It's trendy to advocate multi species pasture with benefits of biodiversity, improved soil carbon capture, and other claims, mainly unproven, relating to beneficial to the environment.
Many factors are at play here. Given many paddocks have a wide range of soil type, for factors such as pH, soil depth, aspect, and fertility. It is good logic to have a range of species for many paddocks, especially those with considerable variability, but for others a few species is best for productivity.
There is lots of research documenting what are the best species for different soils and environments, but it is limited as to what best builds soil carbon. Logic suggests what pasture species are best for a given environment, they will also be best for building soil carbon. More dry-mater production, of quality herbage, also generally the greater the root systems. More leaf and stems and more roots contribute to greater soil organic matter, and hence carbon.
Most native grass pastures, even those upgraded via correcting soil deficiencies and adding legumes, are multi species. A good example is a research paper, authored by Bob McGufficke, then NSW DPI Inverell district agronomist, published in the Rangeland Journal, July 2003. "A botanical Study of two native grazing options on a commercial cattle property", noted species diversity was greater in fertilised native pasture compared to unfertilised.
The research compared fertiliser and sub clover added to native grass against unimproved native pasture 20 km north of Ashford (northern slopes). Improved area received no other treatment different to the unimproved area. Soil type was sedimentary (trap), slightly acid and low in natural levels of phosphate and sulphur. Improved area carried three times more cattle (and ran them better) had more groundcover, a feature associated with better soil protection and increased soil biological activity. Especially relevant is that improved area had more different native and introduced species (more diverse) than the unimproved native area.
Many years ago a leading NSW DPI Wagga Wagga based research scientist highlighted that it was wasteful and pointless to sow a smorgasbord of temperate perennial grasses in a given situation when it was clear one species was vastly superior to the others. His example was where Phalaris especially suited the given environment and species like fescue, perennial rye and cocksfoot did not (but note these are suited other environments and situations).
Even a mono culture of one species, for example Phalaris, will normally grow in combination with other species, either intentionally or unintentionally. For example most improved temperate perennial pastures, like Phalaris, will grow in combination with a rang of naturalised clovers as well as deliberately included ones like sub clover. Annual grasses will also likely coexist, as well as a rang of broadleaf plants, even if some of these are rated as weeds. Summer herbage commonly includes a number of annual and perennial grasses as well as broadleaf plants.
In our own case tropical grass pastures like premier digit and Consol lovegrass dominate our light acid soils (acidic down the profile) as far as summer early autumn growth is concerned. On heavier soils bambatsi coexists well with digit grass. Even if other perennial species are added they fail to long term persist. In more western areas a greater range of species suit for inclusion in the mix, including buffel on lighter soils, bambatsi and Mitchell grass on the heavier soils.
But winter species, in our example, are far more diverse with serradella, biserrula and sub clover, plus a range of naturalised legumes. Their individual dominance is much related to soil aspects like pH. Other winter species like annual grasses and broadleaf plants (again often termed weeds but commonly useful) commonly comprise a part of the pasture mix.
Species composition is only one part of pasture productivity as well as soil carbon. Especially important also is grazing management. Whatever the grazing management system, an overriding aspect is retention of sufficient plant cover.
Next week: Summer crops to help with winter crop diseases.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
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