By Patrick Francis
In December 2020 a group of researchers, Sally Power et al published a paper about “Sustainable pasture systems under climate extremes”. An important topic to review given “Predicted increases in the frequency and intensity of climate extremes pose a new challenge for pasture-dependent industries. The research was funded by the MLA Donor Company.
The driver behind the research was to develop “Knowledge of how key pasture grasses and legumes perform under seasonal drought and elevated temperatures, and understanding of the traits and mechanisms driving species differences,” to assist farmers with planting decisions.
Such knowledge reads like a useful objective given the high rainfall variability being experienced across both renovated pasture and native pasture districts. But the usefulness of the data collected was close to zero due to the research methodology employed. The nine field-grown grasses and legumes, and three sets of two species mixtures, studied over a two-year period were not subjected to livestock grazing. The most critical component to resilient pastures and their ecosystem functions under climate change is grazing management. This applies across all species in all environments.
Applying artificial climate treatments to pasture species without including different grazing regimes is absurd, it makes the research conclusions irrelevant. The message is clear for best practice, pastures will remain resilient in the face of variable rainfall conditions if a minimum of 1200 – 1500kg per hectare of herbage dry matter is maintained at all times. This amount of feed also ensures ecosystem functions in the paddock are protected and healthy.
In 2004 the MLA’s “More Beef from Pastures” guidelines were published. In Unit 7 it describes “…tactical grazing to maximise conversion of pasture into beef:
* Below 1000kg green DM/ha, pasture growth is slow because of insufficient leaf area, prolonged grazing depletes root reserves of perennials so plant survival is at risk and the development of bare areas leads to run-off or water, erosion and weed invasion.
* Between about 1000kg and 3000kg green DM/ha the most rapid pasture growth occurs when sunlight is caught by increased leaf area and converted efficiently into pasture growth.
* Above about 3000kg green DM/ha plants are mature; pasture growth is slowing and quality is lower,; death and decay of plant material can be greater than the regrowth; root reserves are replenished; seed allowed to set.”
Figure 1. Left: Rotational grazing and rest from grazing to ensure a minimum 1500kg/ha green dry matter means perennial plant root systems are massive and constantly restored.
Right: In contrast set stocked pasture plants often have shallow, weak root systems so perennial plants have little resilience under adverse rainfall conditions. Photos: Patrick Francis
These recommendations highlight the importance of grazing management for long-term perennial grasses resilience. So climate change impacts such as increased length of dry periods are not of themselves that important for pasture resilience but how grazing is managed during the adverse conditions.
US grazing consultant Jim Gerrish summarised why grazing management is so important for resilience in a 2020 Maia Grazing webinar. Referring to the role of time in the grazing cycle he said rest from grazing is mostly positive because:
* photosynthesis in the plants in increasing
* plant root systems are expanding
* direct energy flow to the soil is resumed
* nutrients are being slowly released from faeces.
These are the same points as made in “More Beef from Pastures” just expressed slightly differently.
The researchers in this review seem content to ignore the critical impacts of grazing and grazing rest on pasture species instead“… quantifying species’ responses under common growing conditions (background climate, soils),” and suggest that they “… have been able to evaluate their relative sensitivity to severe drought and their capacity to recover when drought is alleviated”.
“More Beef from Pastures” states: “Repeated overgrazing without adequate rest leads to a decline in pasture composition, reduced ground cover and soil and environmental degradation.”
Why the researchers approach has limited value to graziers is demonstrated in the Agriculture Victoria pasture recovery webinar in April 2019. The first four months of 2019 were in most districts the driest first four months of the year on record. The webinar highlighted a case study were are relatively new perennial pasture (sown autumn 2017) had been set stocked and most of the plants killed by overgrazing during hot, dry conditions.
The webinar presenters’ advice was to re-sow the pasture that autumn once the “break” happened. The plants in the case study paddock were not killed by climate change, but by overgrazing during a hot dry period.
On Moffitts Farm we also sowed a new perennial pasture in May 2017. Rotational grazing and rest to ensure a minimum 1500kg/ha herbage dry matter was maintained during the hot, dry first four months of 2019. The pasture plants remained alive and when the “break” happened responded so quickly that within six weeks there was around 3000kg/ha green dry matter ready for grazing. The hot dry conditions were not responsible for this pasture’s resilience, grazing management and rest to ensure minimum levels of herbage are retained in the paddock were the keys.
Figure 2: Two case studies showing impact of grazing management on perennial grass resilience under adverse rainfall conditions. Left: Set stocking killing grasses, requiring the paddock to be re-sown. Right: Moffitts Farm rotation grazing and grazing rest, ensuring perennial grasses survival during prolonged dry and quick regrowth after rain.
What the research concluded
Power et al reached some conclusion after two years imposing higher temperatures and CO2 levels and lower rainfall on a range of species. They were:
* Species differed considerably in their magnitude of productivity decline in response to extreme winter/spring drought.
* Most species recovered quickly following the end of the winter/spring drought.
* Tropical grasses were, on average, more productive under prolonged winter/spring drought than temperate grasses.
* Based on productivity responses across the full two years of this study, species can be categorised according to their drought resistance#, as follows:
High: Rhodes, Digit, Wallaby
Medium: Ryegrass, Phalaris, Lucerne, Kangaroo
Low: Fescue, Biserrula
#Resistance categories correspond to overall yield reductions (all harvests across the 24-month period) in droughted plots,compared to controls: High <20% yield reduction, Medium 20-40% reduction, Low >40% reduction.
* Warming resulted in yield losses for each of the four species examined, at different times of the year.
* Plant nutritional quality was altered by climate extremes, with species-dependent gains and losses.
* Elevated CO2 (eCO2) concentrations increased productivity of tropical legumes.
These conclusions mean little as far as pasture productivity and ecosystem functions are concerned under climate change because outcomes for the first six are almost entirely dominated by grazing management. Other inherent factors impacting grass species resilience are soil type, soil pH, rainfall distribution, and seasonal growth pattern particularly in C3 temperate species which have varieties bred for summer growth and summer dormancy.