2010 stubble loads – what’s the value?

By Ian Menz, I&I NSW District Agronomist Condobolin

The 2010 season has produced a load amount of stubble which we have to be able to manage before sowing next season’s crops. Producers have a number of options open to them on how to manage these stubbles: lower harvest height, mulching, incorporation or the final option burning.
A question that many producers face each season is what to do with the crop stubble from previous year. To answer this question, several factors need to be taken into account and there is no one rule that will work in every situation and for every year. The aim of this article is to outline some of the issues with stubble and its management, and to promote some thought and discussion on the topic.
Why is stubble important?
Trials have shown that stubble cover of greater than 30% leads to an increase of stored moisture during the fallow of 18%. Soil loss will be decreased by 70% of bare soil levels. To increase the percentage of fallow rainfall stored in your soil at planting to 20 – 25% requires standing stubble cover of 70%.
Stubble is a useful way of providing protection to the soil from wind and water erosion. The impact of summer storms, and the impact of driving winds is reduced by standing stubble (with ground cover >30%)
Stubble also insulates the soil from sun and wind, it lowers the soil temperature, reducing evaporation and maintaining surface moisture.
Stubble can be a useful stock feed source. Roughage gained from the straw, combined with the grain and weeds (weed management is important to conserve soil moisture for the following season) in the paddock can keep stock going over the summer months.
Stubble releases nutrients into the soil as it breaks down. These nutrients, approximately 25% of the nutrients used by the crop, can be used by another crop. For example stubble from a 2t/ha crop contains 17kg N/ha, 2kg P/ha, 24kg K/ha and 4kg S/ha.
There are many types of soil microbes and bacteria that use organic matter as food. They recycle nutrients; glue soil particles together, make tunnels, and perform a host of other useful tasks that help make a productive soil. Keeping the stubble encourages them to multiply and build up the soil structure.
If stubble is to be incorporated, it needs to be done as close to harvest as possible, to give the stubble time to break down within the soil before sowing. If this incorporation is delay the nutrients within the stubble maybe tied up and may not be available at the beginning of the next sowing season.
Stubble breaks down faster in summer than winter, and faster in wet than dry conditions. Stubble breaks down faster in the soil, than on top of the soil. Therefore if conditions are not suitable, there still may be a heavy stubble load, causing issues at sowing.
In the case of very heavy stubble, burning maybe the only option, this is an undesirable option as a large proportion of the value of the stubble is lost to the atmosphere. If burning is the only option, the burn should be as late in the season as possible. The will result in a cold burn and will give a chance for some nutrients to be return to the ground. Burning can provide some advantages in that some weed seeds and leaf diseases maybe destroyed. In paddocks, which have a history of high weed and disease, burning maybe a management option?
Issues with keeping stubble
There are a few potential problems with keeping stubble. The main problem is the inability of some sowing equipment to easily work through heavy stubbles. Clearance issues and tine blockages can be common in the stubble from heavy crops. Long straw blocks easier than short straw. For these reasons many farmers choose to disc in the stubble early or to burn heavy stubbles.
Having stubble on the surface creates quite a different environment for crop growth. Although stubble retention improves soil fertility in the long term, it may tie up plant nutrients (particularly N) in the short term.
Stubbles can also be a cause of disease carryover. Crop stubbles can decrease the vigour of crop seedlings, making them more susceptible to disease. Crop rotation is necessary to reduce the effect of diseases; crops should not be sown into their own stubble.

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