Early sowing benefits should outweigh challenges

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Dr James Hunt: early sowing has huge potential in the HRZ.

Early wheat sowing is often essential to achieve the high yields possible in south-west Victoria’s high-rainfall zone (HRZ). However, sowing early can be challenging and needs to fit into a system designed to manage trade-offs with weed control, frost, waterlogging and disease.

The factors that mostly contribute to yield, and interact with sowing time and cultivar choice, form a package comprising:

  • seeding system;
  • crop rotation;
  • summer fallow management;
  • time of sowing and cultivar selection;
  • dual-purpose grazing;
  • foliar disease management; and
  • nitrogen management.

Four years of GRDC-funded research has revealed the important considerations within each factor.

Seeding and stubble retention

No-till systems allow for timely sowing and establishment, particularly systems with high breakout pressure tynes, knife points and press wheels.

With the ability to sow directly into undisturbed soil, the need to cultivate before seeding is removed, freeing up more time to sow after rain. In wet conditions, undisturbed soil is easier to drive on, which is important in the HRZ.

In undisturbed soil, weed seeds are left on the soil surface making them more vulnerable to predation and direct hits from pre-emergent herbicides. This is important in south-west Victoria, where most annual ryegrass populations are resistant to post-emergent herbicides.

If soil is completely dry, crops can be direct drilled before rainfall, helping to sow more farm area on time in years with a late break.

Big stubbles need to be managed (mulched or burnt) early so they do not interfere with sowing.

Crop rotation

The most effective way of reducing annual ryegrass-resistant seedbanks and keeping them low is crop and herbicide rotation.

Given the yield penalties associated with delayed sowing, it is worth keeping weed numbers down so crops can be sown on time without relying on knockdown herbicides.

Crop sequences that give two consecutive years of complete seed-set control – double breaks – are the most effective and profitable way of achieving this (Table 1).

Year one Year two
Faba beans (ideally crop-topped and narrow windrows burnt)
Canola (RR or RT hybrids particularly effective, ideally narrow windrows burnt)
Legume-based pasture (winter cleaned and spray topped, fallowed and/or cut for hay prior to annual ryegrass seed set in spring)
Cereal hay or silage (cut and sprayed prior to annual ryegrass seed set)
Cereal hay or silage (cut and sprayed prior to annual ryegrass seed set)

Table 1: Double break options for south-west Victoria that are effective at reducing seedbanks of annual ryegrass to allow early/dry sowing.

Summer fallow

For south-west Victoria, which regularly experiences winter waterlogging, summer fallow rain has not historically been valued, although its importance may increase.

Autumn rainfall decline over the past 17 years and improved agronomy leading to higher-yielding crops have meant that crop yields are increasingly water limited rather than waterlogged.

However, irrespective of whether summer weed control is worthwhile from a water-conservation perspective, nitrogen availability is critical to high yields. The GRDC Water Use Efficiency (WUE) Initiative demonstrated that for every millimetre of water used by summer weeds, they also remove about 1.5 kilograms per hectare of nitrogen.

Also, early-sown wheat crops are more vulnerable to diseases that may have been hosted by weeds and volunteers growing on summer fallows, such as barley yellow dwarf virus (BYDV), take-all, Rhizoctonia and wheat streak mosaic virus. Keeping summer fallows weed free reduces that pathogen risk.

Time of sowing and cultivar selection

Flowering time is one of the main drivers of wheat yield in environments that have cool wet winters and hot dry summers.

For regions such as south-west Victoria, the optimal flowering period is mid-to-late October.

This period is a trade-off between declining frost risk versus increasing drought and heat risk. In this environment, there is a greater penalty for flowering too late rather than too early. Damaging frosts occur infrequently, while dry conditions and hot north winds are historically more damaging.

Slow-maturing cultivars sown early yield more than faster cultivars sown later. This is due to the longer growing season available to early-sown crops, which allows them to produce more biomass.

However, in south-west Victoria, disease pressure from septoria leaf blotch (Zymoseptoria tritici) limits how early it is possible to sow without incurring a yield penalty.

Four years of GRDC-funded trials by Southern Farming Systems, the Foundation for Arable Research (Australia) and CSIRO have repeatedly shown that the highest yields come from crops sown in late-April to mid-May, and from cultivars that best suit a given sowing time.

Winter wheat cultivars such as SQP Revenue, Manning and SF Adagio can be sown much earlier than late April as they need to experience a winter before they will flower and the earlier they are sown the more forage they produce.

Results have also shown that highest yields come from standard HRZ seeding rates of about 200 plants per square metre.

Dual-purpose grazing

The long vegetative period of early-sown winter wheat provides an opportunity to graze with minimal yield penalty in the ‘safe’ grazing period. This extends from the time plants are well anchored with adequate biomass, and pesticide withholding periods have expired until GS30 (mid-July for early April sown winter wheat).

Experiments and modelling suggests that 1500 to 3000 dry-sheep equivalent (DSE) grazing days per hectare can be achieved during this period with little impact on yields.

Disease management

Several wheat cultivars suited for early sowing, such as LongReach BeaufortA, are susceptible to stem rust, and these cultivars should be protected with foliar fungicides at head emergence.

BYDV sporadically infects early-sown crops and needs to be managed, particularly in crops sown before May.

The winter wheats ManningA and Mackellar carry host-plant resistance to BYDV and are suited to very early sowing and grazing. Cultivars that do not carry BYDV resistance, such as SQP Revenue, need to be protected from aphid vectors. An effective insecticide program should start with a seed-dressing product registered for aphid control. It needs to be backed up with a foliar insecticide at GS13 if aphids persist after that. If planning on grazing, check stock withholding periods on any insecticides used.

Nitrogen management

The high yield potential of early-sown crops needs to be supported with nitrogen. Dryland wheat crops need about 40kg/ha mineral nitrogen per tonne of grain yield at 11 per cent protein.

A 6t/ha crop at 11 per cent protein needs a total nitrogen supply of 240kg/ha.

Some nitrogen is available in the soil at the start of the season and some will mineralise during the growing season, but the remainder needs to be supplied as fertiliser.

The amount of nitrogen available in the soil can be determined from soil cores either before sowing, or in spring following very wet winters likely to result in denitrification and leaching.

Mineralisation is hard to predict. It can range from more than 80kg N/ha following a legume pasture in years with good spring rainfall, to 0kg N/ha following a cereal crop with a large stubble load in a dry spring.

In early-sown crops most nitrogen should be topdressed to avoid excessive early growth and lodging.

A split that has been successful in early sowing trials is 25 per cent of total nitrogen budget applied mid-tillering, 50 per cent at GS31 and the remaining 25 per cent at GS37.

More information:

Dr James Hunt, 0428 636 391,
james.hunt@csiro.au

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