





Crop height (cm)





What does the anthesis table say?

About Farm 1

These observations together indicate that in this season, when the farmer’s normal practice was followed, N limited growth up to flowering.

About Farm 2

On Farm 2 it seems that the farmer’s normal practice is close to optimum prior to flowering. Water, not N, was probably limiting biomass production at higher rates of N. This is suggested from the crop height reduction at high N.

The maturity harvest

What do the maturity data say for Farm 1

Check the table at the bottom of the page as you read.

There was the commonly observed trade-off however that kernel weight and harvest index both decreased in response to N-fertilizer.

In summary, maturity data for Farm 1 indicate that in the farmer’s normal system N is a limitation to growth (and eventually yield) prior to flowering.

Farm 1

Farm 2

N-fertilizer kg/ha









Grain yield (t/ha)









Biomass (t/ha)









Spikes per m2









Sterile shoots/m2









Harvest index (%)









Kernels per m2









Kernels per spike









Kernel wt (mg)









Height (cm)









So, depending on the relative cost of fertilizer and the grain price, adding further N could be beneficial.

Almost certainly the increment in yield of 530 kg ha-1 for 25 kg N ha-1 (just over one bag urea) would be an economic proposition.

However, a prospective further increment of 680 kg grain associated with two to three bags of urea may not be worth the parallel risk of haying-off.

What do the maturity data say for Farm 2?

On Farm 2 it seems that it was fruitless to apply more N than in the farmer’s standard regime as growth was primarily limited by lack of water both prior to and after flowering. Though water limitation prior to flowering was severe enough to halt height growth it did not limit floret fertility. Consequently, there could still be arguments for delayed N applications in seasons with late rain.

In the example, season late drought was so severe that when higher increments of N were applied, kernels were unable to fill even to 30 mg and harvest index fell dramatically. In effect the additional N was harmful for the crop.

Which treatments produced a profit?

Gross margins calculations help you to make decisions about whether N applications are worth the cost and effort. They have been worked out for the two farms of this imaginary study. Your prices will differ from these examples quoted in A$ and based on costs in Australia, but trends will remain the same. Furthermore, your grain receiver may not be concerned with grain quality (protein content or screenings), only with weight. Re-work the examples in accordance with local regulations.

Assumptions in the calculations

The Gross Margins

Farm 1

N-fertilizer kg/ha





Grain yield (t/ha)





Input costs ($/ha)





Grain protein (%)





Screenings (%)





Value ($/t)





Crop value ($/ha)





Gross margin ($/ha)





$ Return per $N




For Farm 1 the gross margin per hectare increased with increasing amounts of supplementary N-fertilizer (the red figures in the table). However the return on money spent on N fertilizer was greatest at the low rate of N being A$3.4 for every A$1 invested.

Farm 2

N-fertilizer kg/ha





Grain yield (t/ha)





Input costs ($/ha)





Grain protein (%)





Screenings (%)





Value ($/t)





Crop value ($/ha)





Gross margin ($/ha)





$ Return/$N




For Farm 2 the greatest gross margin was for the farmer practice, the ‘0’ treatment. Additional N-fertilizer wasted money especially at the highest N rate due to haying-off and associated unacceptable levels of small grains screenings.

Some conclusions

The prime conclusion from these trials is that nitrogen is an essential plant nutrient that positively impacts on growth and yield when used thoughtfully.

However, it must be used in balance with other potential limitations to production, particularly water.

If farm management does not first attend to the basics of good seedling establishment with optimum plant populations, with the best variety sown at the optimum time, with adequate control of weeds and disease, nitrogen applications may provide no economic benefits.

The on-farm trials suggested here provide an opportunity for researchers to show farmers how to budget for a correct nitrogen balance, to work with them, to monitor water availability initially and through the season and match nitrogen and water inputs for optimum productivity.

They provide the tools to actually calculate for the farmer’s unique circumstances, what these N-requirements might be.

They concomitantly provide an opportunity to see what the yield potential of a farm actually is and what the farmer might realistically aim for depending on the season.

Further reading

Angus, J.F. & van Herwaarden, A.F. 2001. Increasing water use and water use efficiency in dryland wheat. Agronomy Journal 93, 290-8.

Angus, J.F., van Herwaarden, A.F., Fischer,R.A., Howe, G.N. & Heenan, D.P.1998. The source of mineral nitrogen for cereals in southeastern Australia. Australian Journal of Agricultural Research 49, 511-22.

van Herwaarden, A.F., Farquhar, G.D., Angus, J.F., Richards, R.A. & Howe, G.N. 1998a. ‘Haying-off’, the negative grain yield response of dryland wheat to nitrogen fertilizer. I Biomass, grain yield and water use. Australian Journal of Agricultural Research. 49, 1067-81.

van Herwaarden, A.F., Angus, J.F., Farquhar, G.D. & Richards, R.A. 1998b. ‘Haying-off’, the negative grain yield response of dryland wheat to nitrogen fertilizer. II Carbohydrate and protein dynamics. Australian Journal of Agricultural Research. 49, 1083-93.

van Herwaarden. A.F., Richards, R.A., Farquhar, G.D. & Angus, J.F. 1998c. ‘Haying-off’, the negative grain yield response of dryland wheat to nitrogen fertilizer. III The influence of heat shock and water stress. Australian Journal of Agricultural Research. 49, 1095-110.

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