‘How much is too much stubble to handle?’
When do you need to reduce or remove stubble from the paddock to avoid affecting crop production and profitability?
Ideally you want to leave enough stubble after harvest and during the summer fallow to benefit from:
- Groundcover protection from wind and water erosion
- Better soil moisture retention to help with timely sowing
- Maintained or improved nutrient retention
- Same or better soil structure by increasing soil microbial activity
- Stubble and residual grain for a livestock feed source
What a retained stubble no-till system won’t do, with or without the addition of high nutrient inputs, is increase total soil organic carbon.
Reducing the stubble load however, must be considered if it compromises the crop production and profitability drivers including:
- poor herbicide efficacy on pre-emergent crop weeds
- trash clearance and blockages at sowing – affecting sowing timeliness
- poor crop establishment – uneven seeding depth, sowing gaps due to blockages
as well as potential risks to crop growing conditions:
- early season nitrogen tie-up
- stubble borne disease carry-over
- harboring pests such as mice, snails and slugs
- increased risk of frost occurring on frost prone areas
Begin decisions regarding stubble load management before harvest, allowing time to get the header organised and to use the harvesting process to produce stubble with desired characteristics for the following season. Continual monitoring of the stubble over the summer fallow period is required to determine if the stubble has the same characteristics required prior to seeding as those determined at harvest, due to changes such as rotations, summer rainfall, grazing intensity or weed spectrum.
Harvest (and the harvesting process) is the best time to consider the type of stubble you require and estimate how much stubble you will need to handle at seeding the following season. Break crops are less likely to cause sowing issues in the Wimmera and Mallee. Canola crops have stems that generally remain upright that can be sown between. Legume stubbles break down faster than cereal stubbles because they have a lower carbon:nitrogen ratio so generally don’t pose a risk to sowing. For example lupins have a C:N ratio of 65:1 whereas cereal is 200:1.
To estimate stubble load
Before harvest, cut 0.5 metre of crop row to ground level, at 5 sample points (totaling 2.5m). Cut the heads off, then dry and weigh the samples.
To calculate stubble biomass: total weight of dry samples (g) = g/m2 2.5m x row width (m)
Every 100g/m2 equals 1t/ha of dry stubble and crop residue.
After harvest, when straw has been cut and spread, place a quadrat whose sides are the width of your sowing row, centrally over a row, and collect the residue inside at 5 sample points. Dry and weigh the samples.
To calculate stubble biomass: total weight of dry samples (g) = g/m2 5 x sowing row width (m) x sowing row width (m)
Again, every 100g/m2 equals 1t/ha of dry stubble and crop residue.
To estimate stubble ground cover
It’s recommended that a minimum of 50 percent (Mallee) to 70 (Wimmera and other areas) percent ground cover (about 1-1.5 tonne dry matter/ha) remains on paddocks to prevent wind erosion.
Figure 1 shows ground cover for wheat stubble at Birchip.
There are also good reference photos of standing, rolled, chained or cultivated stubble for estimating percentage ground cover in Know your Ground Cover – A Guide for Mallee Farmers produced by Mallee Sustainable Farming.
The threshold where machinery can start to have trouble handling stubble varies considerably, but occurs most commonly from 3 to 4t/ha. For some older tined machinery in wet conditions it can be as low as 2.2 t/ha while more modern set-ups and disc seeders can sow into heavy stubbles as of 9t/ha or more. To establish and successfully grow a crop in these high stubble loads requires strategic planning and will be strongly influenced by seasonal conditions, eg. hairpinning of stubbles in wet conditions.
Many Victorian farmers will graze stubbles soon after harvest to utilise any residual grain. Monitoring of stubbles during grazing to determine when stock should be removed based on groundcover, residual grain and green pick, and stubble orientation (standing vs lying).
Once stock are removed, decide whether further stubble reduction is needed. There are occasions when burning will be the most economical solution for trash, weed and disease management. If possible, burning should be avoided due to the loss of groundcover, erosion and moisture conservation reasons, but also public views about air contamination and soil heath. Preferably mulch (slash and leave on surface) or lightly incorporate stubble (shallow tillage to increase soil-stubble contact) soon after harvest to achieve the same benefits and avoid the pre-sowing rush to burn once fire restrictions are lifted. Further discussion about baling, mulching or, as a last resort, burning stubbles safely, can be found here.
Author: Alison Frischke (BCG)
This research is being conducted by BCG as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).