Rotations of field crops

On this page:

• Rotation management
• Costs and benefits
• Soil fertility
• Pest management
• Rotation options
• Ley pastures

Monoculture wheat has been a major farming practice of graingrowers. Under this system, weeds such as wild oats and paradoxa grass, and diseases such as yellow spot, crown rot and common root rot increase, while soil fertility particularly nitrogen and soil structure, declines. Barley is often grown to suppress weeds.

The practice of growing winter and summer crops in a well managed rotation provides many benefits, all of which contribute to reduced costs and increased yield with higher net returns. Chickpea, mungbean, lucerne and medic rotations are becoming increasingly popular with farmers attempting to maintain grain yields and quality. The length and type of rotation depends on soil type, cash flow requirements, capital investment in livestock and cropping, soil structural problems, fertility (particularly nitrogen) and weed and disease levels.

Rotation management

The greater crop diversity created by the introduction of grain legume and oilseed crops in rotation with the cereals, combined with a higher cropping intensity with opportunity cropping, increases the demand on management in several respects, including herbicides, insecticides, crop selection, harvesting and marketing.

In the winter-summer and summer-winter changeovers, the decision to opportunity or double crop depends on seasonal conditions. At least 50-60 cm of stored moisture is required. A homemade soil moisture probe can be used to estimate this. Reduced or zero tillage can give a better chance to opportunity crop, particularly in drier conditions. Retaining stubble on the surface can improve moisture infiltration and reduce runoff and soil erosion.

Because of the limitations of broadleaf weed control herbicides in broadleaf winter crops, it is desirable to plant these crops early in the rotational sequence, before winter weeds have had the opportunity to increase in numbers. Long residual herbicides should be used with caution as they can reduce the flexibility of cropping programs.

Legumes are well suited as opportunity crops because of their ability to fix extra nitrogen in low nitrogen situations, which then benefits subsequent crops. These crops are then best followed by a cereal grain to exploit the improved soil nitrogen conditions. Legumes are relatively easy to establish in a zero or reduced tillage seed bed.

Costs and benefits

The loss of production during the interchange phase, as pastures are established, or as the land is prepared for a return to cropping may have implications on cash flow and the capital costs of the property. Income from livestock may offset the cost of the change, however, it may take longer to get a return than with grain crops. Reduced areas of cultivation can increase the capital cost of machinery per hectare. Ley pastures may also increase the capital investment in livestock.

Rotations out of a monoculture system offer improved long term cost/benefit ratios by reducing the costs of weed and disease control, fertilisers, labour and machinery. The benefits can be increased reliability and reduced risks. Rotations require high levels of management but offer many long term benefits.

Soil fertility aspects of crop rotations

Pulses do not fix as much nitrogen as pasture legumes as a lot of nitrogen is removed in the grain. This results in 'maintenance only' and does not significantly increase soil nitrogen levels. Legumes in a pasture rotation can fix large amounts of nitrogen and may benefit following crops for several years. Where soil nitrogen is high, the legume will not fix nitrogen, but use available N like any other crop. The N input from legumes is approximately 1% of their dry matter production, thus the amount of legume grown, not how long it is grown, is important.

Mycorrhizae (VAM)

Mycorrhizae (known most commonly as VAM) are a combination of beneficial root-infecting fungi and the plant root. VAM help plants to access nutrients, especially phosphorus and zinc, and crop species differ in their dependency on this relationship. Long fallows, either planned or in drought conditions, reduce VAM populations, causing a condition known as long fallow disorder. 

Click here for more information on VAM and long fallow disorder.

Pest management aspects of crop rotations

Crop rotations are often used to combat specific pests, by removing their host for a season or more. Rotations can be used to prevent buildup of a particular pest (e.g. soil-borne nematodes) or increase management options (e.g. planting a broadleaf crop to allow the use of grass herbicides).

Weed populations increase to economically damaging levels in most monoculture systems. Hard to control weeds such as wild oats, paradoxa grass (wild canary), and buckwheat can be managed by crop rotations. If using residual herbicides, ensure plant-back times are carefully observed. A rotation of at least three years is needed for effective weed reduction. If changing from winter to summer cropping or vice versa is not an option, aggressive 'early growers', or crops with a range of effective and affordable herbicides are preferred. Avoid pulses (grain legumes) if heavy broadleaf weed infestations are expected. Pasture rotations may also provide effective weed control. 

Table: Wild oat seeds in the soil after a four year rotation of lucerne

Rotation	Wild oat seeds (per m²)
Initial population	225
Four years continuous wheat	2200
Four years wheat/sorghum	13
Four years lucerne	0

Crop rotation breaks the life cycle of diseases which survive in the soil or on crop residues, including crown rot and root rot of wheat and barely, yellow spot in wheat, and phytophthora root rot in chickpea. Crown rot and common root rot are most severe when wheat follows wheat or barley. Two years in a summer crop will help control the problem. Rotating to any winter crop other than wheat, or summer crop, controls yellow spot. Chickpeas should not be grown on areas previously cropped to chickpea, medic or lucerne for a period of four years, due to increased risk of phytophthora root rot infection.

In soils infested with root lesion nematodes, the desired interval between wheat crops is four, preferably six years.

Soil insect populations can also be reduced by crop rotation. Wireworms favour wheat, sorghum, barley and maize residues, but not oats, soybean and sunflower residue. White fringed weevil infestations are promoted by crops such as lucerne, peanuts, sunflower, soybeans, but are not favoured by cereals, forage sorghum, oats and pasture grasses.

Options for crop rotations

Winter crop/summer crop rotations

Dryland: General plans for rotations in a 2:2 and 3:3 winter to summer crop ratio suitable for the Downs region are illustrated below.
Irrigated: With irrigation, even more intensive rotations can be employed, provided there are adequate water supplies.

S (Summer crops) - Sorghum, Maize, Sunflower.
W (Winter crops) - Wheat, Barley, Canary, Linseed, Canola.
FOC (Fallow or opportunity crop) - Mungbean, Chickpea, Millet, Panicum

Example of a 2:2 rotation
Paddock	Year 1		Year 2		Year 3		Year 4		Year 5
1	FOC	W		W	FOC		S		S	FOC
2	FOC		S		S	FOC		W		W
3	S	FOC		W		W	FOC		S
4		W	FOC		S		S	FOC		W
5	S		S	FOC		W		W	FOC

Example of a 3:3 Rotation

Paddock

Year 1

Year 2

Year 3

Year 4

Year 5

Year 6

Year 7

1

FOC

S

S

S

FOC

W

W

W

2

FOC

W

W

W

FOC

S

S

S

FOC

3

S

S

S

FOC

W

W

W

FOC

4

W

W

FOC

S

S

S

FOC

W

5

S

S

FOC

W

W

W

FOC

S

6

W

FOC

S

S

S

FOC

W

W

7

S

FOC

W

W

W

FOC

S

S

In the dryland situation, vary the crop to suit soil moisture storage. For example, after a wet fallow the soil should be wet beyond the depth of a shallow-rooted crop, and any nutrients (particularly nitrogen) left from the previous crop could be leached to the subsoil. A deep rooted crop such as cotton, sunflower or canola can exploit the extra subsoil moisture and leached nutrients better than fibrous rooted sorghum or cereal crops.

Winter/summer crop rotations spread capital and labour costs, reduce weed and disease problems, but do not arrest the decline in soil fertility or structure. A long fallow into and out of summer crop is necessary to ensure reliable grain yields and this may leave the soil exposed to erosion. Double cropping can minimise the erosion risks but should only be considered if there is a full soil moisture profile.

Legumes/Pulses

Legumes improve the general condition and N content of the soil, and form an essential component of effective crop rotation systems. Many of the grain legumes (pulses) are profitable crops in their own right as markets have developed with improved reliability and stability. Pulses also act as a disease break for cereal crops. They are unsuitable where broadleaf weeds are a problem. Successfully nodulated pulses slow the rate of N decline but they will not rebuild soil nitrogen levels to the extent of lucerne as much of the fixed nitrogen is removed in harvested grain.

Forage crops

Legume forage crops such as lablab and cowpea contribute more nitrogen to the soil than pulses, as no grain is harvested. Only 10 to 15% of nitrogen is removed from the paddock in the animal. The rest is returned to the soil as crop residue, dung and urine. Forage sorghum and oats do not rebuild soil fertility, but may provide short-term relief from annual weeds and provide soil cover. They are often the first crops grown after land is cleared.

Perennial lucerne is one of the best legumes for rebuilding soil N levels. On deficient soils, lucerne can fix 70-100 kg N/ha/annum. Research on low N soils showed a 3½ year lucerne ley increased subsequent wheat yields by 0.5 t/ha or more for 4 to 7 years. The duration of lucerne necessary to restore N depends on the starting soil N levels and amount of lucerne grown. 

On the Darling Downs, annual medics with oats or pasture can fix on average 30 kg and up to 100 kg N/ha/annum in favourable seasons, however the further west and north we go the more unreliable medics are. Unless grown after a fallow, their overall N input is small compared to lucerne. The main benefit of the medic is to improve the quality of the grazing diet.

Butterfly pea has enjoyed substantial success as a ley legume in central Queensland. 

Table: The effects of crop and pasture rotations on key components of the farming system.

Rotation	Weed control	Disease control	Fertility effects	Soil  structure	Erosion control	Cost	Long term $ benefit
Wheat/ wheat	poor	poor	poor	poor	OK	OK	OK
Wheat/ sorghum	good	OK	poor	poor	OK	OK	OK
Wheat/ chickpea	poor	OK	OK	poor	poor	OK	OK
Medic or vetch	OK	OK	good	OK	poor	OK	OK
Lablab/ cowpea	OK	OK	good	OK	OK	OK	OK
Lucerne	good	good	good	OK	good	poor	good
Grass/ legume pasture	v.good	v.good	v.good	v.good	v.good	poor	good

Crop rotation and ley pastures

Ley pastures (a rotation out of continuous cropping into a short phase of pasture) present the best opportunity for achieving long term economic sustainability in many cropping situations. Although the greatest benefit is achieved over 3 to 5 years, there can also be benefits from shorter periods.

Grass/legume ley pastures give the greatest improvement to the soil and to future crops. The legume component increases soil nitrogen levels, while the grass component improves soil structure and may improve the availability of some nutrients through increased organic carbon levels and nutrient redistribution. Grasses in the pasture rotation may increase the amount of fixed N as they 'absorb' excess N, ensuring the legume continues to fix nitrogen. This nitrogen is also released over a longer period than in a pure legume sward.

Mixed pastures tend to degrade after 5 years as the balance between grass and legume changes. Three years under a legume pasture ley would appear to give nitrogen responses for the next three to four wheat crops.

Establishing the pasture ley

The legume component (such as medics or lucerne) of a ley pasture can be undersown with previous winter crop, or planted after a fallow period by itself or with the grass. Undersowing can limit crop yield due to moisture competition, and can greatly affect legume establishment particularly in a dry year. Shallow planting depths are required for mixed pastures. Lucerne established by itself in autumn will grow and produce faster than lucerne undersown with winter crop.

A range of other legumes such as butterfly pea have been effectively integrated into a pasture ley program in some regions. Consult your local agronomist as to the legumes most suited to your area.

Returning to cropping after the ley pasture

The successful return to cropping from a ley pasture requires a fallow period to rebuild soil moisture reserves and to remove the pasture residues so they will not interfere with the planting operation or become a weed problem in future crops. If the crop after the ley pasture phase is to be wheat, it is advisable to commence the fallow in the previous spring.

Advantages of a ley pasture in the rotation

• an increase in soil nitrogen available for future grain crops
• an increase in soil organic matter (grass component) 
• an improvement in soil structure and water holding capacity 
• reduced surface sealing and subsoil compaction
• enhanced soil erosion protection and rainfall infiltration
• better control of weeds, insects and diseases.

Disadvantages of ley pastures

• cost of changing over to a rotation that includes a pasture ley (often involves rebuilding facilities for stock handling)
• depletion of soil water reserves (a suitable fallow period is necessary before returning to cropping to allow soil water to build up).

Last revised 27 September 2006.