Using short term pasture phases in crop rotations to manage weeds and soil nitrogen

Author: Annieka Paridaen (Premier Ag Consultancy Group), Cam Nicholson (Nicon Rural), David Watson (Agvise Pty Ltd), Simon Falkiner (Falkiner Ag) and Zoe Creelman (Southern Farming Systems). | Date: 25 Aug 2016

Pasture and forage systems to enable reductions in populations of key weeds  

Annual ryegrass and wild radish remain the two greatest weed problems for cropping in the high rainfall zone (HRZ). The GRDC funded Pastures in Crop Sequencing project was rolled out through Southern Farming Systems between 2012 and 2015 and was intended to encourage a more integrated approach to the management of annual ryegrass and wild radish, by incorporating fodders into the rotation. Using fodders should be an attractive option to many in the HRZ because growers still retain a large livestock enterprise, retain the equipment to make hay and silage and there is a ready market for conserved fodder, especially to the neighbouring dairy region. However, there is resistance to change, partly based on the convenience of finding a solution in a chemical drum.

The use of short term pastures and fodders and stopping weed seed set through silage, hay, grazing or brown manure before they become viable provides another control tactic on weeds. Bringing weed numbers down through just one cycle of no weed seed set can mean weed populations are manageable again through effective pre-emergent herbicides and late season crop competition. 

Development and understanding of pre-emergent chemistry, stubble management and seedling systems has given many options for early season weed control. However, previous work through the Grain and Graze Program showed that despite very effective early season weed control, late germinating (September to November) annual ryegrass and wild radish were still contributing to the seed bank. These late germinating weeds appear far less important in the lower rainfall zones where most of the work had previously occurred.

Herbicide resistance and paddocks with very large weed burdens can be conquered with the right management. Where the enterprise mix allows, the use of short term pastures and fodders in the crop rotation can have a great fit. Four years of field trials at Lake Bolac and Inverleigh has led to these key findings:

  1. Weed populations can be reduced very rapidly, with reductions of more than 80 per cent after one year and 90+ per cent after two years. However, this was totally reliant on preventing weed seed set. In trials where effective early season weed control was achieved, but no late season seed set control, there was a substantial annual ryegrass population the next year. Figure 1 outlines the reduction in numbers of weeds when a fodder crop was followed by a TT canola.

    Figure 1: The change in annual ryegrass numbers (pl/m2) after three years of different fodder break crops at Lake Bolac (weeds counted in June 2012, 2013, 2014) and in the following canola crop (2015).
    Figure 1: The change in annual ryegrass numbers (plants/m2) after three years of different fodder break crops at Lake Bolac (weeds counted in June 2012, 2013, 2014) and in the following canola crop (2015).
  2. The species used made no significant difference to the effectiveness of weed control, except where species failed to establish.
  3. Hay, silage and grazing followed by spraying were equally effective at controlling annual ryegrass.
  4. Stubble burning was ineffective in reducing seed numbers. Even with large stubble loads the temperature reached was insufficient to kill the weed seeds.
  5. Increasing the sowing rate of the sown fodder did not increase the effectiveness of weed control. This would suggest the common sowing rates used in the HRZ are sufficient to provide adequate competition and that stopping weed seed set is the most critical component.
  6. Delayed sowing to achieve a ‘double knock’ was ineffective, most likely because of the very late germinating weeds.
  7. Over summer or autumn cultivation was ineffective in altering weed seed germination.
  8. Summer fodders made no difference to subsequent weed seed numbers.

Control of wild radish was more problematic because of the longevity of viable seed in the soil. After four years there were still large populations of wild radish plants despite effective weed control in each of the subsequent three years. While plant populations were reduced from the first year, seasonal fluctuations in germination were evident.

What is the most suitable fodder break crop?

These results illustrate the range in dry matter production possible from species across different growing conditions. Therefore, the choice of species will depend on the outcomes sought from the fodder phase. 

The tables below outline production and subsequent crop performance when a short term fodder was used for two years to provide the best bet result on reducing weed numbers. Note that most species were sown two years in a row (apart from sub clover, lucerne and Nil plots). The nil treatment was not sown at all and relied on weed seeds already in the seedbank to germinate. The second year of the fodder rotation provided the bulk of the fodder, indicating the seeds can remain viable, especially in a poorer season.

Table 1: Fodder and crop yields from 2012-2015 at Inverleigh when using a two year fodder break crop.

Duration Species 2012 2013 2014 2015 Total fodder (t/ha) 2012-2014
Fodders (t/ha) Fodders (t/ha) Crop (t/ha) Fodders (t/ha) Canola (t/ha) Wheat (t/ha)
Two year fodder Arrowleaf 4.70 7.75     2.70 2.75 12.5
Persian 5.67 7.49     2.46 2.74 13.2
Balansa 4.81 6.78     2.96 2.88 11.6
Sub clover 1.98 5.41     2.51 2.54 7.4
Peas 5.29 3.90     2.75 2.87 9.2
Lucerne 2.08 3.42     1.62 2.32 5.5
Serradella 3.37 1.96     2.57 2.29 5.3
Grazing oats 6.46 8.95     2.54 2.87 15.4
Ryegrass 6.65 6.98     2.53 3.17 13.6
Nil 1.43 5.50     2.16 2.65 6.9
 
LSD p=0.05 0.54 NS  

Table 2: Fodder and crop yields from 2012-2015 at Lake Bolac when using a two year fodder break crop.

Duration Species 2012 2013 2014 2015 Total fodder (t/ha) 2012-2014
Fodders (kg/ha) Fodders (kg/ha) Crop (t/ha) Fodders (kg/ha) Canola (t/ha) Wheat (t/ha)
Two year fodder Arrowleaf 6.94 8.07
  2.15 4.63 15.0
Persian 5.97 6.03     1.96 4.83 12.0
Balansa 4.46 5.08     2.27 4.90 9.5
Sub clover 2.37 4.24     2.04 4.50 6.6
Peas 6.25 11.43     2.25 4.89 17.7
Lucerne 1.66 3.70     1.84 4.79 5.4
Serradella 4.43 0.00     2.51 5.31 4.4
Grazing oats 9.94 12.31     2.23 4.73 22.2
Ryegrass 7.64 7.61     1.66 5.12 15.3
Nil 3.14 7.39     2.06 5.06 10.5
 
LSD p=0.05 0.45 NS  

The key take home messages for fodder crop selection are as follows:

  • Controlling seed set, especially very late season weeds that germinate and grow outside the efficacy of in-crop herbicides is essential to achieve a significant and sustained reduction in annual ryegrass. Seed set sterilisation, silage, hay and green/brown manuring are all effective techniques at controlling seed set if the decisions are focussed on seed set control of the weed and not maximising fodder production (quality or quantity). 
  • For maximum annual dry matter production and grazing consider annual grass species such as oats and annual ryegrass. If a longer fodder phase is sought, consider short lived perennial ryegrass or phalaris. 
  • Increasing the sowing rate does not provide more biomass; common sowing rates are adequate (Table 3).

Table 3: Plant numbers and dry matter produced for common, double and triple sowing rates.

Species Sowing rate (kg/ha) Establishment (pl/m2) Dry matter (kg/ha)
Balansa clover    
Common 6 113 5176
Double 12 202 5812
Triple 18 248 4031
Peas    
Common 100 43 5637
Double 200 74 6393
Triple 300 81 4785
Forage oats    
Common 100 187 8802
Double 200 279 7824
Triple 300 447 9681
  • If a legume is required then consider annual aerial seeding varieties such as arrowleaf, balansa or persian clover. If a longer fodder phase is sought, consider subclover however be prepared for lower growth in the first year. 

  • If maximum nitrogen (N) fixation is required but no grazing, then consider peas or beans.

  • Avoid using lucerne in short term rotations as dry matter production was lower than other legume species (and does effect subsequent crop yields if rainfall is below average).

  • Controlling seed set in wild radish is as important as control in annual ryegrass, however it cannot be eliminated through a short term (three year) fodder phase. A reduction in plant numbers that carry over into the next cropping can be achieved but wild radish will still persist.

  • Other practices such as later sowing, increasing sowing rate and using in crop herbicides with no weed resistance are effective in early and mid-crop, but do not have an effect on the late germinating weeds.  

  • Effective nodulation and husbandry of legumes is critical to achieve N fixation. Just because a legume grows and looks healthy does not mean it is maximising N fixation.

Legume pastures to provide N

Biological fixation of N through fodder legumes gained considerable grower and adviser interest during the project. Despite considerable testing undertaken in this project, the ability to confidently predict the soil N obtained from pasture fodder legumes grown in a rotation remains difficult (certainly far less predictable than legume crops). 

A range of legume species were tested and taking into account the variability in responses (and challenging climatic conditions), some important conclusions could be drawn:

  • Growing fodder legumes does not guarantee an accumulation of soil N. The trialling strongly suggests a lack of N fixation from most legumes at the one trial site (Lake Bolac) despite growing a lot of biomass and looking visually healthy. There appeared to be a lack of N fixation and when this occurs legumes will use equivalent soil N as grasses. The reason for the lack of soil N accumulation remains unclear because the same seed and inoculation was used at other sites when N successfully accumulated. High soil aluminium, molybdenum deficiencies and use of group B herbicides could all be reasons for poor N fixation 

  • N fixation of legumes may be lower than the common rules of thumb suggest. N fixation work conducted in partnership with the CSIRO showed N fixation was lower than expected, even at the sites where soil N accumulation was measured under a legume. This may also indicate a potential problem with N fixation.

N fixation tests (using N15 ) were conducted in 2012 on Persian clover, lucerne and peas grown at Inverleigh.  The fixation rates are presented in Table 4. 

Table 4: N fixed (kg/ha) per tonne of dry matter grown

Species Shoot N fixed (kg N/t DM) Dry matter grown (t/ha) Anticipated soil N added (kg/ha) Soil N above ryegrass (kg N/ha)
Persian clover 17 4.24 72 48
Lucerne 9 1.99 18 -4
Peas 6 5.29  32  33 
  • Lucerne was the least beneficial fodder break crop. Using lucerne in a short term rotation resulted in lower soil moisture at the end of the fodder phase and depleted soil N. Work in Grain and Graze 2 showed similar results, with the lucerne being highly effective at using existing soil N, which then took until at least the second grain crop to become progressively available for the plants to use.

Conclusion

Moving away from continuous cropping does not have to mean sacrificing production or income from that paddock. Utilising a pasture or fodder in the short term can help solve problems in a cropping system (weeds or soil fertility), provide an opportunity (grazing, fodder conservation or fixed N) with a vast range of options available to cater for the preference of the farmer. 

There are many mixed farming operations with weed problems and livestock to feed, however they seek the ideal break crop in the form of a harvestable grain. Why not grow fodder as the break crop?

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

In particular, thanks must be given to research site host farmers Neil Vallance (Lake Bolac) and John & Stewart Hamilton (Inverleigh). Consulting researchers David Watson and Simon Falkiner, Project Manager Cam Nicholson and various SFS staff (full time and casual) over the project for hours spent counting weeds and taking dry matter cuts, particularly Corinne Celestine, Gina Kreeck, Aaron Vague and Zoe Creelman.

Contact details 

Annieka Paridaen
Premier Ag Consultancy Group
PO Box 2152 Grovedale, VIC 3216
0439 339 433
annieka@premierag.com.au
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