Take home messages

• A pasture phase rebuilds organic carbon (C) and soil nitrogen (N) reserves.
• A pasture phase provides opportunities to reduce the weed seed bank prior to cropping.
• To achieve these benefits, the pasture must be dense, productive and persistent.
• Soil test - ensure that all nutrition issues are addressed at establishment and across the entire pasture phase, in particular phosphorus (P), sulphur (S) and molybdenum (Mo).
• pH stratification and subsoil acidity have the potential to severely compromise pasture production.
• Pasture legumes must have effective rhizobia nodules to fix N effectively.
• Winter cleaning within the pasture phase is a highly effective practice for weed control and lasting crop rotational benefits in terms of N and cereal root diseases.
• Match pasture type to desired outcome and utilisation potential.

Background

Across the southern NSW cropping belt, there has been a multitude of research projects conducted and papers written about the management of pastures in a mixed farming system. The biological processes and systemic effects are very well understood, however a large amount of this research was carried out in the 1990s and as this decade came to a close, one paper concluded that ’the relevance of pasture-crop rotations is decreasing as sheep numbers decline and cropping area increases’ and in 2001 this same paper identified that ’there has been little attention paid to the long term consequences of continuous cropping around the issues of nitrogen, weeds, residues, tillage, lime and gypsum’ (Angus, Kirkegaard and Peoples, 2001). So fifteen to twenty years on, what has changed in southern NSW?:

• The area of continuous cropping has increased.
• Crop rotations are longer on the arable zones of mixed farms.
• Canola is the dominant broadleaf break crop.
• There is not a stable, reliable pulse crop being widely grown.
• Organic N levels have declined under long term cropping.
• Weeds are an ongoing issue.
• Direct drilling using knife point press wheel seeders and retaining stubbles are common practice.
• pH stratification and acidic subsoils are a constraint.
• Phosphorus stratification is not well understood.
• Recently livestock enterprises have reached new profitability levels, often higher than cropping and with less risk. Mixed farmers are capitalising on the synergies of grazing crops, pastures and cash crop systems.
• Well managed pastures are one of the key drivers of the profitability, sustainability and resilience of these systems.

It is this last bullet point that is the focus of this paper and in particular, what is the ability of a well-managed pasture phase to deliver significant benefits to the subsequent cropping phase? The main areas of discussion are on:

• Pasture types on typical mixed farms.
• The positive impacts that pastures have on the soil resource.
• Long term weed seed bank management.
• Transitioning from pasture to crop and from crop to pasture.

A typical mixed farm

A ‘typical mixed farm’ is a very difficult thing to describe given the significant variation in farm size and enterprise mix across southern NSW. Table 1 attempts to standardise the typical differences from west to east.

Just as describing the typical farm is difficult, so too is describing a typical pasture. By far the dominant improved species are lucerne, sub clover and arrow leaf clover. Moving east as rainfall increases, perennial grasses are included in the majority of mixes and the pasture phase is longer.

Whatever the mix utilised from this diverse range of pasture species and varieties, the key to bringing the many positive benefits available from the pasture phase across to the subsequent cropping phase is to use the pasture that is best suited to each paddock and manage it to ensure that it is dense, persistent and productive for the entire time that it is there.

The impact of pastures on the soil resource

Pastures are recognised as providing a long term benefit to the soil resource both in terms of soil structure and soil fertility. There are two major nutritional benefits that flow from a pasture phase to the subsequent cropping phase — residual organic N from pasture legumes and an increase in soil C levels. A well-managed legume based pasture can fix between 100 and 200kg/ha of N annually. This number is highly correlated to the shoot dry matter produced. As a rough rule of thumb, research indicates that most legumes fix between 15 and 25kg of N/t of dry matter. This is, of course, dependent on effective nodulation. Pasture type also influences the subsequent supply of N mineralisation into the crop rotation as shown in Figure 1.

Figure 1. N mineralisation rates in relation to previous pasture type and the number of years since the pasture phase (Source: Angus, Bolger, Kirkegaard, Peoples, 2006).

Building up the soil organic matter pool improves soil structure and provides more regulated soil water holding capacity and nutrient cycling and is a very slow biological process. Long term research at Harden by John Kirkegaard and Clive Kirkby of CSIRO has shown that long term cropping (stubble retained — direct drill) depletes soil organic C by approximately 50kg/ha per year, while a pasture phase can increase soil organic C levels by 200 to 550kg/ha per year. The latter is by far, the fastest way to substantially rebuild organic C levels. However, to do this, the pasture must be productive. Recent research has shown that fertility is vital to drive this process (Chan et al. 2010). Without sufficient phosphorus (P) and sulphur (S), the available C and N in the pasture residues cannot be converted to stable humus, which would increase the organic N and C pools in the soil (Kirkby et al. 2014).

Another benefit of deep rooted perennial pasture species, such as lucerne and chicory, is that they have the ability to improve the macro porosity of the soil well into the subsoil layers. The initial benefit is higher infiltration capacity, as well as greater access to deeper water and nutrients for subsequent crops (McCallum et al. 2014). In the southern NSW, chicory is particularly useful for penetrating acidic subsoils. The downside to this was very evident across the ‘millennium drought’, years (2002 – 2009) during which these deep rooted perennials de-watered the subsoil, with negative impacts on subsequent crops. However, this short term impact is outweighed by the long term residual benefits of greater macro porosity. Tap rooted crops could not achieve the same effect.

(Source: McCallum et al. 2014)

Weed seed bank management

Pasture establishment provides an opportunity to reduce the weed seed bank because ideally the pasture density will be high enough to out compete weeds which can be easily achieved with a planned establishment program; an appropriate sowing rate and species mix.

During the pasture phase there are a number of opportunities for reducing the weed seed bank.

These include:

• Fodder conservation as hay and/or silage.
• Spray topping and/or fallowing.
• Winter cleaning of pastures with grazing and herbicides.

Towards the end of the pasture phase it is then important to implement a weed control strategy that achieves zero weed seed set for the three years prior to returning to the cropping phase. A typical strategy would aim to reduce the seed set by 70 to 80 % in year one by utilising strategic crash grazing (running large numbers of livestock in small areas for a very short period of time), fodder conservation or spray topping. This would then be followed by a winter clean in year two and an early fallow in year three.

Winter cleaning involves the use of grazing and herbicides to control a range of broadleaf and annual grass weeds, including capeweed, radish, fumitory, mustards, milk thistle, prickly lettuce, ryegrass, barley grass, brome grass, wild oats and vulpia.

The process aims to:

• Graze the paddock very hard so the pasture left is very short.
• Apply a low dose of Gramoxone^® herbicide after grazing to reduce any residual bulky areas that stock cannot reach.
• A week later, apply a herbicide mix of Gramoxone^® and Simazine^ɸa (plus broadleaf herbicide).
• An alternative herbicide option is propyzamide^ɸb.

^ɸaGesatop is not registered for use on capeweed, radish, fumitory, mustards, milk thistle or prickly lettuce.
^ɸbLabel states not registered for use on capeweed, radish, fumitory, mustards, milk thistle, prickly lettuce, ryegrass, barley grass, brome grass or wild oats.

It needs to be noted that the above strategy has a substantial impact on the desirable species in the pasture. It will take at least eight weeks for the pasture to rebuild dry matter levels back to levels to allow grazing. Consequently, winter stocking rates need to be managed accordingly and it’s highly beneficial to have grazing crops available elsewhere on the farm.

The added proven benefit from winter cleaning is increased N supply to the subsequent crops, along with control of cereal root disease pathogens, if a cereal is to be the first crop in the rotation. Winter cleaning in the year prior to cropping has been shown to increase canola yields by 80% in canola and 40% in wheat with significant increases in the order of 10-15% for the subsequent three years (Harris et al. 2002).

Fallowing involves leaving the land unseeded for a growing season and is an important weed control opportunity. To maximise the level of weed control, apply robust herbicide rates of a glyphosate based application and fallow early. This is then followed by some grazing to reduce the bulk with the aim of maintaining ground cover over the summer period. If any weeds survive the initial glyphosate based application, then a double knock with Gramoxone^® is advised.

Fallowing in early spring will also provide a greater opportunity for spring rainfall to be captured, particularly with perennial species e.g. lucerne and phalaris, and nutrients conserved for the subsequent crop (Table 5). To maximise this opportunity, the fallow must be kept weed free and ground cover maintained over the summer period.

(Source: Angus et al., 2000)

Transitioning from pasture to crop

The chosen time to transition from pasture to crop varies from farm to farm. The timing ranges from a set rotation to simply when the pasture declines to a point where it is no longer productive. Ideally, the transition should occur while the pasture is still fully productive, providing a significant grazing resource, weed control options and building soil resources. If the pasture phase is extended, lucerne and other perennials will thin out to the point where they become vulnerable to invasion by annual weeds. At this point it’s important to return to crop.

Prior to pasture removal, soil testing will indicate the fertility status of the paddock. If pH levels have declined then lime should be applied to raise the pH of the top 10.0cm to at least 5.5 CaCl₂ and strategically cultivated to fully incorporate the lime (Conyers et al. 2017). This strategic cultivation can also have other benefits as described by Conyers et al. 2017.

Transitioning from crop to pasture

The optimal chosen time to transition from crop to pasture also varies from farm to farm. The time is often dictated by the financial return expected from the crop which is influenced by the fertility level of the soil and the weed burden and their corresponding effect on crop yield and input costs. It would be preferable to rotate the paddock back to pasture without a weed burden compromising pasture establishment.

Pasture establishment

Getting this transition right and establishing a dense, productive pasture are critical for achieving the benefits outlined previously. A number of factors influence the success of pasture establishment:

• Nutrition - especially phosphorus (P), sulphur (S) and molybdenum (Mo).
• Acidity.
• Weeds, insects and diseases.
• Establishment technique – under sowing or direct seeding pasture only.

Nutrition

Often setting the paddock up for the pasture phase from a nutritional aspect is overlooked. It is assumed that things were right for the cropping phase so they will be right for the pasture. Two recent projects looking at legume performance have highlighted that things may not be as they seem.

Firstly, the GRDC supported, NSWDPI project (DAN00191) ‘Boosting pulse crop performance on acid soils’ (Burns, Nortonand Tyndall, 2016) has identified that pH stratification is a significant issue for pulse crop nodulation and performance. They found it reasonable to conclude that the presence of undetected, but severely acidic layers is likely to be a major factor responsible for inconsistent ‘performance’ of acid-sensitive pulses on slightly acid (pH CaCL₂ >5.0) and moderately acid soils (pHCaCL₂ 4.6 to 5.0) of the medium and high rainfall zones. Pasture production would be expected to be compromised in the same way, significantly reducing immediate performance and the carryover benefits.

Secondly, ’The Trouble with Sub’ project, funded by the Riverina Local Land Services (LLS) in conjunction with many collaborators, surveyed 81 clover paddocks in the spring of 2015 across the Riverina and south west slopes region.

Some key findings from this project were:

• Legumes comprised 48% of the pasture base.
• Only 23% of paddocks had sub clover with good rhizobium nodulation.
• Nodulation in 45% of paddocks was poor.
• 22% of paddocks had damaging pH levels in the topsoil layer with Al% > 5.0 %.
• 40% of paddocks had a damaging subsoil pH levels with the pH CaCl₂ of the 10 to 20cm layer being less than 4.5.
• Only 4% of paddocks had a history of Mo in the past 10 years.
• 61% of paddocks have a history of fertiliser applications.

Phosphorous (P) drives overall pasture production by boosting the legume content which fixes more N which helps drive pasture growth. The critical level of Colwell P to maximise pasture production depends on the desired stocking rate, soil type and environment as outlined in Figure 2. This issue is comprehensively covered in the publication, ‘Five easy steps to ensure you are making money from superphosphate” (Simpson et al. 2009).

Figure 2.Critical soil phosphorus values relative to stocking rate. (Source: Five easy steps to ensure you are making money from superphosphate (Simpson et al. 2009)).

The current standard practice on most mixed farms is to build the P level of the paddock across the cropping phase with annual mono-ammonium phosphate (MAP) inputs, then let it run down  through the pasture phase. As pasture phases become longer (greater than three years), regular soil testing to monitor P, S and pH is recommended. A typical program that aims to maintain an ideal level of pH, P, Mo and S for pasture and crop production is illustrated through a case study paddock in Table 6.

MAP applied @ 90kg/ha to all crops, with Single Super applied @ 120kg/ha in 2010.

The key message here is to ensure that all nutritional issues are fully considered. Soil testing regularly will determine if a top up lime application and strategic incorporation may be required, along with an application of Mo to ensure conditions are suitable for legume pasture production. More regular P and S applications may be required as well.

Weeds, insects and diseases

Weeds, insects and disease all need to be managed as per current industry practice. Some new pests such as earwigs and slugs need to be considered as they can quickly bare out large areas of establishing pastures, the effects of which carry right through the pasture and well into the next cropping phase.

Careful attention also needs to be paid to the emerging weed problems of prickly lettuce and milk thistle, particularly in the year prior to pasture establishment, as these weeds can be hard to control in some pasture mixes. It is also advisable to be very cautious of the plant back periods that must be observed for herbicides such as Lontrel^® for control of these weeds. In general, it is important to refer to the label of any herbicide used prior to sowing the pasture to ensure that there will be no risk of residual herbicide carry over effects. In particular, group B herbicides (e.g. Atlantis^®) which are being increasingly used to manage wild oats in cereals as well as chemistry used on Clearfield™ crops. Some common herbicides and their plant backs are listed in Table 7.

Note: As there are often rainfall and soil type requirements that affect these periods, refer to the label.

Establishment technique

Regionally there are two methods of establishing pastures:

1. Under a cover crop
2. Direct seeding pasture alone.

There is always debate about which method is best practice, without a clear outcome. There are many research papers that show that cover cropping increases the risk of pasture establishment failure, and reduces the pasture biomass production in the year after establishment as shown in Table 8.

Source: Hayes et al., 2015.

However, the whole farm economic analysis from this same research project concluded that there was no difference in the probable farm cash balance at the end of a decade between pasture establishment techniques. Hence, across the region the dominant method (greater than 80%) of pasture establishment is under a cover crop. There are, however, some clear directions that come from this research:

• Use a low cover crop seeding rate, relative to environment.
• Lucerne and chicory based pastures are more likely to establish under a cover crop
• Perennial grasses, such as phalaris, fescue and cocksfoot are more likely to fail.
• Short term straight annual legumes are best direct seeded.
• Short term, three year ’rebuilding pastures’ are best established without a cover crop.

From my experience and observation, there are several rules of thumb that can be followed to reduce the risk of pasture establishment failure:

• Cover crop choice —Cover crops that finish early in the spring will give the pasture the chance to utilise spring rains for improved establishment. Short season barley and wheats are therefore popular choices. Canola is also being used more widely where weeds are not an issue. Within species there are options on what is the most competitive crop. For example, the wheat varieties Condo and Spitfire are quick, erect and low tillering.
• Time of sowing — Plan to seed undersown paddocks right at the front of the cover crop sowing window to improve establishment opportunities and avoid heat stress in spring. This is where the wheat variety Lancer, which is very slow and poorly competitive through the winter may have a place, as opposed to a high tillering tall variety like Gregory which should be avoided.
• Direction of sowing —It is now recognised that sowing in an east -west direction significantly reduces weed competition in the inter-row. To improve the growth and establishment of the undersown pasture, sow in a north-south direction.
• Pasture type, seasonal conditions and outlook — If sowing perennial grass based pastures in a dry autumn with the forecast of a dry spring, do not use a cover crop, or consider not sowing the pasture at all.
• Environment — in low rainfall environments (< 400mm), use cover crops with caution.

Case study

This short case study is a real example of how the introduction of a pasture phase can rebuild fertility and get on top of a significant weed burden on a mixed farm in the mid rainfall belt on the southwest slopes of NSW.

A 1,000 hectare block was purchased in 2005. The block had been continuously cropped for the previous 15 years. The new rotation changed from 100% crop to 70% crop with 30% pasture because of ryegrass and poor fertility.

Paddocks are now cropped for seven years and out to pasture for four or five years. Pastures are lucerne/clover based and are winter cleaned* in year 2 or 3, have hay made in year 4 (the year prior to being spring fallowed) and spring fallowed in year 5.The cropping phase began with triazine tolerant (TT) canola, followed by wheat with Sakura^®.

*Winter cleaning was predominantly Gramoxone/simazine/Tigrex^® herbicide mix, but now also includes propyzimide .

The canola area now varies between 33% and 45% of crop area depending on the grain market outlook. Winter cleaning and narrow windrow burning have been critical in getting the ryegrass back in control. In the past, without pasture on this farm, achieving protein levels in wheat over 9.5 to 10.0 % rarely happened. For the 2016 harvest, the wheat yields ranged from 4.0t/ha of APW1 on the last of the long term crop paddocks, up to 5.8t/ha of AH2 on paddocks with the second crop after a pasture phase.

Figure 3. The change in rotation on the case study farm. Current is five years’ pasture, canola, wheat, wheat, canola, wheat, canola, wheat, wheat undersown with lucerne and clover pasture.

Summary

A pasture phase can deliver substantial and lasting nutritional benefits well into the subsequent cropping phase, as well as providing the opportunity to drive weed seed banks to very low levels. To achieve these outcomes, the pasture must be dense, productive and persistent. The pasture phase is the ideal opportunity to address soil constraints that are limiting production and rebuild soil N and organic C levels. A rigorous process of soil testing, identifying all the issues then addressing them with the appropriate addition of required nutrients and/or ameliorants is just as important as the selection of the most appropriate pasture species, variety, seeding rate and establishment technique.

Well-managed productive pastures provide significant synergies to a mixed farm — firstly in terms of a major fodder resource for the livestock enterprises and secondly, by providing a range of tangible benefits that enhance the sustainability and profitability of the subsequent cropping phase.

References

Angus, Kirkegaard and Peoples, (2011). Rotation, Sequence and Phase: Research on Crop and Pasture Systems CSIRO publishing.

Angus et al. (2006). Australian Journal of Agricultural Research 44, 355-365.

Chan et al. (2010). A farmer’s guide to increasing soil organic carbon under pastures. I&INSW ISBN:981742560106.

Kirkby et al. (2014). Soil Biology & Biochemistry 68, 402-409.

McCallum et al. (2004). Australian Journal of Experimental Agriculture 44, 299-307.

Angus et al. (2000). Australian Journal of Agricultural Research 51, 877-890.

Hayes et al. (2015). Evercrop – Improving establishment of perennial pastures. CSIRO Ag Flagship.

Contact details

Tim Condon
Delta Agribusiness,
Harden
0427 426 501
tcondon@deltaag.com.au
www.deltaag.com.au
@timmy64c