So far, my blog posts have (I hope) given some insight into why Tropical Forage Legumes are a vital tool in helping us lessen the contribution of agriculture to climate change. They can also help us adapt our agricultural systems to cope with its effects. I have touched on the role of ‘Adaptive Traits’ in both of these endeavors. Today, I hope to delve a little deeper into this with an important case study that aided the development of my conceptual approach to successful trait selection towards the main objective of ‘Climate Proof’ forage legumes.
Adaptive Traits are those that confer some sort of advantage in a given context. In a drought prone area this could be deep roots that can access the below ground water table. Or, as a previous post discussed, in areas where fire is a significant risk , it could be ‘fire-proof’ seed-pods that can withstand extreme heat of wildfire and go on to germinate.
In the context of livestock production there are other factors to consider and a specific set of adaptive traits needed. A recent Review of Tropical Forage Legumes in Brazil highlighted the importance of context and management via a failed attempt to introduce legumes into a livestock grazing system.
A few years ago in Brazil, in an attempt to improve soil health, cattle farmers introduced a native Stylosanthes legume species into their predominantly Brachiaria grass pastures 1. But far from delivering on the promise of improved productivity, these legumes failed to thrive 1 . After only a few years they had died out and disappeared from the pasture biome altogether – an expensive and worrying problem for the farmers. An investigation found the ‘Crown-Forming’ structure of these Stylosanthes legumes had unforeseen consequences.
If we look at the diagram above, we see that crown-forming legumes like Alfalfa, Sainfoin (and indeed many Stylosanthes species) display a tall, elongated primary axis 2. When planted in the pasture, these legumes grew taller than their neighboring grasses and were left exposed above the pasture canopy. As a result, they disproportionately suffered defoliation by livestock 1. In layman’s terms they were more easily and more often eaten by the cattle until they were ‘grazed-out’ all together. This phenomenon was consistently observed in cattle farms across Brazil and for several years caused significant worry for livestock farmers.
Because legumes play a vital role in soil health and fertility, these Stylosanthes/Brachiaria pastures became degraded and less fertile over time as they reverted back to solely Brachiaria. This resulted in lower quality grazing and lower quality cattle 1 . This had knock on effects for the economic security of the farmers who are paid in weight per carcass 3. It also lead to lower systematic efficiency across the board – a big problem for global Food Security and Climate Change.
Unlike these tall, crown-wearing legumes, other legume crops such as many of the Clover cultivars (Trifolium spp.) exhibit shorter axes which spread outwards instead of upwards and keep below the pasture canopy 2. They are therefore likely to suffer less defoliation stress and may persist in the pasture biome for longer. Had farmers perhaps used a well matched Trifolium spp. instead of a Stylosanthes spp., this costly problem could have been avoided.
This case study highlights why commercial legume cultivars must be matched not only with the climate, but also with the specific agricultural context. I discuss this in my literature review and have developed a preliminary, conceptual approach to trait selection in legumes for downstream commercial breeding pipelines. I identify three major dimensions that I consider critical for trait selection and breeding of superior cultivars – Climate Change, External Inputs and Natural Endowments. Some of my previous posts have already discussed climate change. Lets now explore the other dimensions…..
External Inputs
The ‘External Inputs’ dimension could be broadly considered as everything farmers can control and manage 4,5. It includes factors like the irrigation system in place, animal density per hectare and the relative pasture biome. Defoliation stress by over-grazing for example is related to both herd density and pasture biome and falls within this dimension.
In smaller, lower-input farms with fewer animals, the problem of over-grazing and defoliation might not be such a problem. However, in a larger more industrial scale context, where herds are bigger and grazing is more intense, this should be a major concern. For developing countries in the tropical zone undergoing major livestock intensification, this example is particularly relevant.
Irrigation also has a major contextual impact. A farm that has an irrigation system in place will have a less urgent need for drought resistance legumes compared to a ‘rain-fed’ farm wholly dependent on the weather. This farm will be far more vulnerable to the effects of drought and will need legumes selected or designed to withstand periods of high stress.
Natural Endowments
This dimension broadly encompasses everything that cannot be controlled to any major degree. Depending on the context or discipline, it may be considered as ‘Natural Assets’, ‘Natural Capital’ or ‘Ecosystem Services’ 6. It can include elements of the natural landscape such as proximity to natural water bodies, elevation and slope and soil type (i.e. predominantly clay or sand) 4,5.
These dimensions interact with each in a multitude of ways; the number of computations across the many variables is vast. A farm located on a steep hill with sandy soil for example will experience higher rates of erosion and poor soil quality than one located at the bottom of a valley in predominately clay soil (particularly if located in a region experiencing higher rainfall as a result of climate change). Each context will require different legumes and solutions…..
Trade-Offs
There are also trade-offs involved between agricultural yield and environmental considerations. To strike the right balance between food security and climate change outcomes, genomic approaches to crop breeding should aim to match different cultivars with particular farming systems, ecological contexts and local climates.
As the field of legume genomics advances and commercial breeding strategies improve, we may begin to more accurately target commercial cultivars to suit a specific set of needs. In the meantime, by taking these other dimensions into consideration, we are more likely to prevent costly and largely avoidable mistakes as that experienced in Brazil.
Direction over Perfection
However, as the Sixth IPCC report makes strikingly clear, we are running out of time to meet climate targets 7. We cannot wait for the ‘perfect crop’ before we take action. In this (and all) sustainable agricultural endeavors, we should aim for ‘Direction over Perfection’.
By using this contextually based directional approach to trait selection in legumes and other important crops, we are more likely to be successful in developing superior and successful cultivars for climate change action without sacrificing food security.
1 – Boddey, Robert M, Daniel Rume Casagrande, Bruno GC Homem, and Bruno JR Alves. 2020. ‘Forage legumes in grass pastures in tropical Brazil and likely impacts on greenhouse gas emissions: A review’, Grass and Forage Science, 75: 357-71
2 – Faverjon, L., Escobar-Gutiérrez, A. J., Litrico, I., & Louarn, G. (2017). A conserved potential development framework applies to shoots of legume species with contrasting morphogenetic strategies. Frontiers in Plant Science, 8, 405.
3- Ritchie, Hannah, and Max Roser. 2017. ‘Meat and dairy production’, Our World in Data
4 – Mendelsohn, Robert, and Jinxia Wang. 2017. ‘The impact of climate on farm inputs in developing countries agriculture’, Atmósfera, 30: 77-86.
5 -Pandey, CB, RC Srivastava, and RK Singh. 2009. ‘Soil nitrogen mineralization and microbial biomass relation, and nitrogen conservation in humid‐tropics’, Soil Science Society of America Journal, 73: 1142-49
6 -Martinez-Harms, M. J., Gelcich, S., Krug, R. M., Maseyk, F. J., Moersberger, H., Rastogi, A., … & Pascual, U. (2018). Framing natural assets for advancing sustainability research: translating different perspectives into actions. Sustainability science, 13(6), 1519-1531.
7 -IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.