By Rohan Leach, Regional Agricultural Landcare Facilitator
After a brilliant season for much of the Central West in 2020, producers will undoubtedly have high levels of stubble cover on their cropping paddocks. While groundcover and moisture conserving stubbles are a sight for sore eyes, producers will in turn be faced with the increased difficulty that heavy stubbles cause when sowing.
Heavy stubbles can cause seeder blockages, hair-pinning of seed and inconsistent stubble and chaff levels and may lead to poor establishment, poor weed control and uneven nutrient uptake in crops. Removing the stubble is a fast solution. However, some producers will be hesitant to do so after ploughing or burning in 2017 and then not having cover on paddocks for the following three years. There are a number of options open to producers, each with their own advantages and disadvantages.
Burning may be an attractive option in some situations this year having “done its job” over summer of conserving moisture and protecting soil. An autumn burn eliminates the possibility of blockages and provides the opportunity of even seed establishment. It also reduces the incidence of fungal diseases like crown rot, which is important after 2020 as fungal inoculum levels will be high after the wet season. While burning removes groundcover, after the widespread March rainfall in the Central West, the risk of crop failure will likely be low, ensuring groundcover will be regained going forward.
Nutrient loss is an unfortunate side effect of stubble burning as nutrients are lost as gases or ash. For example for a 5 tonne/ha wheat crop in 2020 a 7.5 t/ha stubble is expected. In this situation, when this stubble is burnt around 46kg of nitrogen (N) and 2.6kg of phosphorous (P) is lost per hectare. This is the equivalent of around 100kg of urea and 30kg of Single Super. How much is your stubble worth? Soil organic carbon (C) will also decline faster when stubbles are removed. Burning 7.5t/ha stubble releases 2,760kg of carbon into the atmosphere (Midwood & Birbeck, 2011).
Incorporation and cultivation
While incorporating stubbles can have benefits to the seeding operation in terms of reduced blockages and providing an even seedbed, if looking solely to improve the sowing operation, it is not your best bet. The benefits of incorporation come from the associated outcomes such as controlling difficult to kill weeds like windmill grass or fleabane or being able to incorporate lime or gypsum. The soil disturbance will also encourage some weed species to germinate allowing for a pre-sowing flush of weeds and subsequent excellent knockdown of those weeds.
The disadvantages of incorporation are the same as those for conventional farming of soil: reductions in soil structure, crusting or sealing of soil surface, moisture loss and faster decline in soil carbon. What farmers may not think about immediately is nutrient immobilisation caused from incorporating stubbles, in particular N. Soil microbes use stubble as a food source but because of stubble’s relatively low N (especially cereal stubbles) the microbes will also access all the available mineral N in the soil. This “ties-up” N in the short term, leaving the soil deficient in plant available N. This N will mineralise and become available later in the season but a short-term nutrient deficit is caused at the start of the growing season, a crucial point in the crops life and a yield determinant (Gupta et al., 2018). To reduce the impact in this situation, additional N fertiliser must be used or avoid the incorporation procedure.
Chopping, slashing or knocking down stubbles soon after harvest can increase the rate of stubble breakdown while also capturing moisture and providing groundcover. This can be a good option when trying to conserve the nutrients in stubble or in zero till systems. The effects of mulching are maximised when done as far ahead of sowing as possible.
Mulching practices may need to be varied, especially in wet conditions as the chopped stubble may still ball up and cause blockages. More straw laying over in the wet can also increase hair-pinning with disc machines. Mulching close to sowing also reduces the efficacy of many pre-emergent herbicides as the targeted soil is obscured by stubble.
Sowing and machinery practises
If burning, incorporation or mulching are not suitable, producers should look to their sowing rigs. Disc machines can cope with greater stubble loads but farmers must ensure that discs are not worn and that moisture conditions for implement use are ideal.
For tined machines, moving tines to wider spacings is a potential option. These wider spacings allow tines to pass through stubble more efficiently and reduce blockages. However, this would be at the cost of reduced yield in cereals (Midwood & Birbeck, 2011). If RTK, GPS guidance is an option, inter row sowing would be ideal. This has the added benefit of potentially reducing incidence of fungal infections in crop.
What to consider for next time
Managing stubbles can be considered at harvest, post-harvest or pre-sowing. This article has looked primarily at pre-sowing management but harvest management and post-harvest management are equally important. Strategies to consider for next time include:
- Baling straw;
- Lowering harvester height and cutting stubble lower;
- Using livestock to graze over summer;
- Chaff and straw management at harvest; and
- Mulching stubble post-harvest
A note on stubble management and mice
In GRDC’s recent webinar update on mice (February 12, 2021), Steve Henry (CSIRO) talks about the effectiveness of stubble management techniques on mice populations. In summary, shallow cultivation or burning will have little effect immediately on mice populations, as mouse burrows are 30-40cm deep and protect them from injury. What it does do is remove or bury the food source that the grain and stubble is providing. Thus, mice numbers will gradually drop as they look elsewhere for shelter and food. This may take weeks or months, so producers sowing within several days/weeks after such an operation, are unlikely to see a reduction in the impacts of mice on the newly sown crop. To see the full update click here: GRDC Grains Research update, online – Mice – GRDC – See [41:00] for comments on stubble management practises effects on mice.