Wheat on wheat? consider the risks

Mean yield, nitrogen uptake and gross margins
First cropWheatCanola
First crop yield (t/ha)3.631.91
Following wheat yield (t/ha)3.292.91
First crop N uptake (kg/ha)92107
Following wheat N uptake (kg/ha)95112
Two-year gross margin ($/ha)552699

Gross margin = yield x price - variable costs.
The assumed farm-gate prices were $150/t for wheat and $330/t for canola.A grain protein premium and discount of $15/%/tonne was assumed for wheat protein above and below a 10 per cent benchmark. Variable costs were $280/ha for wheat and $313/ha for canola.

Wheat on wheat, a recipe for disaster or a sound management plan, at least for a short term? Agronomists are still divided on the subject. Here we give you the pros and cons and some long-term trial results.

The case for

Wheat vs high-risk alternatives

Economic studies such as the FAST program have shown it is advisable to have less than 20 per cent of the cropping area sown to high-risk crops. For many areas, farmers would consider wheat on wheat less risky than growing many pulse crops. Cereals are usually cheaper to sow and maintain throughout the season, which keeps costs down, and despite a drop in yield, wheat on wheat can provide a better economic option.

Limited options

In many high-rainfall areas, where the soil is prone to waterlogging, and in marginal areas of very low rainfall, the option of using pulses is either not available or restricted to one or two varieties. Wheat on wheat has been used with varying degrees of success in many of these areas, with a slight drop in yields considered a small risk, as opposed to possible low income from the pasture phase, or low yield from a high-risk legume crop.

Gross-contamination

In many situations, growers will choose to grow wheat on wheat rather than wheat followed by an alternative cereal to avoid the problem of removing contaminants such as barley or oats in the wheat crop.

The case against

Disease carryover

The main factors against growing wheat on wheat are carryover of disease inoculum for take-all and crown rot, the build-up of nematodes such as cereal cyst nematode (CCN) and root lesion nematodes in the soil, and the increased risk of the leaf disease yellow leaf spot.

Herbicide resistance

The increased use of selective herbicides for grass weed control in a continuous wheat rotation may lead to herbicide resistance. One of the keys to managing herbicide resistance is a good rotation. A range of crops allows for variation in seeding time, harvest time, herbicide choice, and the advantageous use of the different competitive ability of crops.

Yield loss

Some agronomists budget on a yield loss of 20 per cent for the second wheat crop, compared to wheat following a break crop. This figure is definitely not standard, and may be higher in a severe disease situation or much lower, depending upon the paddock and seasonal history Some agronomists even consider wheat on wheat may cause a form of self-induced drought, with a high risk of the second wheat crop not being able to access all the available moisture.

Fertiliser

Fertiliser nitrogen inputs are usually increased for the second wheat crop as opposed to wheat following a legume or fallow. Cereal-legume rotations also support a larger, more active and more diverse soil microbial community than a cereal-cereal rotation.

Spreading risk

Economists advocate that generally there is safety in diversity of production. A mix of cereals, oilseeds and pulses with a contribution from meat and/or wool can spread the risk among several industries and markets. This also helps buffer against natural occurrences such as frost, and major leaf and root disease outbreaks.

If you go ahead with wheat on wheat:

Management is the key

Sound management will determine whether or not the option of wheat on wheat is risky for your situation. This needs to start while the first wheat crop is still in the paddock.

Yellow leaf spot

Yellow leaf spot is a stubble-borne leaf disease, which may not be observed in wheat plants in the first year. It can infect plants late in the season, particularly in wet years, leading to problems in subsequent crops. The disease is favoured by long hours of leaf wetness, so is usually observed in the winter months. The level of yellow leaf spot infection in the second crop will be in direct proportion to the amount of infected stubble; one management option is a late burn to reduce stubble levels. But burning must be weighed up against other problems such as wind erosion.

Root disease

A thorough paddock check for root disease problems is required following the first wheat crop. A wet spring can increase take-all levels in the soil, whereas crown rot is more likely when a wet start to the season is followed by a dry finish.

Disease levels can be assessed from soil samples. In March-April, prior to sowing the second wheat crop, have your local accredited agronomist send a soil sample to the Root Disease Testing Service (RDTS) at the South Australian Research and Development Institute (SARDI). The RDTS currently tests for the root lesion nematodes {Pratylenchus thornei and P. neglectus), cereal cyst nematode (CCN), take-all and Rhizoctonia. A test for crown rot should be widely available through the RDTS from the year 2002.

Deep soil sampling is also an important management tool for making the correct decisions on how much, if any, additional nitrogen is required.

Varity choice

After you've considered the RDTS results and the potential for yellow leaf spot, and you want to proceed, decide on the best variety. Many recently released varieties have tolerance and/or resistance to the serious diseases found in the southern region and growers are advised to consult with local agronomists or their state Department of Agriculture for the most appropriate varieties for their area.

Weed control

To avoid herbicide resistance problems from the continued use of selective herbicides, wheat on wheat also demands a careful weed management program.

A pre-sowing cultivation may stimulate dormant weed seeds, the seedlings of which can then be controlled prior to or at sowing. (See other stories this Ground Cover and previous issues on integrated weed management strategies.)

Trial results

A recent compilation of trials carried out by CSIRO, NSW Agriculture and SARDI has shown that the average yield of wheat grown after canola was 19 per cent greater than wheat on wheat. The researchers also calculated that, based on recent grain prices and growing costs, the two-year gross margin for canola-wheat was 27 per cent more than for wheat on wheat.

The researchers analysed results from 26 separate on-farm trials in southern Australia over 10 years, where a wheat on wheat sequence was compared to wheat following a break crop. Canola, Linola and Indian mustard were the break crops. Results for Indian mustard and Linola are not shown here but provided similar results to canola.

Of the additional gross margin of $147/ha, 73 per cent was due to the following wheat crop and only 27 per cent to the canola crop itself. Nearly three-quarters of the benefit of a canola crop is in the added yield and protein of the following wheat crop.

Advantages of the rotation

The average nitrogen uptake for wheat after wheat was 95 kg/ha, compared to 112 kg/ha for the wheat after canola.

The wheat yield advantage appeared related to control of wheat root diseases, particularly where the second wheat crop was grown under high rainfall. The results varied from a doubling of yield in some areas in the wet season of 1996, to a yield reduction for wheat following canola at a couple of places in 1991 and 1992. However, of the 26 comparisons, 14 showed a yield benefit of at least 0.5 t/ha for wheat after canola.

The cases with no wheat yield advantage with rotation were in low-rainfall environments. The reductions were due to 'haying-off', when the additional growth by wheat after break crops led to excessive early use of the available soil water and nitrogen.

Long-term trials show risks inherent with disease, A number of long-term, wheat-on-wheat trials have also been undertaken in South Australia, with up to 20 years of continuous wheat. These trials also found that, where diseases were a significant issue, continuous wheat was generally not economically viable. Where diseases were not a significant issue, continuous wheat yielded about 10 per cent less than wheat after an alternative crop and was very profitable.

In these long-term trials grass weeds, particularly brome grass, became the greatest limiting factor to continuous wheat.

Further trials are underway in areas such as the Victorian southern Mallee, where farmers have successfully grown wheat on wheat, with yields rivalling and better than those of wheat following a pulse or canola.

The upshot?

Growing wheat on wheat has the potential to increase both root and leaf disease problems. The options for weed control are limited, and the yield has been shown to decrease in many instances.

Some flexibility may be required by farmers using no-till and stubble retention systems to accommodate the best means of reducing stubble-borne and other root and leaf diseases.

Whether considering wheat on wheat as a short-term option to reduce risk and increase cash returns, or if it forms part of a rotation with very few alternatives, precise management is essential for obtaining the highest yield.

Contact: Mr David Roget, CSIRO 08 8303 8528 Dr Gordon Murray, NSW Agriculture 02 6938 1999 Mr Rob Sonogan, Department of Natural Resources and Environment (Vic) 03 5033 1290 Dr John Angus, CSIRO 02 6246 5095 Dr Hugh Wallwork, SARDI 08 8303 9382

Region North, South, West