Growing maize in the coastal Wet Tropics
Over recent years maize has been successfully grown in north Queensland for grain and silage.
Maize has a number of features that make it well suited to north Queensland, including:
- high rainfall requirement
- cheap weed control options
- few pest and disease problems
- can tolerate rain at harvest
- a growing market in the north with the expansion of feedlots.
Maize is not an ideal break crop for sugarcane because both crops belong to the grass family, which hosts root lesion nematodes. However, while high numbers of nematodes are found in the soil after maize crops, the following planted sugarcane crops have been surprisingly good. In southern regions, some growers harvest sugarcane early, plant maize in the winter and then plant a legume over the summer before returning to sugarcane.
On this page:
- Paddock selection and land preparation
- Planting time
- Planting depth and row spacing
- Planting rate
- Crop nutrition
- Weed and disease control
- Harvest time
- Gross margins
Maize is grown mainly for the domestic market, either as grain for feed mixes or silage in areas close to dairies and feedlots. Markets need to be close to the area of production as transport costs can be prohibitive. Growers also need to consider the availability of harvesters in their area for both grain and silage.
Identifying a market for the grain or silage prior to growing the crop is essential so appropriate management decisions can be made regarding variety, planting and harvesting times. Most grain buyers insist on pre-delivery contracts with a pre-determined price. Crops have yielded around 5 t/ha of grain but there is potential to yield to be as high as 8 t/ha. Prices vary, around $240 - $300/t, depending on the supply at different times of the year. Sourcing a harvester with a front suited to your row spacing prior to planting is a sensible precaution.
Selling maize for silage is less common but prices vary from $30 - $35/t fresh weight, at 60 - 70% moisture. Yields of silage chop range from 30 - 60 t/ha. To estimate silage yield, use this rule as a guide:
- tonnes of silage chop/ha = estimated grain yield (t/ha) x 6.
Summer maize crops
There are many varieties of maize but only a few are suited to tropical conditions. In 2004, the varieties QX6, QX8 and 31H50 were all grown successfully. The QX varieties have been superseded by AT1 and AT2 and there are even better hybrids in the pipeline. Varieties with good tight husk cover will help prevent grain damage, such as sprouting and rots, if there is rainy weather leading up to harvest. Check with your local supplier for suitable tropical varieties.
Winter maize crops
More temperate varieties can be grown during the winter in the tropics. In 2008 nine varieties were planted in replicated trials at Silkwood, near Innisfail. One block was planted in mid-June and another in mid-August, the second having irrigation. At the time of writing, the crops are looking good and yield data will be published after harvest in November-December.
Paddock selection and land preparation
Maize will not tolerate waterlogged or saline soils. For summer cropping, avoid low-lying areas and soils that have poor drainage. Clay soils with good levels of stored moisture are best for rain-fed winter crops.
Being large seeded, maize does not need a fine seedbed. However, large clods will reduce the effectiveness of pre-emergent residual herbicides. Maize can be zero-tilled into cane trash, avoiding cultivation prior to the wet season.
The row spacing needs to match the harvester front and it may be necessary to remove compaction with a ripper where a maize row coincides with an old sugarcane inter-row.
The traditional time to plant summer maize is December, but experience has shown that December-planted crops on the coast mature in the wet season and are susceptible to weather damage such as sprouting and rotting.
Crops planted in January are less susceptible to weather damage because the cobs are still immature during the wetter months. However, cultivation in January is unwise as it leaves the soil prone to erosion, so zero-tillage techniques are recommended. Planting a summer crop fits in well with the sugarcane system as the crop will be harvested around June in time for planting cane again.
A winter crop of maize can be planted as soon as the wet season has finished if the paddock has been fallowed over the summer (ideally with a legume). A May-planted crop will be harvested around October which is considered late for sugarcane planting. It is possible to cut the fallow paddock in the first round, plant maize in June and harvest the maize in November. A legume crop can then be planted through the summer.
Planting depth and row spacing
Maize is planted with a row crop planter into moisture at a depth of 5 - 8 cm.
Row spacing depends on the spacing on the harvester front. Most harvesters on the Atherton Tablelands are on 900 mm spacing but there are 750 mm spaced fronts available.
Aim to get 50,000 - 60,000 plants established per ha. The planting rate needs to take into account the seed size, germination rate and expected field emergence. A summer crop could have a slightly higher plant population due to good moisture, while a lower population is recommended for the drier winter period.
Use the following formula to calculate planting rate:
- Planting rate (kg/ha) = 50,000 / 3,000 seeds/kg / 0.85 (germ rate %) / 0.90 (emergence %)
= 21.7 kg seed/ha
Good nutrition is important for good yields of high quality grain. Big crops need large quantities of nitrogen, phosphorus and potassium. A soil test is recommended to help in calculating the fertiliser rate.
- Acidity: maize grows best in soils with a pH of 5.6 - 7.5. If the pH is less than 5, apply lime.
- Nitrogen: as a rule 130 - 180 kg N/ha. If a winter crop is planted after a summer legume the nitrogen rate can be reduced.
- Phosphorus: as a rule 0 - 35 kg/ha. Many sugarcane soils are high in phosphorus and may not need any to be applied.
- Potassium: sugarcane soils are commonly low in exchangeable potassium. If the soil test is low, an application of 50 kg potassium/ha is recommended.
- Trace elements: if the soil test shows deficiencies, these should be rectified.
Weeds compete for nutrients and water, create harvesting problems and may lead to cleaning charges with a grain crop. Weeds need to be controlled during the first four or five weeks after planting. After this time shading from the crop will prevent weed germination.
Zero-tillage maize uses the cane trash to suppress weeds.
There are some relatively cheap and effective herbicides available for use with maize:
|Post-plant||Mainly annual grasses||S-Metolachlor
|1.0 - 2.0 L/ha|
Annual grasses and broadleaf
S-Metolachlor and Atrazine
|2.5 - 3.3 kg/ha|
Annual broadleaf, some grasses
|Atrazine||Gesaprim®||2.5 - 3.3 kg/ha|
|Annual broadleaf weeds||2,4-D
|0.7 - 1.1 L/ha|
0.5 - 0.75
One strategy would be to apply Atrazine post-plant / pre-emergent at a rate of 1.1 kg/ha and another 1.1 kg/ha 25 days later when top-dressing.
Always read the label and check registrations before spraying.
Common diseases include rots, leaf blights, rusts, smuts and viruses. Selecting resistant varieties is the best way to reduce the incidence of disease.
Soil insects can be a problem for maize but this has not been experienced in northern sugarcane fields to date.
Corn earworm (Helicoverpa armigera) is a moth which lays its eggs on the top two-thirds of the plant and can damage the tassels and the silks, and so reduces pollination and grain set. Spraying is rarely worthwhile as the caterpillars are protected inside the cob and spraying will destroy any beneficial insects present.
Aphids will often be found on maize but do not warrant control measures.
Most coastal growers will not apply irrigation to summer crops but winter-grown maize would benefit from extra water. Maize requires 4 - 8 ML/ha depending on seasonal conditions. This equates to 400 - 800 mm of rainfall. Summer rainfall is far in excess of this in the north tropics and the crop will tolerate extra rainfall if it is on well-drained soil and the grain is immature at the time of the rainfall.
Mild water deficits during vegetative growth stages will decrease leaf area and more severe moisture deficits will reduce leaf number. Both will reduce the plant's photosynthetic ability and have severe consequences on grain yield.
Flowering is the critical stage for determining final crop yield, and irrigated crops should be well watered from three weeks before flowering through to physiological maturity.
Moisture stress at flowering can slow pollen shed and sustained moisture stress will increase the interval between pollen shed and silk emergence, decreasing pollination rate and therefore grain formation.
Excessive soil moisture at seedling and flowering stages can have disastrous effects on maize establishment and yield. Selecting a well-drained paddock is crucial.
Different varieties have different times to maturity. Under optimal growing conditions, slower varieties will yield more than quick varieties. A slower variety will also stretch the harvest time further into the drier months.
The target moisture content of the grain for storage is 14% but the grain can be physically harvested up to moisture content of 25%. This is not recommended as grain drying is expensive. Crops harvested at 17-18% moisture content will attract a drying charge of around $15/t.
Silage maize is normally sold direct to feedlots and dairies at an agreed in-field price per tonne. Farmers growing for stock-feed grain and silage need to be aware of the stringent requirement of most buyers in relation to chemical residues.
The best indicator of dry-matter yield and silage quality is in the milk line score. At late grain-fill stage, there is a visible line of separation between the starchy tip and the milky base of individual grains. The line moves down the grain towards the core of the cob over a period of up to six weeks.
When this milk line is half way down the length of individual grains, its milk line score is said to be 2.5. Prior to the line being visible the milk line score is zero and once the line is at the base of the grains the score is 5. For most corn silage buyers, a milk line score of 2.5 is ideal. This occurs between three and four and a half months after planting, depending on the variety grown and seasonal conditions.
The gross margin for a crop is the gross income earned, less the variable costs incurred. For machinery operations, only include the cost of fuel, oil, repairs and maintenance.
The sensitivity analysis shows how returns would vary with a change in yield and price.
Levy (0.7% farm gate price)
|Variable expenses - land preparation (FORM)||Applns||Price $/unit||$/ha|
|Rip ($/ha)||1 @||$25/ha||$25|
|Rotary hoe||1 @||$50/ha||$50|
|Total land preparation expenses||$99|
|Seed||17 kg @||$5.20/kg||$88.40|
|Total planting expenses||$108.40|
|CK55||1 applns@||350 kg @||$1.20/kg||$420|
|NitraKing (S)||1 applns@||400 kg @||$0.90/kg||$360|
|Fertiliser applicator ($/ha)||1 @||$15||$15|
|Total fertilising expenses||$795|
|Weed control||Applns||Rate||Price $/unit||$/ha|
|Atrazine||1 applns@||2 1 @||$12/1||$24|
|2,4-D||1 applns@||1 1 @||$5/1||$5|
|Sprayer ($ha)||2 @||$5/ha||$10|
|Total weed control expenses||$39|
|Header||1 applns@||kg @||$20/t||$120|
|Drying||1 applns@||kg @||$15/t||$15|
|Transport to Atherton||1 applns@||1 @||$30/t||$30|
|Total harvesting expenses||$390|
|Total growing expenses||$1431.40|
Market price maize
Gross margin/ha at different maize prices and yields (not including levy)