Investing in the farm system - legume manure
Author: Peter and Hazel McInerney (3D-Ag Whole farm consulting) | Date: 14 Feb 2023
Take home message
- Understand the long-term implications of rotational choices
- Legume manures provide whole system benefits including:
- A ‘free’ natural source of nitrogen
- Improved soil condition
- Increased diversity of Integrated Pest Management (IPM) strategies
- An aid to affordable carbon capture.
- Farm systems including a legume manure phase are profitable.
Setting the scene
Historically, soil erosion was a major problem in Australian farming systems. In the mid-late 1970s, researchers at Condobolin, Cowra and Rutherglen began working on cropping with reduced tillage. This led to minimum tillage (min-till), and now, zero/no till. This transition was made practically feasible with modified seeding equipment and herbicides that allowed growers to spray and seed.
Until the advent of these new technologies, most farms had been mixed farming systems with crops and legume-based pasture in a rotation. This pasture history meant that soil organic matter (SOM) levels were relatively high.
The early 1990s saw the advent of canola as a major break crop.
In 1991, the wool floor price scheme was stopped and ‘overnight’ half to two thirds of most farms were losing money!
Combine these factors with the drought of 1994 and interest rates rising to 20%+; and the financial pressures created were the catalyst for rapid uptake of min-till cropping systems. Wider, fit for purpose seeding equipment made it an ‘easy’ decision for many farmers to move to cropping intensive systems.
The result was intensification of cropping on large numbers of farms while the area of pasture and sheep numbers declined dramatically. The loss of legumes in many farm systems has increased reliance on ‘bagged’ nitrogen fertilisers and overtime, has contributed to a decline in soil organic matter.
Farming is a nutrient exporting business. For the business and the key asset of the soil to be sustained, not only do those nutrients need to be replaced, but also sufficient must be available in a specific ratio (Kirkby et al 2011) to capture and store carbon.
In many areas after thirty years of no till continuous crop farming systems, there is considerable evidence that the soil resource is being run down, particularly with regard to SOM i.e., your asset is depreciating. SOM is a key to soil health and is directly related to the amount of carbon within the soil.
The decline or absence of legumes in the system contributes to an ongoing decline in soil nitrogen. Without a surplus of nitrogen (beyond what is required for farm produce), the capacity for affordable carbon capture and storage in soil is impacted.
In addition, more and more synthetic fertiliser is now required just to maintain yield potential, with rates of up to 400kg/ha of urea now not uncommon. This adds to escalating input costs and without an associated yield improvement, puts further pressure on farm margins.
Another factor of concern is that continuous cropping is still largely based on cereal and canola. The lack of diversity in such tight sequences has increased pressures on weed control and overtime has contributed to the rise of herbicide resistance in an increasing number of weed species.
In other words, asset management is now more expensive, more difficult and there is an urgent need for change.
There is a tool available that can help address several of these challenges.
Legume manures are an excellent tool for assisting the farmer to:
- Provide a natural source of soil nitrogen
- Increase available strategies for weed control and herbicide resistance management
- Reducing soil borne disease levels
- Over the rotation as a whole - lower input costs, lower risk, more flexibility and potential for profit.
The choice of species or combination of species to target maximum biomass will depend on your location, rainfall, and seed availability and cost. Treat it as any other crop and prioritise which paddock/s based on fertility status and weed control issues as fits your long-term rotational plan.
A legume (pulse) manure phase provides 20-25kg N/ha/t biomass depending on rainfall, i.e., 160-300kg N/ha. In a drought year, this contribution may be reduced to 100kg N/ha, but it is still nitrogen that does not have to be purchased. This natural source of nitrogen is a slow-release form available to following crops or to facilitate carbon retention. To retain a tonne of carbon, 80kg nitrogen is required (Kirkby et al 2011). Home grown nitrogen is obviously a more cost-effective means of building soil carbon. Improving soil carbon in turn improves soil condition making your soil resource more resilient under a wider range of seasonal conditions (i.e., your asset is appreciating).
A wider range of IPM strategies becomes available with an increase in the diversity of rotation. A legume manure phase is an excellent opportunity to manage herbicide resistance issues through termination of the manure crop prior to weed seed set. With a follow up control of any survivors this can provide 98% reduction in resistant individuals. If available in suitable numbers for the required grazing pressure, animals can be used in a spray graze scenario as an alternate strategy. Using non-selective sprays also saves selective herbicides which develop resistance faster from over-use and the risk of them losing efficacy.
A legume manure provides flexibility as an alternate break ‘crop’ to canola and reduces the potential risk of disease build up under a ‘double break’ of two broadleaf crops in succession. In addition, soil borne diseases such as take-all and crown rot can be significantly reduced, especially if the crop following the manure is not a cereal!
Despite the growing evidence, there remains a perception that having a paddock produce no revenue for a season must be a drag on profitability. It’s time to think beyond season by season and consider a legume manure phase as an investment, rather than a cost. As a relatively low-cost investment it provides many benefits to the farm system. Over the life of a well-designed rotation, where subsequent species complement those that follow, the whole system improves over time. The value of a legume manure in the system comes from the benefit to successive seasons with home grown nitrogen for following crops, better soil condition and improved capacity to manage weed and disease.
While the maximum benefit of legume manure is achieved through termination of growth at the appropriate time, if seasonal or financial circumstances dictate, growth can be utilised by making silage or hay or grazing animals to make a cash contribution to the business. The loss of biomass and export of nutrient in product will necessarily compromise additions to the nitrogen bank and progress of SOM improvement.
Figure 1. Gross margin of manure hay crop
Assessing impacts overtime… To assess continuous crop scenarios with and without legumes, a modelled case study follows, based on actual client information and supplementary data, initially prepared in 2014 then updated for subsequent years.
A continuous crop system without legumes (CC-L) is compared with a more diverse rotation including legumes (CC+L) over a 30-year period. Each is based on six phase cycle. The cropping sequence for CC-L is canola-wheat-wheat and repeat initially, moving to canola-wheat-barley repeat, in more recent times. The rotation for CC+L is wheat-manure-wheat-canola-wheat-faba beans throughout.
Revenue and costs are an expression of an amalgamation of client and district data with appropriate market values for the time. From these gross margin values are calculated and presented as a table (Figure 3) and graph (Figure 4).
Figure 3. Mean annual gross margin values per 6-year crop sequence based on cropping sequences with (CC+L) and without (CC-L) a legume manure phase.
The trend over time is clear – average revenue for CC-L is 5% less relative to CC+L.
On a similar basis, expenses for CC-L are 48% greater than those of CC+L.
Overall the average gross margin for CC-L was 21% less than for the CC+L rotation
Figure 4. Mean average gross margin comparison per 6-year crop sequence with (CC+L) and without (CC-L) legume manure crops in the system.
Legumes in all forms;
- Increase diversity of crops grown
- Provide opportunities for strategic weed, pest and disease control
- Reduce requirements for bagged fertilizer N
- Lower cash requirement, particularly up front
- Allow flexibility and lower risk
- Provide flow on benefits to following crops
- Support a stronger balance sheet
- Contribute to nitrogen bank and potential carbon capture.
Why hasn’t the use of legume manure crops become standard best practice?
Kirkby CA, Kirkegaard JA, Richardson AE, Wade LJ, Blanchard C and Batten G (2011) Stable soil organic matter: A comparison of C:N:P:S ratios in Australian and other world soils. Geoderma Vol 163 p 197-208
3D-Ag Pty Ltd
Ph: 0428 317 746
Publish date: February 2023
Was this page helpful?