- The components that make up total crop biomass at flowering (peak biomass) are consistent under different management and agronomic practices when expressed as a percentage of total biomass
- Total Crop biomass will differ between practices and varieties, however the percentages remain similar, which may provide some insight into what may makes up the crop residue at harvest.
- Taking measurements prior to, during or post-harvest, obviously are the best way to determine the true crop residue to be managed however, often this doesn’t allow enough time to rectify or implement alternate strategies to improve stubble handling.
During 2013-2018, GRDC responded to industry wide feedback about the lack of knowledge around stubble retained farming systems. Grower groups across Victoria and Tasmania (BCG, VNTFA, SFS and ICC) were engaged to identify and summarise the specific key practices / learnings that reduce the barriers to adoption and provide growers with the tools to see them retain stubble profitably. These practices have been published into ‘locally’ relevant guidelines. Being a National project there were a lot of shared solutions between different groups and states however, where some solutions were not possible, alternatives were identified. The project explored various aspects of stubble management such as weed & pest control, nutrition, machinery set up, crop sequencing and improving establishment.
Many of the issues that contribute to growers having trouble with stubble is when the problems are not identified prior to harvest. When considering what tools growers and advisers could use to determine stubble issues prior to harvest, it was evident that there was little knowledge of what percentage of total biomass was made up of the various components (stem, leaves and head (grain and chaff).
Over multiple years, biomass data was collected from wheat variety trials in the Victorian Mallee/Wimmera. This data was collected in order to determine what makes up crop residues. The percentage weight of components of wheat plants (leaves, stems and chaff) were determined prior to harvest, at flowering. By collecting this information on partitioning of wheat plants it was intended that we can identify principles that growers and advisers could use to determine what management practices to employ to avoid crop residue (stubble) issues arising. Monitoring over different years and varieties will allow variations to be identified.
Data was collected at flowering (GS65) in 2014, 2015 and 2016. These were three very different seasons in terms of environmental conditions and rainfall.
Method for Determining Canopy Composition
Ten individual plants were collected from replicated cereal trials sown in different regions at flowering in 2014, 2015 and 2016 seasons. Through random selection, the plants were collected from each replicate of specific treatments in GRDC funded stubble management trials throughout the Mallee and Wimmera. Each plant was cut at the base of the stems off at ground level, tillers were removed and then the main stem was partitioned into the various fractions (stem, leaf and head). Each component was then oven-dried at 70°C for 72hrs and weighed. This weight was then converted into a percentage of the total biomass (combined weight of the different components).
Results and Discussion
What components made up cereal crop canopies?
As expected, it was found that stems provide the largest portion of biomass at flowering (58%) while heads and leaves only accounted for close to 20% each. Somewhat surprisingly, both wheat and barley had the same breakdown of the components (comparison only made in 2014).
Were differences found between varieties?
It is commonly understood that varieties differ in their total biomass and grain yield. However, what proportion of that biomass is made up of stem, leaves and head may aid growers in their understanding of varieties in stubble retained systems. As part of this project, commonly grown varieties in the region were evaluated for differences between them that make up total biomass and yield using the partitioning methodology. There are plenty of other processes that growers can use to determine a variety’s suitability to stubble retained systems such as its susceptibility to lodging, height and disease rating. However, combining that knowledge with some measurements in-crop may help them to determine the risk of potential issues. By taking these measurements at flowering time, the influences of grain and subsequent total yield do not skew these results. In addition, the grain in theory should be removed, thus resulting in no need for the data to be incorporated.
It was found that though the varieties varied slightly, the differences were only minor. It would be expected that some varieties, that appear to be more “leafy” than others would have a greater proportion (or %) of weight attributed to leaves. Those varieties are usually associated with shorter plant heights, where stems will be reduced. For example, Corack still had a greater percentage of leaves, and lower stem % on average compared to other varieties however this was within 2-5% difference in the years of data collection. Greater repetition of data could help to reliably quantify varietal differences across poor, average and good seasons.
In 2014, commonly grown barley varieties were also measured, with similar results. The greatest difference occurred in Latrobe at it had significantly greater proportion of weight in its heads compared to the stems. Latrobe and Hindmarsh types in other management trials conducted by BCG has been found to behavior differently to other varieties, potentially supporting this finding, however as the dataset for barley is limited, further investigation is required before this can be confirmed. Overall, it could be assumed that whilst this data suggests there is some variation, in general barley varieties appear to represent similar compositions. Significant differences in grain yield and total biomass would change between these varieties, so using those key parameters with the composition data may help growers predict.
What influence did time of sowing have on crop canopies?
Sowing early promotes early growth and subsequent biomass, whereas later sown crops can have less time to grow and are subsequently shorter with less biomass. Two wheat time of sowing trials at Horsham (2014) and Kalkee (2016) were used to determine if sowing time, changes the percentages of the specific crop components.
Despite changes in the percentage of leaves and heads, the stems were quite consistent (61% and 57%). The greatest difference was observed in the heads with only 19% of total biomass being attributed in the earlier sown crop to 27% in the later. As the majority of the issues with stubble exist in regard to the stems, it would be plausible to conclude that despite difference occurring between the proportion of leaves and heads, that, on average, the stems still represent ~60% of total biomass at flowering.
- Wheat and Barley was found (limited to one year of data) to have the same composition (as a percentage of total biomass) of stem (60%), leaves (20%) and head (23%) at flowering (mean of all treatments).
- Minor differences (5-10%) were observed between some varieties, with the earlier maturing varieties producing slightly less weight in its stems and more in its leaves compared to later maturing varieties.
- There was little trend or evidence to suggest that these components change in varieties with different plant architectures (e.g. tall or short) in wheat as all varieties tested averaged similar percentages of each component.
- From a smaller dataset, row spacings and rainfall zone (low or medium) had little to no influence on these percentages.
- A trial in the low rainfall zone (Mallee) in 2014 showed applying N at sowing increased the percentage of weight in the leaves compared to the nil and in-crop treatments.
Whilst the data in this paper is not in itself going to improve stubble handling, the findings can be used to help growers and advisers when estimating potential issues. Below is an equation where this information could be used to help predict stubble loads:
Flowering biomass x 0.6 = potential stubble load
By also taking the height of the crop at flowering, cutting height could be adjusted to further refine this estimate by expressing the cutting height as a percentage of the total height. For example, assuming a crop height of 1m and a cutting height of 30cm, it the “potential stubble load” was multiplied by 0.3 then this would represent the standing stubble and the remaining 70% is the amount needing to be processed.
Using this calculation as a guide could help growers with post-harvest stubble management planning.
Author: Simon Craig (Consultant)