A study shows climate change may lead to a decline in pasture quality

An increase in average temperature may render forage crops more fibrous and poorer in protein content. In the process, cattle will need to consume more food to reach slaughter weight and will produce more methane.

By Peter Moon  |  Agência FAPESP – The projected increase in average temperature of at least 2 °C in coming decades may have an unforeseen impact on the livelihood of cattle farmers. New studies suggest that one of the effects of climate change will be a decline in the quality of pasture, which will take longer to digest as the protein content of forage crops falls and they become more fibrous.

Cattle will therefore need to consume more food to reach slaughter weight and will produce more methane, a potent greenhouse gas.

These are key findings from an experimental study conducted by a group of researchers led by Carlos Alberto Martinez y Huaman, a professor in the Biology Department at the University of São Paulo’s Ribeirão Preto School of Philosophy, Science and Letters (FFCLRP-USP) in Brazil. The group included researchers based in Brazil and affiliated with the São Paulo Botany Institute (IBt-SP), São Paulo State University (UNESP), in Jaboticabal and the Goiás Federal Institute (IFG) in Rio Verde, as well as foreign colleagues affiliated with the University of Illinois in the United States.

“We set out to understand how forage crops will respond physiologically and productively to future climate conditions, including higher average temperatures and levels of CO₂, as well as water shortages,” Martinez told Agência FAPESP.

The crops most frequently grown for cattle grazing are classified as C3 and C4 plants based on their leaf anatomies and the enzymes they use to fix carbon in photosynthesis. Soybeans and dry beans, for example, use the C3 (three-carbon) pathway. Tropical grasses, such as sugarcane, corn and forage grasses, use the C4 pathway.

To determine with precision the physiological changes that grazing crops are likely to undergo in the future, Martinez decided to eschew hothouse experiments because the necessary simulations would be limited. Hothouse plants are grown in pots, he explained, and cannot put down deep roots, so they grow less than they do in the field. Other variables that cannot be reproduced in a hothouse are the brightness and temperature variations due to the action of wind on leaves and soil depth, which affects the extent to which roots can find water.

“The hothouse model is valid for some experiments, but field trials are also required to simulate future climate change,” Martinez said. “We were able to warm plants in the open air using infrared heaters. In addition, we enriched the air with CO₂ in an outdoor environment thanks to a system called Trop-T-FACE installed in the field with support from FAPESP’s Research Program on Global Climate Change [RPGCC].”

In these open-air experiments, the plants were subjected to normal conditions in terms of temperature, luminosity, wind and humidity. The soil was deep, enabling the roots to grow as much as necessary in search of water.

The researchers planted guinea grass (Panicum maximum), a tropical fodder crop of African origin that photosynthesizes via the C4 pathway. Widely used in Brazil for pasture owing to its high nutritional value, P. maximum is common in São Paulo and other states.

“We placed the infrared heaters in 16 plots, warming the plants 2 °C above the ambient canopy temperature. The devices detected the ambient temperature every 15 seconds, adjusting themselves as needed,” said Eduardo Habermann, first author of the articles published in Physiologia Plantarum and PLOS ONE. Habermann was supported by a scholarship from FAPESP.

“The experiment was performed in November 2016, a very warm period. The ambient temperature was 38 °C. Canopy temperature reached 40 °C,” Habermann said.

During the trial, the researchers measured leaf gas exchange with the atmosphere, photosynthesis, chlorophyll fluorescence, leaf, stem and total biomass production, and nutritional quality.

“We observed that under dry conditions, the plants attempted to economize water from the soil. This control was performed by the stomata. Stomata are tiny pores in leaves that open to absorb CO₂. As they do so, they lose water. When the soil is dry, the roots detect this, and the stomata close so that the plant transpires less. The effect of water economy is a reduction in photosynthesis, leading to a loss in plant quality,” Habermann explained.

In addition to FAPESP’s support, the study was funded by the National Council for Scientific and Technological Development (CNPq) and the National Water Agency (ANA).

More fibrous leaves

Other responses of guinea grass to water stress detected by the study were an increase in leaf fiber content and a decrease in crude protein, both of which represent a loss of nutritional quality.

According to the researchers, under future temperature conditions, more fiber will mean less digestibility and an increase in methane production by cattle.

“The animals will need to consume more fodder to reach slaughter weight,” Martinez said. “To maintain the same level of production, farmers will have to add feed to the cattle’s diet and irrigate pastureland, significantly raising production costs.”

Another option, which is not always feasible, is expanding the area dedicated to pasture, which could increase deforestation or lead farmers to give up growing other crops.

The research group also conducted experiments with C3 crops, such as Estilosantes Campo Grande, a mixture of the species Stylosanthes capitata and S. macrocephala, protein-rich tropical forage legumes that capture nitrogen from the atmosphere and fix it biologically in the soil, reducing the need to invest in agricultural inputs, mitigating environmental impacts and contributing to faster cattle weight gain.

“Climate change experiments with the C3 legumes yielded the same results in terms of a reduction in nutritional quality,” Martinez said.

The article “Increasing atmospheric CO₂ and canopy temperature induces anatomical and physiological changes in leaves of the C4 forage species Panicum maximum” (https://doi.org/10.1371/journal.pone.0212506) by Eduardo Habermann, Juca Abramo Barrera San Martin, Daniele Ribeiro Contin, Vitor Potenza Bossan, Anelize Barboza, Marcia Regina Braga, Milton Groppo and Carlos Alberto Martinez can be read at journals.plos.org/plosone/article?id=10.1371/journal.pone.0212506.

The article “Warming and water deficit impact leaf photosynthesis and decrease forage quality and digestibility of a C4 tropical grass” (https://doi.org/10.1111/ppl.12891) by Eduardo Habermann, Eduardo Augusto Dias de Oliveira, Daniele Ribeiro Contin, Gustavo Delvecchio, Dilier Olivera Viciedo, Marcela Aparecida de Moraes, Renato de Mello Prado, Kátia Aparecida de Pinho Costa, Marcia Regina Braga and Carlos Alberto Martinez can be retrieved from onlinelibrary.wiley.com/doi/10.1111/ppl.12891.