Rain forecast for this weekend means Riverina farmers are in the middle of preparing for, and sowing, the 2016 winter crop, an integral part of Australia’s agricultural output. Riverina farmers produce $2.1 billion worth of total agricultural output annually (ABARES) with grain production alone worth $795 million annually.
As a part of the preparation for this year’s crop, farmers have been using stubble burning as an important tool in the management of their farming systems. But why do farmers burn?
There are lots of reasons why farmers burn crop residue, but the overriding issue is that this stubble residue will adversely affect their cropping program, which can be compromised if the stubble -
• causes blockages to seeding equipment
• causes uneven crop emergence
• perpetuates crop diseases
• provides habitat for crop pests
• affects the efficacy of weed control
• affects the uptake of nutrients by the new crop
Farmers retain their stubble for as long as possible to provide grazing for sheep, to reduce erosion risk and to retain moisture.
Southern NSW produces some of the highest crop yields and best quality grain in Australia. Average wheat yields of 3-4 tonnes/ha are common, and could be double that in a good year. A direct result of that is that there is large amounts of stubble to deal with before the next crop can be sown. Typically stubble residue can be 1.5 x the grain yield. Research has shown that if the amount of stubble exceeds 3 tonne/ha then many seeders will have difficulties in sowing crops evenly and there will be blockages as the stubble tends to clump up. Getting crops sown to emerge within the correct time period for each variety is so important for the end result. Agronomists spend a lot of time planning crop rotations with farmers and selecting varieties suitable for each paddock to avoid frost and heat stress in critical times in spring, and this depends on the sowing operation going smoothly.
Many crop diseases and pests are harboured in the old stubble residue, so depending on the crop rotation the stubble may need to be removed to prevent the problems. Fungal leaf diseases such as Yellow Leaf Spot in cereals and Blackleg in canola are carried over in stubble residue. Stubble burning reduces these risks in the paddock so that crop seedlings are not exposed to the disease at emergence. Pests such as earwigs, millipedes and slugs rely on stubble cover to improve their habitat in cropping paddocks and can damage emerging seedlings, especially fragile canola seedlings. Removing the stubble by burning can help prevent these from causing a problem.
Weed competition is the greatest threat to crop productivity worldwide, so early control is essential for ensuring that crop yields are not compromised, and the emerging crop can become competitive itself. Stubble can interfere with efficient application of herbicides. Soil active pre-emergent herbicides will not be effective if stubble cover absorbs the spray, preventing even application. Post emergent weed control may also be compromised if standing stubble is shading seedling weeds so that spray droplets do not reach the target so burning stubble before herbicide application will give better results.
The technique of windrow burning as a way of destroying resistant weed seeds is also becoming an important part of the integrated weed management package. Instead of spreading harvested weeds seeds across the paddock, stubble residues are simply dropped via a shute at the back of the header so they can be burnt in late summer or early autumn in a hot burn that destroys all the weed seeds. This allows the majority of the stubble to be conserved, but herbicide resistant weed seeds are destroyed.
Stubble residues can also affect the uptake of nutrients to crops in some circumstances. Cereal stubble from high yielding crops can have a carbon : nitrogen ratio of 80 : 1. When this residue is incorporated into soil it will provide a feedstock for soil microbes. Carbon from crop residues is the primary driver of soil microbial activity and this in turn leads to the release of plant nutrients. The problem here is that the microbes (bacteria and fungi) need other nutrients, particularly nitrogen to digest this carbon. Nitrogen is stripped from soil reserves to do this, hence there will be a short term tie-up of nutrients that are not available to the emerging crop. Farmers have the choice of adding more nutrient for the crop, having the stubble residue in contact with soil for a longer fallow period, or removing the stubble by burning to avoid these nutrient problems. Local research showed that grazing stubbles over summer helped to avoid this nutrient tie-up and redistribute the nutrients across the paddock. Care needs to be taken to not over graze the stubble and increase the risk of soil erosion by wind and water.
Farmers do their best to avoid any impact of smoke on nearby communities by observing some basic principles when conducting stubble burning, including -
• checking with the Bureau of Meteorology and local Rural Fire Authorities when weather conditions will be suitable for burning
• avoiding calm weather conditions that are associated with temperature inversions where smoke will hang in the air for long periods and can drift into local towns.
• burning stubble in the heat of the day with a good cross breeze to give a quick, hot burn.
• awareness of when winds will take smoke into neighbouring towns.
Burning stubble can be done at various strategic times for many reasons, but there is a need to be aware of the “big picture” how it fits into the farming system and how it can be done to minimise the effects on the community.
FarmLink is continuing its work in the GRDC Stubble Initiative to ensure farmers have the support of their local communities thanks to improved stewardship of stubble burning and ongoing sustainable farm management practices.