P.PSH.2009 - Impacts of climate extremes on productivity, nutritional characteristics and persistence of perennial legumes and mixtures.

Summary

Perennial legumes and herbs play an important role in pasture sustainability and methane emissions reduction by virtue of their relative drought tolerance, high nutritional quality and anti-methanogenic properties. However, their responses to predicted changes in climate are largely unknown, as is the role of management in modifying these responses. To address these key knowledge gaps, we exposed four pasture legumes, in combination with herbs and/or grasses, to year-round rainfall manipulation in combination with warming or contrasting management regimes at the Pastures and Climate Extremes (PACE) facility at Western Sydney University.  

Both reduced rainfall and warming decreased pasture productivity, usually in an additive manner. Sainfoin-cocksfoot swards were the most climate-sensitive and digit-desmanthus the most climate-resistant. Although productivity of biserrula was strongly suppressed by warming and low rainfall, it produced the lowest methane in-vitro.

Temperate mixtures (-58%) were more affected by reduced rainfall than tropical (-44%) mixtures, with the largest losses occurring in spring. More frequent harvests (simulating grazing) decreased sward-level productivity but boosted plantain growth. In spring, frequent harvests and low rainfall strongly suppressed legumes and increased senescence, implying lower forage value. Understanding how rainfall interacts with warming and grazing helps inform pasture management decisions for improving sustainability under future climates.

Objectives

The project’s main objectives were to:

1. Quantify the impacts of future climates (+3°C warming and contrasting rainfall extremes) on the productivity and carbon allocation strategies of perennial legumes and grass-herb-legume mixtures across multiple seasons. 
2. Compare the performance of new “opportunity” species (e.g., desmanthus, sainfoin, chicory) with legumes (e.g., lucerne) traditionally used in southern pasture systems under contrasting climatic conditions. 
3. Determine the effects of climate warming and extreme rainfall conditions on plant nutritional chemistry, including anti-methanogenic properties of perennial legume and herb species. 
4. Evaluate the role that grazing management can play in the climate resilience of new perennial pasture mixtures in terms of productivity, persistence and belowground carbon storage.

Key findings

* Based on individual species’ productivity responses to low rainfall and higher temperatures, overall climate sensitivity can be categorised as follows:

a.    High sensitivity (highest risk to graziers): Biserrula, cocksfoot, sainfoin
b.    Medium sensitivity: Lucerne, phalaris
c.    Low sensitivity (lowest risk to graziers): Desmanthus, digit, chicory
* In vitro methane production varied strongly among species and climate treatments. Biserrula had by far the lowest emissions (85% lower than lucerne), confirming its anti-methanogenic potential, including under warming and reduced rainfall.
* Tropical mixtures were more than twice as productive as temperate ones at our warm-temperate study site, and less negatively affected by low rainfall. Their high summertime productivity may help address summer feed gaps, although the nutritional challenges (lower protein, higher fibre contents) associated with C4 grasses would necessitate a higher proportion of companion legumes in such mixtures.
* More frequent (simulated) grazing reduced overall productivity, with no evidence of compensatory growth in response to frequent biomass removal. Frequent grazing under dry springtime conditions significantly reduced seasonal sward-level biomass production, over and above the rainfall-related productivity decline. 
* Belowground carbon allocation was altered by reduced rainfall, resulting in higher root biomass and lower carbohydrate stores. While frequent grazing increased proportional carbon allocation to roots under high rainfall, the opposite effect was seen under dry conditions. The combination of frequent biomass removal and low rainfall also resulted in reduced rates of pasture regrowth (post-harvest), a finding that was strongly associated with low carbohydrate stores. This highlights the importance of moderating grazing pressure under dry conditions, to allow replenishment of belowground carbon reservoirs to fuel aboveground biomass production.

Benefits to industry

* Pasture mixtures including deep-rooted legumes alongside tap-rooted herbs appear to be a good option for increasing climate resilience and boosting productivity. Inclusion of warm-season (C4) grasses in sub-tropical and warm temperate areas is also likely to boost growing season productivity. The lower nutritional quality of such grasses, relative to temperate species, may necessitate an increase in the proportion of legumes.
* Biserrula was associated with very low methane emissions compared to other legume and herb species. Although it was less productive under low rainfall and higher temperatures at this study site, it remains a promising species for reducing enteric methane production and, by extension, increasing feed conversion efficiency. Alongside biserrula, chicory also seems promising for reducing methane emissions intensity.
* Tropical mixtures provide a highly productive feed base that is less affected by low rainfall than temperate mixtures and can help address a summer feed gap in warm temperate regions that have sufficient summertime rainfall (e.g. northern NSW, southern Queensland, South Coast and Western Plains NSW). Targeting higher legume percentages in such mixtures can help offset the lower protein and higher fibre concentrations associated with the C4 grass component.
* Gains of up to 28% in the amount of available forage can be made by reducing the frequency of grazing (simulated by harvesting in this study). Such benefits can be even greater during spring (up to 51%). Reduced grazing pressure during spring resulted in large productivity and resilience benefits in our temperate mixed pastures, particularly under dry conditions. Allowing plants time between grazing events to build up the belowground starch reservoirs that drive re-growth following defoliation and/or alleviation of climate stress can improve both resilience and persistence.

MLA action

MLA is using these findings to inform a new project in the same space: P.PSH.1564 - ​​Perennial mixed pastures for improved productivity and reduced methane emissions in southern Australia​.

Future research

* Evaluation of the climate sensitivity of a wider range of candidate species - and their performance in different mixtures and proportions - is an essential next step towards providing robust recommendations for growers across the region.
* Tropical species have the potential to play an increasing role in more southerly, warm-temperate regions (e.g. NSW Riverina), as demonstrated by their high productivity and relatively lower drought sensitivity in the current study. Further research is needed to evaluate different combinations of species, and legume: grass planting proportions, to achieve appropriate targets for nutritional quality in such mixtures.
* Having identified the importance of reducing grazing offtake under dry spring conditions for forage resilience, overall productivity and persistence, further work is needed to evaluate the consequences and farm-scale economics of a wider range of grazing regimes (considering both grazing intensity and timing), under varying climates.
* Noting the importance of belowground carbon stores for fuelling post-grazing regrowth, a key area for future research is to examine how repeated and/or prolonged drought cycles influence root dynamics, carbohydrate reserves and post-stress recovery. 
* A better understanding of how protein content, protein digestibility (ruminal and post-ruminal) and dry matter digestibility are influenced by warming and drought is needed to enable grower adoption of warm-adapted species across temperate regions. 
* Two species in this study showed particular promise to reduce ruminant methane emissions (biserrula, chicory). However, additional research is needed into the plant chemical and structural mechanisms behind this, as well as evaluation of whether these are linear or threshold responses between plant biomass consumed and methane reduction. Further, confirming in vitro findings in vivo is key to developing a pathway for pasture-based methane emissions reduction.