Despite two years of what most people label as drought for much of the state, with way below average rainfall, tropical grass pasture on light country has commonly been a godsend.
Across much of NSW, light country has commonly been labelled poor country. Low in natural fertility and soil acidity are common features of these soils.
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They are also unable to store large amounts of soil water, a very important factor with cropping, as stored soil water more years than not allows crops to flower, set and fill grain in typically dry warm conditions.
However, in a grazing business, while heaps of stored water can’t be saved following an above average rainfall period, light soils are able to use periodic low rainfall events, common in droughts even if well-spaced.
In contrast, after a long dry period and when pasture has used up soil water, low rain events mainly fail to trigger pasture growth on heavier soil types.
Even a low rainfall event of 5.0mm can trigger useful growth in a tropical grass pasture between the months of September through to late May (nine months) for much of NSW.
In contrast a rainfall event of 20mm or more is commonly required to stimulate useful growth on clay loam soils, and often even 30mm or more on a typical heavy clay soil.
Note dormancy period for tropical grasses vary from area to area.
Once regular frosting occurs, growth stops with dormancy duration as short as only a month or so in hotter environments (warmer winter can experience no total dormancy), but up to five months in higher altitude colder environments.
The ability of tropical grasses to respond to light, as well as better rainfall events, depends on many management aspects, including strength of the pasture related to previous grazing management and soil fertility.
Pasture previously allowed to periodically head and not grazed into the ground, generally has good root reserves and is able to respond quickly to rainfall.
In contrast, heavily grazed especially for long periods with little ground cover, has two disadvantages.
Across much of NSW, light country has commonly been labelled poor country. Low in natural fertility and soil acidity are common features of these soils.
- Bob Freebairn
Firstly, with little ground cover much of rainfall runs off rather than into the soil unless it falls slowly and lightly. Secondly, with low root reserves regrowth is slow.
In contrast to lucerne, when dry sets in, even after a low rainfall event, tropical grasses generally retain their leaves, with good grazing possible from the dried off or drying off plants.
While not precise, 30 to 40 or more kg/ha dry matter growth is possible per 1.0 millimetres rain that falls on tropical grass. That is useful for assessing likely grazing value following rainfall.
Feed quality of tropical grass, as well as productivity, is closely related to soil fertility.
Many sheep and cattle businesses have, over several years, recorded growth rates of weaners for example, equal to what can be achieved off other high-quality pastures.
Soil nitrogen is a key element for high quality and quantity, either supplied via legumes, nitrogen fertiliser, or a combination of both.
Importantly, there are tropical grass species that can perform well in acidic soils, both top and sub soil.
In a perennial grass system, research has indicated further declines in pH are unlikely to occur.
In contrast, annual crop and pasture systems are vulnerable to regular pH decline unless addressed via amendment, such as periodic use of lime.
Long lasting is a further feature of carefully chosen tropical grass species. For example, there are lots of examples of stands now 30 years or so old and continuing to perform well.
Importantly there are also stands in rainfall environments as low as 400 millimetres (long-term average) that have persisted well, and have been far more productive than native pastures.
Growing tropical grasses in conjunction with winter legumes – incorporated separately and in autumn follow grass establishment – is a sound logical long-term strategy.
Legumes help provide winter feed as well as supply nitrogen for the grasses.
No issue is more important than careful planning for tropical grass establishment.
If these stands are to last indefinitely, care to establish a good stand from the first year is good logic and good economics.
Next week: A successful St Barnaby’s thistle control program.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.