Considering sowing tropicals this spring?

01 Oct 2021

What you need to know

Tablelands Telegraph - October 2021

Clare Edwards, Senior Land Services Officer, Pastures

There has been increasing interest in introduced tropical grass pastures over the last few years. Many producers in the Mudgee area have been trialling small areas of these types of pastures, so I have had many enquiries on where these pastures might fit in producers’ landscapes and how they might be beneficial to their animal enterprises.

Introduced tropical pastures are not new in the area. In a recent tropical pasture survey, I was able to examine some digit pastures that were 15-20 years old and consol lovegrass paddocks planted 30 years ago. Often, these paddocks were sown to reduce the weed issues such as the summer growing introduced weed spiny burrgrass or onto paddocks that had been farmed and consequently had little remaining grass species.

More recently, landholders have been considering tropicals for a range of different reasons, such as minimising vegetable matter in wool, seed availability, weed issues (including blue heliotrope, khaki weed, African lovegrass and cineraria), suitability for lighter soils and the potential to give good groundcover. Overall, the ability for these pastures to respond to summer rainfall, their drought tolerance and potential to fill feed gaps were the main reasons why producers were using these pastures.

If you are thinking about sowing tropicals this spring-summer, here are my top 5 considerations.

1. What is your current pasture or crop?
2. Preparation – Weed control
3. Soil moisture and the likelihood of follow-up rain
4. Soil temperature
5. Sowing – seed depth and seed quality

There are also other considerations, such as how well do these pasture systems fit in your landscapes, enterprise production goals and overall farm feed budget? Likewise, how you manage them to be sustainable and productive is also important. For further information on introduced tropicals please find a link to the NSW DPI booklet ‘Tropical perennial grasses for northern inland NSW’ here.

Current pasture or crop

What is your current pasture species or crop? Are the pasture species winter growing or summer growing? What are the potential weed species? How much legume – such as sub-clover – has been seen in this paddock? Is the pasture species mostly summer growing natives, such as redgrass? Will grazing management, increased soil fertility and a legume work well and be more cost efficient?

Before you remove or change your native grass-based pastures, it is important to consider that native groundcover may be subject to approval under Part 5A of the Local Land Services Act and Federal biodiversity conservation laws and regulations, unless otherwise authorised. Landholders are encouraged to contact their Local Land Services office for advice on their specific circumstances.

Preparation – Weed control

To see a good, successful introduced tropical pasture established, be sure to include preparation for reducing the weed population that would otherwise be aggressive and compete against a newly sown pasture. Some of the more aggressive weeds are summer-growing and include liverseed grass, barnyard grass, spiny burrgrass and blue heliotrope. These will germinate around the same time as the newly sown tropicals. It is recommended that there be at least 2 years of weed control prior to sowing tropicals to reduce that weed seed burden and increase the chance of seedling establishment. Many producers use a forage cropping phase for a couple of years to prepare for sowing.

Soil moisture and follow up rainfall

Firstly, having good soil moisture at sowing is important. This year, many areas will have a full soil moisture profile in early spring – but check as the season goes on because top-up falls may not have been adequate to counteract evaporation rates and plant usage. Grazing hard and/or spraying out forage crops a little earlier than usual – while sometimes hard to do – will start ‘banking’ soil moisture for pasture sowing. Avoid sowing into dry or in very hot conditions.

The second consideration is the rainfall at germination. Many experienced producers will monitor for oncoming weather systems and sow just before a couple of days of forecast rain. 20mm to 50mm after sowing is ideal. However, achieving this goal is sometimes difficult. Secondary root development will depend on rainfall but also on sub soil moisture. Note that storm rainfall can be intense, and may not penetrate the soil profile, hence why sub soil moisture is critical.

Soil temperature

Many of the introduced tropical grass species have certain soil temperature ranges that suit their germination. For example, hodes and digit will germinate on lower soil temperatures than the panics, bambatsi and purple pigeon grass. Having daytime temperatures consistently above 200C and night temperatures above 100C for 7 to10 days is often the ‘sowing window’ for tropicals.

That being said, we also look at steady rising temperatures at 9.00am in the morning. In late October to November, soil temperatures are in the range of 16 – 18 degrees around Mudgee in most years. For higher elevations, these temperature triggers might be late November to December. Note – sowing too early with the potential of late frosts may have an impact on establishment. Most sowings occur before Christmas.

What about a late summer – early autumn sowing? There are some paddocks that have achieved good results, but there are more critical factors to be aware of. The level of weed competition (especially if there is any ryegrass, sub-clover, barley grass, etc present), early frosts and reducing germination, plants not having established their root systems or reached maturity before the onset of cooler weather, and the time lag before being able to graze the paddock (it could be up to 7 months) are all potential problems.

Seed considerations

There are several factors relating to seed that need to be investigated to ensure success. Firstly, seed quality is something you should ask about. Does the seed have a seed analysis certificate? Sometimes, seed harvest occurred in a ‘soft’ year or finish. So, it is important to not only get a germination test which looks at the percentage of normal seeds (or what is likely to come out of the ground), but also read this in conjunction with a tetrazolium test. Note that some species cannot be sown in the year they are harvested. For example, some of the panics and purple pigeon grass.

Secondly, there is a lot of discussion around seed coating and ease of sowing. This is worth investigating, and will depend on your situation, sowing equipment and method, and availability of seed. Thirdly, do not sow too deep. Aim for 10mm depth. Many of our paddocks are a little uneven and this will make sure that the seed isn’t buried. Take the time to check sowing machinery depth.  Finally, don’t forget to soil test and sow with adequate amounts of soil fertility for the pasture to perform and persist.

One of the main issues from the tropical grass paddock survey, taken early in the year around Mudgee, was the companion legume species and diversity of these tropical pastures. This issue is not uncommon in the research world of tropical grass pastures. Discussion includes sowing with a ‘like’ legume eg another tropical, or introducing a cool-season legume such as sub-clover, vetch, arrowleaf or serradella.

Research has also looked at lucerne, which is a perennial legume, as an option. There is still work to be done in this space. That being said, I came across some awesome tropical grass paddocks on my small survey that had good populations of white clover, sub-clover and serradella. All of these had been managed with the legume component paramount in the producers’ thinking to ensure that it persisted as a valuable component of the tropical pasture.

For further information, or if you would like to discussion your options on tropical pastures this year, please feel free to talk to Phil Cranney or myself. We are hoping to hold field days on tropical pastures later in the year and I hope to expand on the survey of Mudgee tropical grass results as well.