Agronomy & Irrigation
Agronomy and irrigation of sunflower crops
Sunflower growth stages
Days to critical growth stages, such as flowering, have been recorded for several commercial sunflower hybrids. This information assists in matching hybrid maturity to sowing time.
The exact number of days required to reach specific growth stages can vary depending on temperature, day length, moisture and hybrid, so this information provides a guide only.
Flowering is a critical growth stage that growers can use to match hybrid maturity to planting date. Doing this helps to avoid high risk periods such as heat stress, frost risk and disease. The critical time for heat stress is 12–15 days after flowering.
Sunflower plants usually flower for 5–10 days, however high temperatures shorten the duration of flowering. Temperatures greater than 35°C during and after flowering can affect yield, fatty acid composition of the oil and the hull to kernel ratio. The temperature within a sunflower head is usually 5°C warmer than the ambient temperature.
Choose paddocks that have:
no herbicide residues
been previously sown to a winter cereal or a sorghum crop
a good level of stubble cover
a full profile of moisture after rainfall or a long fallow
minimal broadleaf weeds
deep soil with minimal sub-soil constraints
not had another broadleaf crop sown in the last three years.
Sunflowers are sensitive to several common residual herbicides, including sulfonylureas and picloram. The time since application, soil type, herbicide rate and rainfall all affect the plantback length.
To minimise any potential TSV risks, growers in Central Queensland should also consider selecting paddocks away from areas heavily infested with parthenium weed.
When targeting the oil market, choose hybrids firstly on their yield potential and secondly on oil content. Use trial results as a guide but always try the hybrids on your farm and grow those that produce the best average results.
Other important considerations for hybrid choice include disease tolerance, maturity, head inclination, height and good agronomic type. Commercial availability of sunflower hybrids can vary by region, according to the time of the year and seed supply.
High oleic (monounsaturated) sunflowers are best sown in the spring to take advantage of the warm temperatures needed to produce oleic acid during grain fill. Controlled temperature studies show that low night temperatures suppress the production of oleic acid.
Monounsaturated sunflowers are required to have greater than 80% oleic acid content on delivery.
Ausiclear 20 – high oleic mono unsaturated oilseed – herbicide tolerant (Intervix)
Ausigold 62 – high oleic mono unsaturated oilseed
SuperSun 66 – poly unsaturated oilseed
Ausistripe 14 – grey stripe birdseed
For spring sowings, the soil temperature at 10 cm depth should exceed 10–12°C at 8.00 am (Eastern Standard Time) and the period of heavy frosts should be over. Aim to plant on rising soil temperatures.
In contrast, extremely high soil temperatures can reduce establishment, particularly if the air temperature is greater than 40°C, when soil temperatures can exceed 60°C. Lighter soils with a higher percentage of sand will be hotter than clay soils. Retained stubble and soil moisture assist in keeping the soil temperature relatively cooler and more even throughout the profile.
Sunflower establishment will be best when 7–10 days of favourable growing conditions follow planting. Extremes of heat or cold may result in patchy plant stands. Sunflowers can tolerate light frosts in the early and late stages of growth, and can tolerate high temperatures during the vegetative growth phases, but not during flowering and seed-filling.
Available moisture, the planter and the soil type dictate sowing depth, which may range from 2.5 to 7 cm, but is most commonly 3–5 cm.
Sunflowers may be successfully sown with precision planters, air seeders or even combines. However the preferred option is the precision planter, which can place the seed more accurately, with minimal double seeds or gaps.
Sunflowers have distinct advantages over many other summer crops due to the two sowing windows in most regions. The early, or spring, plant (mid-August to end of October) enables sowing of a percentage of the summer crop area before the main window for crops such as sorghum opens. Conversely, the late summer plant (November to end of January) window allows double cropping after a winter cereal in favourable seasons and the ability to plant after other summer crop windows have closed.
Sowing time will always be a compromise. Early planted (spring) crops are at risk of late frost and low soil temperatures during establishment and heat during flowering and seed-fill. Crops sown in the spring use more water due to the summer heat and the longer growing season.
Late planted (summer) crops often experience extreme temperatures during establishment, and sowing after the end of January increases the risk of disease such as sclerotinia and powdery mildew. Late planted crops are also at risk of frost damage and slow crop dry down at the end of the season.
Establishment of a uniform plant stand of adequate density is critical for a successful crop.
Target a plant population based on the depth of wet soil at sowing, the likely in-crop rainfall and growing conditions in your area. Seed suppliers can advise the optimal plant population for your situation.
When calculating your seed requirements, allow an extra 25% for establishment losses. Depending on planting conditions, machinery losses can be in the range of 20–50%.
Row spacing and configuration
Sunflowers may be planted on row spacings ranging from 50–150 cm, depending on the location in which they are grown, likely yield and planter configuration.
The majority of sunflowers sown on the Liverpool Plains in northern NSW and in southern Queensland areas are planted on 75 cm solid plant rows, whereas in Moree, NNSW and Central Queensland production regions, 100 cm is the preferred row spacing due to higher average temperatures and lower expected rainfall.
Row configuration impacts yield and oil content. NSW DPI trials (2007/08) indicated that yields of rainfed crops were reduced by sowing on wide rows (150 cm) and reduced further by single skip configurations on 100 cm row spacing. Row spacing and configuration did not affect oil content until sunflowers were planted on single skip configurations at 100 cm, when a significant reduction occurred.
Research and grower experience indicate that sunflowers respond to irrigation with 2–3 fold yield increases over dryland production. Target yields of 2.5–4.5 t/ha are typical in irrigated systems.
Sunflowers have a lower water use than other summer crops, such as corn, cotton and soybean, and are suited to a range of irrigation systems, including spray, furrow and subsurface drip irrigation. Water application will depend on soil type and environmental conditions, and on water availability. However as a guide, surface irrigated sunflowers will use approximately 4 ML/ha in total, including pre-irrigation, and irrigations at budding and at petal drop. Spring sown sunflowers are likely to have higher water requirements of up to 7.5 ML/ha (NSW DPI 2013).
Row spacing in irrigated systems on raised beds is usually 100 cm, with some variation depending on the bed width, which is usually 1.8–2.0 m.
Calculate planting rates to achieve the optimal population for the region (see Plant Population section in this guide for more information). If full irrigation is planned, the target plant population is 50–75 thousand plants per ha. If irrigation is limited aim for a plant population of 35–50 thousand plants per ha.
Irrigation scheduling has a significant impact on the yield and quality of sunflower seed. Targeting irrigation to critical growth phases in NSW DPI trials (2012) achieved yield gains of 40%.
Sunflowers have a relatively low water demand until 10 days after buds are visible. Demand then increases rapidly, until approximately 26 days after 50% flowering. Adequate soil moisture is required through until maturity.
During the vegetative stages water stress does not significantly affect sunflower yield potential and in most cases delaying irrigation will increase the harvest index. The greater the leaf area at flowering the higher the yield potential, but it is the top half to one-third of leaves that contribute the most to yield.
Research conducted in the USA suggests that maximum yield and highest water use efficiency is obtained from three irrigations, timed to coincide with pre-planting, budding and petal drop. In limited irrigation systems, irrigating pre-plant and at full bloom optimises yield potential and total water use.
Irrigation scheduling depends on three factors:
extractable soil water (ESW) capacity, measured as mm/m of soil
daily evaporation rates (pan evaporation), measured in mm/day
effective rainfall (in-crop).
The optimal irrigation interval is the time taken for the crop to use 50% of the ESW (may be 60–65% in limited irrigation situations). Daily root growth (3.2– 3.5 cm per day) is also a consideration.
Irrigating prior to planting, or starting with a full profile, should provide sufficient moisture to avoid crop stress until after bud initiation.
Prioritising available irrigation water
If there is only enough water for one irrigation – apply pre-plant.
If there is sufficient water for two irrigations – apply at pre-plant and full bloom.
If there is sufficient water for three irrigations – apply at pre-plant, budding and petal drop.
Monitor crops to ensure excessive moisture stress does not occur. Sunflowers may show signs of moisture stress (wilting) up until bud initiation during the heat of the day but should recover at night. Commence irrigation if recovery does not occur in the evenings.
Excessive vegetative growth can pose two key risks for irrigated sunflower crops. Firstly, it increases the risk of lodging and secondly, a dense, humid canopy can be conducive to disease development.
Where very high plant populations are targeted, root lodging can occur when the root system in wet soil is unable to support the weight of the heads and the plants begin to lean over.
Diseases such as sclerotinia, rhizopus head rot and powdery mildew are of most concern in overhead irrigation systems, such as centre pivots or lateral move irrigators (see Disease management section of this guide). Avoid applying overhead spray irrigation during flowering as the spray can wash pollen off the flowers and disrupt pollination.
Avoid waterlogging while sunflower seeds are germinating and the seedlings are young as the plants in these growing stages are more susceptible to pythium and rhizoctonia under wet, cool conditions and poor drainage. This is common on heavy vertosols, especially in low lying or compacted areas of the paddock. Affected seeds and seedlings will rot and die, leaving bare areas where weeds can thrive.
Once sunflowers are growing well they are quite tolerant of large rainfall events and temporary waterlogging for 24–48 hours. These events do not usually cause plant death, however temporary waterlogging events during grain filling can cause reductions in grain yield of 10–60%. The impact is greatest in heavier soils that take longer to regain adequate soil aeration after being inundated. Waterlogging causes the root growing tips to die and disrupts the aerobic photosynthetic processes —restricting plant growth and crop yield.
Sunflowers are moderately tolerant of a range of soil constraints but prefer a friable soil surface for best crop establishment. They grow best in neutral soils but can be successfully grown on soils ranging from slightly acid to alkaline.
They do not tolerant acidic soils with pHCa 5.0 or below. These soils tend to have more available aluminium and manganese, which can induce toxicities in sunflower.
Sunflowers tolerate high concentrations of manganese (Mn) in the root environment and are moderately tolerant to salinity, being less tolerant than cotton, wheat or sorghum but more tolerant than soybean or maize. The threshold soil salinity level for sunflowers is 4–5 dS/m and the rate of yield decline is about 5% per dS/m above this level.
High sodicity does not appear to effect oil content but may delay germination and flowering. Sunflower is considered to have moderate tolerance to sodium (ESP 30–40%) however sodic top-soil will constrain production through soil surface instability, inhibiting crop establishment and restricting water infiltration. Sunflower roots do not efficiently extract water from sodic sub-soil so crop yield potential is restricted.
Ensuring the nutrient needs of the crop are met is critical to maximise profit. Nutrient management seeks a balance between grain yield and oil production and decisions should be based on soil test results.
The relationship between starting soil nitrogen, soil water and yield expectations is important. A nitrogen fertiliser program should target a realistic yield, and consider soil tests, plant available water at sowing and previous crop yield and protein.
Excess nitrogen promotes excessive vegetative growth and can cause a reduction in oil content. Insufficient nitrogen will limit crop yields through poor crop establishment, uneven crop growth and maturity, and increased susceptibility to disease and pests during the season.
Phosphorus is considered the second (after N) most frequently limiting deficiency to occur within sunflower crops. If arbuscular mycorrhizal (AM) fungi levels are low, such as following a long fallow, canola or rice, supplying adequate phosphorus and zinc fertiliser is very important. An application of foliar zinc can be beneficial in alkaline soils where zinc availability is often limited.
In older cropping soils, potassium can be limited or may accumulate in the surface layer. Potassium is fairly immobile in the soil so it is important to test K levels down the profile, not just in the surface layer. Sunflowers require sufficient potassium to ensure stalk strength and to boost the plant’s ability to cope with drought conditions.
In sunflowers there is an important interaction between nitrogen and sulfur. Seed weight, seed numbers per plant, quality and oil percentage are likely to suffer if the N:S balance is wrong.
Germinating sunflower seed is very sensitive to fertiliser burn. To avoid poor germination results, place fertiliser away from the seed at planting. If row crop equipment is used, side-band the majority of the phosphorus and potassium 50 mm beside and 50 mm below the seed at planting.
Some or all of the nitrogen can also be applied pre-plant or side-banded (as above), provided that the total amount of side-banded fertiliser does not exceed 330 kg/ha. Nitrogen can also be side-dressed before the plants are 30 cm tall.