Sugarcane fallow options trial
Source: GRDC Grower Solutions Group – Coastal Burnett
By Neil Halpin, Bill Rehbein, Ken Bird, Angela Marshall and Steve Ginns (DAF Bundaberg)
In the coastal Burnett region legumes are grown in rotation with cane as a cash crop and also for their soil health and farming system benefits.
There is currently no standard agreement on what is the ‘best’ legume fallow crop to grow in this farming system, either in terms of economic return, or optimisation of the following cane crop. This study compares the effect of ten different fallow management options on the nitrogen addition and soil pathogen load as well as the return on investment.
Statistical analysis demonstrated that peanuts had the highest gross margin irrespective of variety ($3176/ha); soybean and pigeon pea were the next most profitable ($664/ha). The extremely low cane productivity (~40 t/ha) combined with the low sugar price in 2018 resulted in a negative gross margin for the continuous cane treatment. Sunrise pigeon pea generated the greatest crop residue levels (9.3 t/ha) and mungbean, irrespective of variety, the least (5.0 t/ha).
All legume options reduced the pressure of lesion nematodes relative to continuous cane. Although the choice of legume break crop will depend on a growers risk profile, farming system and pest pressure, this trial has provided data that will assist growers make a more informed decision to optimise their farming system.
During the 2017 Steering Committee meeting, growers commented that they only grow legumes to maximise their sugarcane productivity. Some growers observed better sugarcane growth (10 tc/ha) following Kairi peanuts than Holt; others believed that sugarcane productivity was superior following peanuts than soybeans.
In an attempt to address this issue, a replicated field trial (randomised complete block design) was established to answer the broad research question ‘what is the best legume to grow in my sugarcane farming system?’
However, ‘best’ could be:
- the most financially rewarding,
- the crop that provides the most nitrogen to the subsequent crop, or even
- the crop that minimises plant parasitic nematode populations.
The trial investigated the following 10 treatments:
- Kairi – peanuts
- Holt – peanuts
- 2B35-808 – soybean
- A6785 – soybean
- Jade – mungbean
- Onyx – mungbean
- Sunrise – pigeon pea
- Red Caloona
- Bare Fallow
- Continuous Cane
Plots were five cane rows (1.83 m) wide by 30 m long and treatments randomly allocated to plots within three replicate blocks. Prior to establishing the legume crops the site was sampled and gypsum was applied at 2 t/ha to improve the soil calcium/magnesium ratio.
The cane stool was destroyed by two rotary hoe operations, with solubor (2 kg/ha) and sodium molybdite (500 g/ha) sprayed on the soil surface in between rotary hoe operations. The final bed geometry was achieved by ripping the bed area only with a three-tine ripper and waisted crumble roller, and the wheel tracks left un-cultivated.
The legume crops were planted with four rows/bed configuration and fertilised with LegumeMax at 370 kg/ha. The site was irrigated with a high pressure travelling irrigator; a summary of crop inputs is listed in Table 1.
The peanut, soybean, mungbean and pigeon pea crops were grown through to grain and harvested. Red Caloona cowpea was grown as a green manure crop and therefore didn’t generate an income.
Table 1: List of inputs (number of operations) for the different crops
Photo: multiple fallow crop plots, including continuous cane plot in background.
Total dry matter production at physiological maturity was determined by destructively sampling a 1.83 m2 quadrat from each plot with samples placed in a dehydrator at 60°C until constant dry weight was achieved. After obtaining the dry-weight of the sample, a sub-sample of approximately 12 plants was taken and weighed, the plant hand-threshed into grain and residue, grain weight recorded and expressed as a percentage of total dry matter, thereby providing a harvest index. The grain and residue components were then ground to <2mm and sent for analysis to determine N concentration (TKN).
Crop productivity was determined using a KEW small plot thresher to provide ‘commercial yields’. The grain from the soybean, mungbean and pigeon pea plots were sent to Bean Growers Australia to provide a grade / crop price to allow gross crop value and gross margin determination. The peanut samples were taken to the DAF Kingaroy Research Facility, cleaned of dirt and extraneous matter, weighed and a 1000 g sub-sample hulled and graded to determine payment pricing based on the 2017 PCA contract; enabling yield, gross crop value and gross margin calculations.
Photo: KEW harvester harvesting mungbean plot
Gillens Creek Rd, Bundaberg
Statistical analysis demonstrated that peanuts had the highest gross margin irrespective of any variety ($3176/ha); soybean and pigeon pea were the next most profitable ($664/ha). Mungbean variety Onyx ($119/ha) was significantly more profitable than Jade ($-597/ha). Since both Red Caloona cowpea and the bare fallow didn’t generate an income, they had a negative gross margin. The extremely low cane productivity (~40 t/ha) combined with the low sugar price resulted in a negative gross margin for the cane treatment (Table 2).
Sunrise pigeon pea generated the greatest crop residue levels (9.3 t/ha) and mungbean, irrespective of variety, the least (5.0 t/ha) (Table 2).
Red Caloona cowpea provided the greatest amount of nitrogen in crop residue (158 kgN/ha) due to the grain not being harvested, while soybean and mungbean provided the least N (58.2 and 68.7 kgN/ha, respectively).
The peanut crops returned 94 kgN/ha to the farming system in the crop residue (Figure 1).
Please note that these values are only the nitrogen contribution in the above-ground biomass. The Six-Easy-Steps process also factors in another 30% nitrogen to account for the N in the root system and nodules.
Table 2: Treatment effect on gross margin, yield, amount of crop residue and nitrogen contribution of the legume residue. Values in columns followed by the same letter are not statistically different (P=0.05)
Figure 1: Nitrogen contribution from the residue of the different legumes
The two most important plant parasitic nematodes in sugarcane farming systems are root knot and lesion nematodes.
All the legume options reduced the pressure of lesion nematodes relative to continuous cane (Table 3). There was a large variation in the population of root knot nematodes with the various legume options; peanuts were the only legumes, along with bare fallow, to have statistically fewer root knot nematodes than continuous cane (Table 3).
Table 3: Treatment effect on plant parasitic nematode populations/200 mL soil. Values are log (x+1) transformed counts with back-transformed means in parentheses. Values in columns followed by the same letter are not statistically different (P=0.05).
Stunt, Stubby, Ring and Dagger nematodes are considered moderately pathogenic. Stunt and Dagger (data not shown) nematodes were of low numbers and there were no statistically significant treatment effects to report. Whilst not statistically significant, there was a clear trend for peanuts to increase the densities of Ring nematode relative to continuous cane and there was a trend for Kairi to host more Ring nematodes than Holt (Table 4). Cowpea, pigeon pea, mungbeans and soybean variety A6785 hosted significantly less Ring nematodes than the cane and peanut plots. Whilst there was a trend for all legumes to reduce Stubby nematode populations, Onyx mungbean and both of the soybean varieties were not statistically different to the continuous cane treatment. Variety Holt peanut hosted the lowest levels of Stunt nematodes. Table 4: Treatment effect on plant parasitic nematode populations/200 mL soil. Values are log (x+1) transformed counts with back-transformed means in parentheses. Values in columns followed by the same letter are not statistically different (P=0.05).
All legumes significantly reduced Lesion nematodes compared to continuous cane; remember nematode surveys in the 1990s demonstrated nearly every cane paddock in Queensland had Lesion nematodes.
Peanuts provided the highest gross margin; contained 90 kgN/ha in the crop residue (and another 30% in the roots and nodules); hosted the lowest numbers of Root Knot and Stubby nematodes of all the legumes tested but peanut variety Kairi hosted the highest number of Ring nematodes.
Soybean A6785 out yielded 2B35-808 at this site and provided 60 kgN/ha in the crop residue (remember that another 18 kgN/ha will be in the below-ground material).
Cow pea provided the largest N contribution to the farming system, because the grain wasn’t removed.
Mungbean, particularly Onyx, exacerbated Root Knot nematode populations.
At the end of the day the choice of legume break crop will depend on a growers risk profile. However, this trial has provided growers with data that will assist them in making a more informed decision.