Break crops in stubble retained cropping systems – Best practice guide for CWFS districts.
Farmer Case Studies
- Case Study 1: North Parkes mine – using peas to manage weeds and improve soil nitrogen.
Property manager: Geoff McCallum
Property size: 4000 ha
Soil type: Clay and red soils.
Average rainfall: 450-530mm pa
North Parkes mine is located approximately 50 kms North West of the township of Parkes, within the Central West of NSW. Surrounding the mine is a commercial cropping operation which is managed by Geoff McCallum who has over thirty years of farm management experience in both the medium and low rainfall zones of NSW and Victoria.
Geoff grows roughly a quarter of the acreage to legumes, a quarter to canola and the rest cereal, usually a quarter wheat and a quarter barley.
Fifteen years ago Geoff introduced conservation farming methods to the farm with all crops now inter-row sown into retained stubble via a tine air seeder on 12” row spacing’s using 2 cm GPS guidance. Keeping a sharp eye out for new ideas that could benefit their cropping activities, three years ago Geoff decided to implement the outcomes of GRDC research which identified the benefits of brown manuring the grain legume field peas within a pea-canola-cereal-cereal rotation.
In this rotation field peas provide a fallow period following the brown manuring. Brown manuring field peas for a fallow period has been shown to increase soil moisture & soil carbon, provide significant nitrogen into a system and also has the ability to assist in addressing herbicide resistance via a complete kill of all weeds during the manuring process.
Geoff likes the field peas compared to other break crops as they provide the option to use a greater array of herbicides during the manuring process and they consume less soil moisture and nutrients leaving more for the following crop in the rotation. This assists in reducing the next season’s nitrogen budget and provides the ability for the next crop to survive and produce during a dry season due to the available soil moisture. However the issue of removing one year’s income from a cropping rotation is a serious consideration and investigation in to the different options available to provide cash income from the pea crop is being conducted.
Morgan peas are the predominant variety grown due to their upright behavior and production of biomass in either dry or wet years. They handle well in an auger, set pods well, and produce a large biomass. In 2013 the variety “Hayman” was trialed. This variety has the ability to grow more dry matter than the Morgan. However they are a later variety and may require earlier sowing, good rainfall or irrigation to ensure a good pod fill. In any given year approximately a quarter of the total farm area is sown to peas with approximately a fifth of that area retained for seed production for use either the following season or for sale. However due to the distance from the point of sale and the glut of peas at the time of harvest Geoff believes that a better option is to store the seed and sell when prices are at a premium or retain for future usage in the event of a seed crop failure.
At the start of the cropping rotation peas are sown into the previous year’s cereal stubble with no fertilizer and at a sowing rate of 80 – 100kg per ha – well above the recommended 35kg per ha. This is done to achieve the highest pea biomass early in the season to ensure a good germination rate, and limit weed growth via shading and competition for nutrients. At the time of sowing pea inoculant is sown with the seed and applied at 1.5 times above the recommended levels. This is done to leave no margin for error; ensuring high rates of root nodulation and providing the maximum opportunity for optimum nitrogen fixation.
Sowing the next rotation of canola into the thick mat of the previous year’s pea stubble has required some creative thinking as well as additions to the sowing equipment. The residual mat of pea biomass must be dry when sowing into it. This was identified on a previous occasion when the combination of wet pea residue combined with the wet clay soils found on the mine site required the seeder to “lift and loop” up to 8 times per run – impacting upon evenness of sowing and the cost effectiveness of the sowing operation. To get through the pea matt, Coulter points have had to be installed on the Tyne seeder and serious consideration has been given to the timing of the brown manuring. When brown manured on the point of flowering the pea stems are un-lignified and break up relatively easily when dry. Brown manuring post flowering and at pod set, the stems are lignified and do not break as easily when dry. Although brown manuring at a later stage can produce higher amounts of biomass and nitrogen it has been found to be of limited value if you can’t plant into it the following season. Consequently the brown manuring is targeted to occur at flowering.
After 17 years of conservation farming, herbicide resistance has been encountered at the mine site. The brown manuring process has been introduced to assist in addressing this issue. Brown manuring provides a high knockdown rate of all weed species at their infant stage, when they are most susceptible to an herbicide application. The timing of brown manuring has been trialed using three different stages of the pea’s life cycle. This trial demonstrated that at the mine they are better to spray on the point of flowering to gain the moisture effects for the fallow, achieve a high level of weed control and reduce the lignification of the pea mass.
In 2013 brown manuring was required on two occasions as approximately one weed per square meter survived, which was considered to be due to the peas impacting upon herbicide coverage. Although this increased expenditure it was considered worthwhile to achieve full weed control. The field pea-canola components of the rotation further provides two broadleaf break crops in succession which enables the ability to control populations of grassy weeds and diseases prior to the cereal rotations.
Brown Manuring Podcast by the GRDC
Geoff has some concerns that the peas aren’t fixing as much nitrogen into the soil as expected. The soils are not acidic and rhizobium nodulation is advanced and healthy. He is considering using faba beans to trial whether or not it will fix more nitrogen and would like to see more research by grower and research groups into this.
The cropping rotation of pea-canola-cereal-cereal allows for the correct period of time between crops to break the cycle of disease between the rotation components. Prior to the introduction of the pea & canola rotations crown rot was an issue within cereal crops. However, the introduction of 2 years of broadleaf break crops has seemingly reduced the carry-over of crown rot between the cereal rotations; one of the main reasons Geoff started using peas as a break crop.
Peas are susceptible to a variety of diseases and there is some concern about the possibility of disease carry over between pea crops facilitated by a number of dry years reducing the ability of pea crop residue to breakdown, but as of yet no issues have been experienced. Some on farm time of sowing trials have occurred with early sowing showing an increase in the risk of black spot and late sowing providing limited N as well as a reduced pea biomass. Consequently for the mine site the best time for sowing has been identified as early to mid-May. This strategy has reduced the risk of disease and has the added benefit of providing optimum rates of nitrogen fixation.
Stubble height of the previous crop is believed to be an issue that requires consideration when sowing peas into retained stubble. At the juvenile stage a pea is susceptible to damage via abrasion which will increase the chance of infection, consequently a stubble height of approximately 150mm is considered to be optimum height. However at the mine stubble height is achieved via the general rule that stubble post-harvest should be the same as the row spacing which is 12” or 30cm and this approach hasn’t caused any disease issues to date. However for the mine, the consideration is that 80% of the pea crop grown is grown to die so disease is not a major concern.
On the mine further on farm investigation into peas is occurring with green manuring to be trialed and the outcomes of the Hayman pea trial to be fully investigated. But for now the brown manuring process and the Morgan peas has proven their worth and are considered to be a valuable component of the cropping rotation.
On the mine peas have been harvested via direct heading, desiccation then direct heading and windrowing. A harvest of 1.5-3t per ha of pea seed is considered to be a good outcome however if a hard dry spring with temperatures greater than 25 degrees is experienced yields have been found to be “ordinary”. Windrowing is the preferred method of harvest followed by desiccation then direct heading. However, due to the clumping nature of the pea, wind damage to the windrows is a serious issue. Consequently desiccation followed by direct heading is the main method used. If timed properly windrowing and desiccation of the field peas allows for the harvest of the peas prior to the windrowing of canola which fits in well with the mines harvesting program and ensures that the peas are harvested at their optimum time.
Harvesting of peas via direct heading has its own individual challenges; the Morgan variety displays upright growth behavior, however due to the large biomass produced once seed is set they tend to lay down which means that the header has to harvest at a low height and slow speed which costs in contractor’s time and risks damage to the header. Due to the damage that has occurred in the past an older header that is dedicated to the pea harvest is used by the contractors. If the pea crop is rained upon the mat of pea biomass develops the consistency of “chewing tobacco” which blocks the header – consequently harvest must occur when the crop is dry.
Soil Health and farm profitability and sustainability:
Overall Geoff feels the soil structure and overall health has improved since using direct drill, organic material is more visible and soil carbon in the 0-10cm range has not decreased in fifteen years. The soil is noticeably softer and crusting is no longer an issue. The use of peas as a break crop has not only contributed that improvement, but has helped to maintain profitability within his stubble retained system through better and more efficient weed and disease control and maintenance of soil nitrogen.
Project Name: Maintaining profitable farming systems with retained stubble in Central West NSW
Project Number: CWF00018