Phosphorus fertiliser an advantage for specific native grass species

By Patrick Francis

A review of phosphorus fertiliser trials on native pastures has demonstrated that most of the native grass species across southern Australia benefit from the application. This conclusion is at odds with some regenerative agriculture champions who contend phosphorus fertiliser degrades native pastures. Some even contend no fertilisers should be applied to native grass pastures.

The review “Phosphorus fertiliser management for pastures based on native grasses in south-eastern Australia” by Meredith Mitchell et. al. published in Crops and Pasture Science October 2019, examined eighty-seven site-years of trial data where different levels of P were applied to native-based pastures.

The focus of the trials is south-eastern Australia’s temperate perennial pasture and annual legume/perennial grass zone where average annual rainfall is above 600mm and is winter/spring dominant. Approximately 22% or 3.1million hectares of this pastoral zone is classified as native pasture. According to Mitchell a feature of the native pastures areas is low phosphorus and moderately acidic (pHCaCl2 less than 5.5) soil in generally non-arable parts of the properties and so considered unsuitable for the sowing of introduced species. Many of the pastures have a low to moderate subterranean clover content.

The review “tests the hypothesis that fertilising native pastures in south-eastern Australia with superphosphate is both profitable and sustainable in the long-term, provided certain conditions are met.”

The reviewers first examined the associations between soil fertility and native grasses. One project collected native grasses from 210 sites in NSW and Victoria, mainly roadsides where fertiliser would not have been applied, and species occurrence was related to soil phosphorus (P) and soil acidity (pH).

“Microlaena stipoides was found at both low and high P fertility sites, Rytidosperma spp. (Wallaby grasses) were found at acidic and alkaline pH levels, whereas Bothriochloa macra (Red grasses) and Themeda triandra (Kangaroo grasses) were found mainly at low P fertility and sites with acid topsoils.”

Native grass species distribution

They found that across diverse farm landscapes the dominant native grass species may be an indicator of paddock production potential. In central NSW higher quality species (in terms of crude protein percentage and digestibility) such as Microleana and sub clover were more prevalent on the lower slopes and in areas with deeper soils so had higher livestock production potential. In contrast, the rocky upper slopes were dominated by Rytidosperma (Wallaby grass) species and had lower production potential.

“The low production zone on the upper slopes had above ground net primary production of 4.6 t dry matter (DM)/ha.year from a plant available water store of 45mm,whereas the high production zone on the lower slopes produced 9.9t DM/ha.year from a water store of 65mm. This difference in production occurred despite higher P fertility on the upper slopes (Colwell P: 55 v. 21 mg/kg), which was likely caused by sheep transferring P to the upper slopes.”

An important fertiliser management strategy lesson from this data was that fertiliser should only be applied to the parts of the landscape that are responsive to P based on soil test. This is the same best management principle for fertiliser application to introduced pasture species.

The reviewers contend “increased use of this current, best practice approach to soil P fertility management (i.e. use of soil testing and soil-test targets for management) will ultimately help to demystify the use of P fertilisers on native grass-based pastures”.

When researchers examined more closely what P fertiliser responses were happening in native pastures they discovered there is considerable variation between varieties of the same species. For example across 40 or so varieties of Rytidosperma (Wallaby grasses) examined in one trial all showed “…a positive

response to the addition of nitrogen (N) and P fertiliser; however,there was a considerable range of growth responses.”

Figure 1: Most varieties of Wallaby grass (Rytidosperma species) will have a growth response when shifting from low to moderate soil plant available phosphorus level, but there is considerable variation in the responses. Photo: Patrick Francis.

The problem with this information is most farmers don’t know which varieties of Wallaby grasses are present in their paddocks so cannot determine whether or not what they have will respond to P fertiliser. But this variation in response does provide landholders with a potential opportunity to alter Wallaby grass species composition using fertiliser applications.

The reviewers make a useful point associated about species and varieties varying responses to P fertiliser. They “illustrate the benefits of a more precise definition of native pastures, and instead should be referred to by the dominant perennial grass species, such as a ‘Microlaena stipoides pasture”. The identification of specific grass or legume varieties is standard practice with introduced perennial pasture paddocks.

Current native pasture recommendations

Generic management guidelines have been produced for native pastures based on experimental research. Mitchell summarises them as:

* Identify native grasses and invasive annual species.

* Use rotational or cell grazing.

* Target minimum herbage mass (800 kg DM/ha) and groundcover (80%).

* In spring apply extra grazing pressure to ensure subterranean clover does not smother native grasses.

* Reduce grazing pressure in summer to allow seedset.

* In late summer retain plant litter to minimise bare ground.

These recommendations are in place Meredith says to protect groundcover and perennial plants from over-grazing while enabling animal production. They are not focused on fertiliser management because there is some uncertainty around native pasture species composition and varieties responses to P fertiliser.

Figure 2: Native grasses management recommendations to now have been about grazing management which protects the plants and retains ground cover (left) versus management which prevents seed set, weakens the plants and removes ground cover (right) threatening the paddock’s ecosystem functions. Photo: Patrick Francis.

This review makes an important point about P fertiliser recommendations available to farmers, these don’t account for native pasture species but apply to pasture domintated by introduced species such as ryegrasses, tall fescuse, cocksfoots and phalaris’s growing in conjuction with sub clovers and perennial clovers. The reason for this approach is the clovers are highly dependent on moderate available P nutrient levels for optimum growth. The impact of strong clover growth is increased high nutrient quality herbage in the clovers themselves plus increased soil nitrogen fixation (by the rhizobia bacteria on the roots of the clovers) which in turn boosts perennial grass species growth and combined boost animal productivity per hectare.

“It is because P management is determined by the high P requirement of the legumes in a pasture that the calculators presently do not differentiate between ‘pasture types’”. The assumption here is that all perennial grasses including native species will benefit from the boost in clover growth.  That is now only partly true, some native species are fertility tolerant and will increase their growth as a result. When this extra growth is combined with higher clover growth and is utilised by strategic stocking, production benefits in terms of higher carrying capacity and or livestock finishing to market specifications can be achieved.

Native species which respond to higher soil P targets have been identified in many trials, table 1. “Fertility-tolerant native grasses such as M. stipoides (microlaena sp), B. macra (Red grass sp) , R. caespitosum, R. fulvum, R. richardsonii, R. duttonianum and R. racemosum (all Wallaby grass species) were able to maintain their basal cover in the high-fertility treatments at an Olsen P level of 10–13 mg/kg, while also showing production benefits such as higher carrying capacity”.

“However, there is insufficient evidence to extend these findings to pastures with a base of fertility-intolerant species such as T. triandra (Kangaroo grass sp), R. carphoides,R. auriculatum and R. erianthum (Wallaby grass sp) and we recommend a lower target fertility of <6 mg/kg where these are the dominant species to avoid the risk that these grasses are displaced by other more P responsive grasses and/or legumes.”

Table 1: Final soil-test P levels and outcomes from trials examining native pasture response to fertiliser in the high-rainfall zone of south-eastern Australia.

P targets rather than P quantity

When considering phosphorus fertiliser application to any pasture the reviewers recommend farmers base their decisions on soil P targets rather than recommendations of phosphorus quantity to apply annually or bi-annually. That’s because soil level targets are applicable across different soil types and ensure application adjustments are based  on paddock soil test status  In other words if a paddock soil tests 10 – 13mg/kg Olsen P there is no need to apply any more to maintain production per hectare until the figure drops below 10mg/kg Olsen P. 

“The preferred management is to know the starting soil P fertility, set a viable target for management and work towards it with modest applications of fertiliser appropriate for the species, and plan an increase in livestock numbers as feed supply increases so that competition (particularly from extra clover growth) can be managed.”

An important finding from research at several native pasture sites over multiple years is that there can be a doubling of annual dry matter production from native grass species through the application of non-limiting P fertiliser. But the increase is not necessarily from native grass species but from the response of the other species present such as clovers and introduced grasses present.  This fact highlights the importance of grazing management with sufficient stock numbers and paddock subdivisions to ensure the native grasses are not displaced by the more P responsive introduced grasses, legumes (particularly sub clovers) and annual weeds (particularly capeweed).

Figure 3: Applications of superphosphate without sufficient livestock grazing pressure in spring can result in a proliferation of annual species such as sub clover and capeweed which smother the native grasses. Photo: Patrick Francis.

This grazing management is the same as is used for introduced pasture species to ensure optimum growth and resilience based on recommended minimum ground cover percentage and herbage mass per hectare. This means that if fertiliser results in more pasture production the carrying capacity of the paddock should be adjusted to utilise it without compromising soil health and plant resilience.

There are differences in farmer extension programs as to the targets for ground cover percentage and minimum herbage mass per hectares. This review concluded that “at the higher P levels, sheep need to be removed temporarily under conditions of low feed availability should herbage mass fall to 800 kg DM/ha or groundcover to 80 percent”.

But other farmer extension recommendations for herbage mass provide higher and lower levels. For example the MLA Pasture Ruler encourages 1200kg green DM/ha as a minimum but then confuses the user by referring to much lower levels depending on the type of sheep or cattle grazing the paddock, e.g. for “dry sheep 400kg DM/ha in a 75% digestible pasture” is considered satisfactory. This figure is based on the Prograze “Minimum herbage mass to maintain satisfactory production levels table”.

MLA’s “More Beef from Pastures” indicates that for best practice grazing management “the preferred pasture mass is between 1500kg DM/ha and 2500kg DM/ha” (module 4 page 12).  The booklet then goes on to confuse readers by stating “the preferred pasture mass for stopping grazing on improved perennial pastures is 1000kg DM/ha depending on pasture type and season” (module 4 page 12).

Take home messages

The reviewers concluded that the recommendations for pastures based on fertility-tolerant native grasses are essentially the same as for pastures compromising introduced perennial species such as phalaris, cocksfoot and tall fescue. That is a soil P level of 10 – 15mg/kg Olsen P. But farmers need to be aware that some Wallaby grass varieties and most Kangaroo grass varieties are not impacted by higher soil fertility and these could be adversely affected if grazing management to control other grasses, weeds and clovers that respond to the higher fertility is not implemented.

“A sound knowledge of species identification is vital for landholders to make more informed decisions (around P fertiliser applications). A more sophisticated approach needs to be fostered where the dominant native grass is referred to,… instead of being referred to as a ‘native pasture’, it could be (for example) a ‘Microlaena pasture’ “.

Reference: The full published paper is available to read at https://www.publish.csiro.au/CP/CP19217