Perennial pasture establishment – Reducing the weed seed bank

Rohan Leach, Mixed Farming Advisor

As we roll into the autumn sowing period, I typically receive calls from producers that are keen to sow improved pasture.

My first question for them is always, what management have you undertaken on the paddock so far?

If the answer is along the lines of “no management”, then the best option is, instead of sowing a pasture this year, to start preparing the paddock for a pasture in a few years.

While there are many ways to establish a successful pasture, the option with the least risk and highest chance of success involves careful management and attention in the lead-up to sowing.

There are many factors to consider when sowing a perennial pasture such as soil characteristics, geographical location and rainfall, pasture goals, enterprise mix, and pest management. Answers to these and more are available in the DPI’s “Temperate perennial pasture establishment guide”.

While all the above issues are important, quite often the biggest factor of success or failure of a pasture is competition with weeds.

Weeds that emerge during the year of establishment compete with the small pasture seedlings which rob moisture, nutrients, and light from our intended pasture.

Commonly our desirable perennials exhibit slow growth while they are establishing when compared to weeds, which are commonly fast-growing annuals.

Weed seed bank

To reduce this competition from undesirable weeds, we must reduce the weed seed bank.

The weed seed bank is the build-up of weed seeds over many years in a paddock as plant species have developed differing dormancy levels in their seeds to ensure a competitive survival advantage.

By achieving absolute control of those germinating seeds, we can limit the number of new seeds being produced and reduce our weed competition.

Due to differing seed dormancy levels of weeds, it is essential to do this for several years in the lead up to sowing to run the weed seed bank down.

The good news is we can have a high proportion of our annual weeds cleaned up within 2-3 years of management.

An indication of viable weed seed numbers present in a pasture has been demonstrated by many different researchers, but Bower (2002) showed around 10,000 “weed” seeds per square metre in improved pastures near Wagga, with Mitchel et al. (2021) showing around 53,000 seeds in a long-term native pasture in southern NSW. Grundy (2002, Ch. 3, Pg. 39) estimates the seed bank density ranging from below 1,000 seeds/m² in intensively managed arable fields, up to 80,000/m² in less intensively managed fields.

With this information, consider that when sowing our new pasture, at say a few kg/hectare, we are applying around 50-100 seeds/m².

If we are sowing these few seeds into an unmanaged paddock and expecting a productive pasture to establish, we are hardly creating a fair fight for our establishing pastures.

If we can take that 80,000 seeds/m² to under 1,000 seeds through absolute control of germinating weeds, we give our pastures a much greater chance of successful establishment.

Cropping program for productive pastures

The following is a general winter cropping program to set up a perennial pasture phase.

Please be aware that the advice in this program is general in nature and to engage local agronomists for more situational advice.

YEAR 0

Spring/summer (September - October depending on location/season and how quickly plants are going to seed)

• Start fallow period (chemical fallow as per minimum tillage best practice) to stop paddock weeds from setting seed (commonly referred to as a spring fallow).
• If you have missed this spring fallow, the earlier you can start the better, with February/March fallows simply resulting in a less productive following crop as well as potentially extending the cropping rotation for another year.
• Soil test and determine nutrient and lime requirements. Lucerne prefers a pH of 6-8 (CaCl₂) and phosphorus (P) rich soils – aim for Colwell P above 30. Build this up over the next few years of cropping program if possible, with starter fertiliser when sowing (starter fertiliser rates of 80-100kg/ha to build P, 40-50kg/ha to maintain P levels).
• Cultivation may be necessary and is dependent on trafficability, weed spectrum and management of soil constraints (plough in lime for e.g.).
• May require 2-3 summer fallow sprays, more depending on summer rainfall.
• If cultivation is required leave until January/February and chance of gully raking storms has diminished. This also allows for required ameliorants (lime/gypsum) to arrive on-farm and to be incorporated in the same pass. Incorporate lime after spreading, surface apply gypsum.

YEAR 1

Autumn

• Sow winter crop, matching crop variety to sowing window. If fodder cropping only, sow oats/barley/fodder brassica in March or as early as opportunity presents.
• If grain cropping, full chemical program to control weeds as per local agronomist recommendations. A program with robust knockdown/double-knock sprays with quality pre-emergent chemistry is now considered best practice in many cropping areas.
• Fodder cropping only has the potential for a lesser requirement for in crop weed control as crop termination in Spring will likely achieve absolute weed control. Depending on the situation, an early broadleaf spray to minimise chemical cost (no grass spray) and to prevent broadleaves from setting seed prior to spring fallow.
• Fodder crop only requires a strict approach because as the season progresses the temptation to take the crop through to grain may arise. This will create problems as the plan prior to this has been to minimise weed control costs due to a planned spring fallow. In this situation, the decision to carry through to grain will compromise the pasture program. I would recommend picking early what you intend to do with the paddock and sticking to it.

Winter

• Manage grazing to maximise crop growth or cropping program as per agronomist recommendation for grain production.
• Nitrogen (urea) may be required to maximise plant growth for grazing and grain harvest.

Spring

• Spray fodder crops out in September before grain fill has started and before weeds have set seed (hence why no expensive grass sprays were required in autumn/winter).
• Start fallow as soon as necessary after harvest in grain crops. This will be dependent on rainfall and weed emergence.
• No need for further soil testing for several years. Liming only required once at any stage of program prior to lucerne seeding.
• In grain cropping systems, I would plan for a broadleaf crop to rotate away from cereal in this second year with a return to cereal for the third and final year of the rotation.
• In fodder crop systems, crop species is less important with ideal scenarios avoiding cereal on cereal and avoiding a legume fodder crop. Grazing brassicas are a great option for controlling grass weeds and don’t harbour many soil pathogens that legume crops do however they do require more intense grazing management to match stocking density correctly.

YEAR 2

• Repeat the above process at a minimum for a second year and potentially a third and fourth year pending; weed spectrum, weed burden, and timeliness of spray/operation in the early part of the program.
• Avoid residual herbicides in the final year of program prior to sowing pasture such as CLOPYRALID and other Group 4 herbicides and CHLORSULFURON and other Group 2 herbicides

YEAR 3

Autumn

Lucerne

• Sow lucerne - April or May are ideal, June - August is possible in western environments if planting on a good moisture profile. Avoid sowing in spring as the risk of failure increases drastically.
• Sow lucerne by itself at 4-6kg of coated seed. I advise against undersowing lucerne with a cereal crop to maximise establishment chances in drier years and minimise risk.
• In terms of variety, sow dormancy rated 5-7 in the central west, and then whatever variety is readily accessible by your local ag merch store and inexpensive.
• Use trifluralin pre-emergent herbicide as per label recommendations. Pending weed emergence, grass sprays are cheap with broadleaf sprays almost prohibitively expensive (think $60-80/ha for some broadleaf sprays for mixed results at best).
• Insect pests such as red-legged earth mites may affect establishment. Keep a close watch on seedling plants for several weeks after sowing.

Temperate grasses - winter growing - cocksfoot, phalaris, tall fescue, etc.

• Follow the above program.
• Mixing with sub clover or other legumes is essential and also quite practical as you are able to sow all together in autumn.

Tropical grasses - summer growing - premier digit, bambatsi panic, rhodes rass, etc.

• Similar program to the above is recommended however the attention to detail with the summer fallow needs to be much closer as this is when the competitive weeds are setting seed.
• Dry sowing in November can be quite successful in the central west with the sowing window closing around February.
• Temperate legumes can be oversown the following autumn with disc seeders, established the autumn prior to sowing the grass and allowed to set seed or if using hard seeded legumes, sown as pods with the tropical grass during summer. This is a complex issue and worthy of an article itself.

Further information / bibliography

Bowcher, A. J. (2002). Competition between temperate perennial pasture species and annual weeds: the effect of pasture management on population dynamics and resource use.

Grundy, A. C., Jones, N. E., & Naylor, R. E. L. (2002). What is the weed seed bank? Weed management handbook, 39-62.

Mitchell, M. L., Virgona, J. M., Durling, A., & Dempsey, F. W. (2021). Germinable Soil Seed Bank of Bothriochloa macra Dominated Pasture in South-Eastern Australia.

Belinda Hackney’s annual temperate pastures webinar series:

• Webinar 1
• Webinar 2