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Maximising the benefits of summer rains

Adrian Smith, Senior Land Services Officer (Mixed Farming Systems)By Adrian Smith, Senior Land Services Officer (Mixed Farming Systems)

With many paddocks receiving good summer rains, many question the value of these rainfall events.

Whilst there is no doubt summer rainfall can cause headaches, and can be costly in terms of reducing the grazing quality of dry standing feeds and stubbles, controlling flystrike outbreaks and the weeds that quickly germinate, the real question should be how can we make the most out of these summer rains – after all, we can’t do much about if, when and how much falls!

You can use numbers and statistics to prove just about anything you want, but the reality is in the southern Riverina, we do actually receive on average around 40% of our annual rainfall in what many would call the non-growing period (November through to March). And whilst it can vary hugely, it does suggest we need to better capture the potential benefits this rainfall can provide to our farming systems.

Prior to intensive agricultural development in our area, the landscape was dominated by perennial species – apart from the trees and larger shrubs, this included a range of grasses, shrubs and forbs. These perennials had the ability to grow all year round – and quickly respond to any (summer) rainfall.

More intensive production, both grazing and cropping, coupled with poor management of these perennials (overgrazing or no spelling to allow them to recover), has seen the removal of much of the ‘perenniality’ from our landscape.

Cropping

For those with irrigated summer crops or pastures, the benefits are obvious – immediately it means reduced irrigation demand. And with low water availability, and the high cost of temporary water this year, there is an immediate benefit to your bottom line!

A simple conversion to remember is that a rainfall event of 10 mm is equivalent to 0.1 Megalitre per ha (Ml/ha); 50 mm is 0.5 Ml/ha, and for those under the right clouds, 100 mm of rain is equivalent to 1 Ml/ha.

For those looking further ahead to their winter cropping programs, do these rainfall events provide any benefit?

The answer is yes - provided you control those summer weeds. A rough rule of thumb is for every 10 mm of rainfall that is conserved, about one-third or 3-4 mm is then available for the following winter crop.

When you consider that every millimetre of stored soil moisture results in around 20 kg/ha of wheat, or 15 kg/ha canola, then conserving 30 mm relates to a yield increase of around half tonne per hectare, then the value of conserving any summer rain becomes an important economic consideration.

How you control those weeds is also an issue – chemical fallowing will retain the highest volume of soil moisture, and conserve stubbles which reduce surface evaporation losses and the impact of heavy rainfall. Mechanical control will open up the soil, and result in greater loss of moisture due to evaporation – but in some instances (and for a range of reasons) it may be the best option.

Grazing

For those with perennial pastures, the results will be immediate – the native grasses such as windmill grass, white top and native millet respond almost immediately, as will lucerne.

Whilst we don’t know exactly how much dry matter the native grasses produce, we do know the feed is highly digestible, is typically high in protein and is generally very nutritious.

Lucerne in our environment is very similar. Depending on the amount of rainfall received, a good dryland stand in our area may produce over 3 t/ha dry matter. Add to this the nitrogen fixing benefits, and the strong competition against summer weeds (particularly heliotrope), and you can quickly see how lucerne could play an important role in your farming system. Certainly producers need to be aware of the risks of grazing lucerne (such as bloat in cattle and redgut in sheep), but appropriate management will offset many of the potential downsides.

Once you do have perennials in your farming system, it is important to retain them – as they can be expensive to establish. This is a whole other topic on its own, but it is essential that perennials’, whether they be lucerne, native grasses or forage shrubs, are given the chance to recover, regrow, set seed or build up root reserves. The quickest way to remove them from the system is to overgraze them. Managing them correctly is essential.

What’s the key message?

Whatever your view of summer rainfall events, the bottom line is they do, and will continue to, occur. They can be extremely variable - within a locality and from year to year – but do contribute on average a significant proportion to annual rainfall in our region. The real trick for producers lies in making the most out of them!